Are human males and females more genetically different than members of other species?

Are human males and females more genetically different than members of other species?

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I'm looking at this Ted talk about a Saudi Arabia woman who dared to drive a car in the last few years. This reminds me that until the last century or so, women (all over the world?) enjoyed less rights and might've been pigeonholed into roles predetermined by society. Those roles might've encouraged certain traits, and discouraged others. Those who did not conform might've been punished, like the woman in the talk above received death threats and was jailed.

This sounds to me like selective pressure, did it really exist, and did it have any effect on the genetics/traits of modern women?

This makes me interested in the question - compared to other species, are men and women more genetically different because of selective pressure put on women to conform to male-dominated world for thousands of years before 19th century?

I'm not sure I buy your premise: firstly, the degree and form of male-female differentiation in social roles has varied widely across time and culture in human history so I doubt it forms a uniform evolutionary driver such as you describe. Secondly, the degree of male-female differentiation appears to me to be much greater in species such as gorillas, lions and peacocks than it does in humans so I'm not convinced that humans would stand out on this front as a species we'd expect to have greater genetic differences.

Even so, the only genetic difference between male and female humans is the Y-chromosome. The X appears in both males and females and doesn't have exclusively female lineage so it can't acquire separate genes for male and female. The Y chromosome contains very few genes so its not capable of manifesting a major genetic gap and because there is no recombination in the Y chromosome it is not a fertile ground for new genes anyway.

So males and females have essentially the same genes. However, this isn't the whole story because how, when and whether genes are expressed is about as important as what genes are encoded anyway. It is these differences that enable big differences between the sexes not actual coding differences.

Essentially, what makes a (mammalian) man a man is a small region on the Y-chromosome called SRY. If this region is deleted, a female phenotype having XY-chromosomes develops. If this region is translocated to an X-chromosome, a male phenotype with two X-chromosomes develops. Other than that, the Y-chromosome does not carry a lot of genes.

So apart from the Y chromosome, males and females are genetically very similar and more similar compared to other species.

Are human males and females more genetically different than members of other species? - Biology


There is not one gene, trait, or characteristic that distinguishes all members of one race from all members of another. We can map any number of traits and none would match our idea of race. This is because modern humans haven't been around long enough to evolve into different subspecies and we've always moved, mated, and mixed our genes. Beneath the skin, we are one of the most genetically similar of all species.

Lots of animals are divided into subspecies. Why doesn't it make sense to group humans the same way?

Subspecies are animal groups that are related, can interbreed, and yet have characteristics that make them distinct from one another. Two basic ingredients are critical to the development of separate subspecies: isolation and time. Unlike most animals, humans are a relatively young species and we are extremely mobile, so we simply haven't evolved into different subspecies.

The earliest hominids evolved from apes about 5 million years ago, but modern humans (Homo sapien sapiens) didn't emerge until 150,000-200,000 years ago in eastern Africa, where we spent most of our evolution together as a species. Our species first left Africa only about 50,000-100,000 years ago and quickly spread across the entire world. All of us are descended from these recent African ancestors.

Many other animal species have been around much longer or they have shorter life spans, so they've had many more opportunities to accumulate genetic variants. Penguins, for example, have twice as much genetic diversity as humans. Fruit flies have 10 times as much. Even our closest living relative, the chimpanzee, has been around at least several million years. There's more genetic diversity within a group of chimps on a single hillside in Gomba than in the entire human species.

Domesticated animals such as dogs also have a lot of genetic diversity, but this is mostly due to selective breeding under controlled conditions. Humans, on the other hand, have always mixed freely and widely. As a result, we're all mongrels: Eighty-five percent of all human variation can be found in any local population, whether they be Kurds, Icelanders, Papua New Guineans, or Mongolians. Ninety-four percent can be found on any continent.

Animals are also limited by habitat and geographical features such as rivers and canyons, so it is easy for groups to become isolated and genetically distinct from one another. Humans, on the other hand, are much more adaptable and have not been limited by geography in the same way. Early on, we could ford rivers, cross canyons, move great distances over a relatively short time, and modify our environment to fit our needs. We are also extremely mobile as a species. Even the remotest island tribe in the Pacific originally came from elsewhere and maintained some contact with neighboring groups.

We may think global migration is a recent phenomenon, but it has characterized most of human history. Whether we're moving halfway around the world or from one village to another, the passage of genes takes place under many circumstances, large scale and small: migration, wars, trade, slave-taking, rape, and exogamous marriage (marriage with "outsiders").

It takes a long time to accumulate a lot of genetic variation, because new variants arise only through mutation - copying errors from one generation to the next. On the other hand, it takes just a very small amount of migration - one individual in each generation moving from one village to another and reproducing - to prevent groups from becoming genetically distinct or isolated. Humans just haven't evolved into distinct subgroups.

But I can see obvious differences between people - don't those translate into deeper differences, like propensity for certain diseases?

The visual differences we are attuned to don't tell us anything about what's beneath the skin. This is because human variation is highly non-concordant. Most traits are influenced by different genes, so they're inherited independently, not grouped into the few packages we call races. In other words, the presence of one trait doesn't guarantee the presence of another. Can you tell a person's eye color from their height? What about their blood type from the size of their head? What about subtler things like a person's ability to play sports or their mathematical skills? It doesn't make sense to talk about group racial characteristics, whether external or internal.

Genetic differences do exist between people, but it is more accurate to speak of ancestry, rather than race, as the root of inherited diseases or conditions. Not everyone who looks alike or lives in the same region shares a common ancestry, so using "race" as a shorthand for ancestry can be misleading. Sickle cell, for example, often thought of as a "racial" disease afflicting Africans, is actually a gene that confers resistance to malaria, so it occurs in areas such as central and western Africa, the Mediterranean, and Arabia, but not in southern Africa. In medicine, a simplistic view can lead to misdiagnoses, with fatal consequences. Racial "profiling" isn't appropriate on the New Jersey Turnpike or in the doctor's office. As evolutionary biologist Joseph Graves reminds us, medicine should treat individuals, not groups.

On the other hand, the social reality of race can have biological effects. Native Americans have the highest rates of diabetes and African American men die of heart disease five times more often than white men. But is this a product of biology or social conditions? How do you measure this relationship or even determine who is Native American or African American on a genetic level? Access to medical care, health insurance, and safe living conditions can certainly affect medical outcomes. So can the stress of racism. But the reasons aren't innate or genetic.

Believing in race as biology allows us to overlook the social factors that contribute to inequality. Understanding that race is socially constructed is the first step in addressing those factors and giving everyone a fair chance in life.

The Resources section of this Web site contains a wealth of information about issues related to race. There you'll find detailed information about books, organizations, film/videos, and other Web sites. For more about this topic, search under "human variation," "evolution," "genetics" and "biology." Explore the HUMAN DIVERSITY interactivities in the LEARN MORE section of this Web site.

Is Gender Unique to Humans?

Evidence from our closest evolutionary relatives suggests that we might not be the only animals with a sense of gender identity.

T his summer, in the introductory course I teach on the evolution and biology of human and animal behavior, I showed my students a website that demonstrates how to identify frog “genders.” I explained that this was a misuse of the term “gender” what the author meant was how to identify frog sexes. Gender, I told the students, goes far beyond mere sex differences in appearance or behavior. It refers to something complex and abstract that may well be unique to Homo sapiens. This idea is nothing new scholars have been saying for decades that only humans have gender. But later that day I began to wonder: Is it really true that gender identity is totally absent among nonhuman species—even our closest evolutionary relatives, chimpanzees and bonobos?

B efore tackling this question, it is necessary to define “sex” and “gender.” Sex refers to biological traits associated with male and female bodies. Sex isn’t a perfect binary, but it is relatively simple compared to gender.

G ender is multifaceted, complex, and a little abstract, and not everyone agrees on exactly what it means. That said, there are a couple of aspects of gender that most experts say are essential. The first is the existence of socially determined roles. Gender roles refer to the range of behaviors that society deems normal or appropriate for people of a particular gender based on their designated sex—the norms that (at least in many Western cultures) cause us to expect men to be assertive and brave, and women to be caring and accommodating, for instance.

I t’s common for people to believe that gender roles are natural or innate, ranging from religious claims that they are God-given to the argument made by evolutionary psychologists that they are the biological result of natural selection. On the contrary, while some aspects of gender-correlated behaviors are probably largely genetic in origin (researchers don’t have a great sense of which are and aren’t), most experts agree that many gender-related expectations, such as that girls play princess and boys pretend to be soldiers, are socially determined—that is, we learn them from our culture, often without even being aware of it. This socially learned aspect is as fundamental to gender as the roles themselves.

A nother fundamental aspect of gender is an internal sense of gender identity. Most people don’t just act in accordance with the roles associated with their gender identity, they also feel something inside of themselves that tells them what their gender is. For many, this sense of identity aligns with their biological sex (cisgender), but that’s not true for everyone. Plenty of people are biologically male, but they identify as women, or vice versa (transgender). Some individuals have a gender identity that is somewhere in between masculine and feminine, or it’s a mix of both or neither (androgyny). Still others are intersex, having both male and female biological traits just like those who fit on either side of the sex spectrum, intersex people fall across a range of gender identities.

Alana McLaughlin grew up as “Ryan,” and before she transitioned, she served as a soldier in the U.S. Army Special Forces. Here, she holds up a photograph of herself from before her sex reassignment surgery. She reportedly says that she has always felt female. Barcroft Images/Getty Images

S o, two criteria substantiate gender: socially determined roles and an internal sense of identity. Neither of these by itself is enough to fully encompass what gender is, but most experts appear to agree that each is a necessary aspect of gender. Therefore, to assess the common claim that gender is unique to humans, we need to look at how other species fare with respect to these two criteria.

T his is a tough endeavor—most of what we know about human gender originated from talking to people, and we usually don’t have the ability to ask other species what they think. Nonetheless (as I’ve written about before on the topic of primate vocal communication), we do have some access to animals’ minds through observing their social behavior. The evidence accrued from numerous studies, while not decisive, shows that gender might, in fact, exist in other species.


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F irst, let’s look at the question of socially determined roles. Plenty of nonhuman species show sex-based differences in behavior. From beetles to gorillas, males of many species are more aggressive than females, and they fight with one another for access to resources and mating opportunities. Males are also often the more flamboyant sex, using showy body parts and behaviors to attract females—for example, take the peacock’s tail, the mockingbird’s elaborate song, or the colorful face of the mandrill (think Rafiki from The Lion King). Females, on the other hand, are in many cases more nurturing of offspring than males after all, by the time an infant is born the female will have already devoted significant time and energy toward forming, laying, and subsequently protecting and incubating her eggs—or, in the case of us mammals, she has gone through an intense process of gestation. The costly nature of reproduction for females limits the number of infants they can have that’s why it generally behooves females to be conservative, expending their time and energy on mating with only the highest-quality males. Being choosy in this way has, over evolutionary time, generally yielded fitter offspring. As a result, females of many species have evolved to be the choosier sex, and their mate choices can direct the course of evolution (an idea that scandalized Victorian England when first proposed by Charles Darwin).

A diver swims near a pregnant male long-snouted seahorse in the Adriatic Sea, Pag Island, Croatia. Borut Furlan/Getty Images

T here are exceptions to every rule, of course. Male seahorses get pregnant. Female spotted hyenas dominate males and sport a pseudo-penis (enlarged clitoris) that is capable of erection and can be as much as 90 percent the size of a male’s penis. As matriarchal as spotted hyena society is, it doesn’t quite reach the level of the northern jacana, a wading bird species whose common territory ranges from Panama to Mexico. Female northern jacanas patrol a territory full of males and fight off intruding females the smaller males engage in less territorial behavior than females, instead spending that time caring for a nest full of the resident female’s eggs.

Turning to our closest relatives, chimpanzees and bonobos, we see additional illustrative examples of the natural variation that exists in sex-correlated behavior. Although the two species are 99.6 percent genetically identical (and equidistant from humans), they are quite different . In general, adult male chimpanzees, like males of many species, are aggressive, domineering, and status-seeking. Much of their time is spent either patrolling territorial boundaries to deter or even kill members of other communities, or vying for social power within their own group. Adult females are generally less political and less violent—they have other priorities, like caring for offspring—but they can still influence the state of social affairs by breaking up male fights or leading rival males to reconcile. After all, as is the case in many species, much of what males stand to gain from high status is access to mating opportunities with females.

It’s been said that if chimpanzees are from Mars, then bonobos are from Venus . Bonobo society is generally female-dominated . Unlike female chimpanzees who mostly, though not always, keep their noses out of politics, female bonobos reign by forming male-dominating coalitions. They bond partly through genito-genital rubbing (it is what it sounds like), forming stronger relationships than female chimps typically have with one another. As for male bonobos, they are much less violent on average than male chimps. Unlike with chimpanzees, lethal aggression has never formally been observed in bonobos (though there has been one suspected instance ) bonobos are more likely to share food (and maybe sex) with a stranger than to fight.

Some scholars look at the sex differences in behavior described in the above paragraphs as clear examples of nonhuman gender. But none of the evidence I have covered so far proves that behavioral differences between male and female chimpanzees, bonobos, or other nonhuman species are socially determined. Again, gender necessarily entails socially determined roles. Do we have any evidence that chimp and bonobo behaviors are determined socially rather than biologically?

T hat is the question Michelle Rodrigues, a postdoctoral researcher at the University of Illinois, and Emily Boeving, a doctoral candidate in psychology at Florida International University , set out to answer . They found that there is flexibility in some of the sex roles previously observed in chimpanzees and bonobos—specifically, in grooming. In both chimpanzees and bonobos (as well as in many other primates), grooming serves as a way of strengthening social bonds. In the wild, most of the grooming in both species is male-on-female or vice versa. Where the species differ is that among wild chimpanzees, male-male grooming is generally more common than female-female grooming—an imbalance not seen in bonobos.

Bonobos groom each other at the Columbus Zoo. Michelle Rodrigues/Springer Japan KK

R odrigues and Boeving wondered whether chimps and bonobos living at zoos would show the same grooming patterns. To investigate this, they observed chimpanzees and bonobos at the North Carolina Zoo and Columbus Zoo, respectively, paying special attention to grooming networks. In contrast to data from the wild, zoo-living apes’ grooming seemed to be more related to individuals’ histories and personalities than their sex: Neither species showed the sex-typical grooming patterns displayed by their wild counterparts.

T his is solid evidence that certain sex roles are at least partly environmentally determined in these species. But is environmental determination the same as social or cultural determination? Not exactly. Social learning could be responsible for the flexibility we see in chimpanzee and bonobo sex roles. In this hypothetical scenario, wild female chimpanzees groom less than males because growing up, they receive less grooming from other females, and they witness little, if any, female-female grooming. They are socialized in these ways not to spend as much time grooming. In the zoo, then, the “culture” around grooming is atypical, and females are socialized differently. However, an equally plausible (but not mutually exclusive) possibility is that sex-based behavioral differences in the wild are simply the result of individuals finding ways of coping with their environment: Females in the wild have the responsibility of infant care. As a result, they are too busy foraging to spend much time socializing. At the zoo, with humans providing food, females groom more simply because they have the extra time—no social learning of sex roles is required.

A gain, these two explanations are not mutually exclusive. Both could play a part. I spoke with Rodrigues about what evidence would be necessary to conclude that chimpanzee or bonobo sex roles were socially determined.

“ We would need to see evidence that adults are actively treating male and female infants and juveniles differently, and actively [socializing] them differently,” she said. Rodrigues pointed out that some chimpanzee behavior is suggestive of different treatment of male and female offspring: For example, she noted, “data on young chimpanzees indicates that female chimpanzees spend more time observing their mothers termite-fishing and, in turn, are able to master termite-fishing using their mother’s technique at a younger age.” Researchers aren’t certain whether this is due to active socialization by mothers or an innate preference among female offspring to observe their mothers’ techniques. Even so, this observation is consistent with the idea of social determination of at least some, but probably not all, sex roles in chimps.

Chimpanzees use a stick to fish for termites. Manoj Shah/Getty Images

F lexibility in chimpanzee sex roles is not limited to the grooming patterns discussed earlier. Females occasionally participate in males’ political coalitions or go ranging with a mostly male group. Likewise, some males seem to prefer ranging with smaller groups of mostly females, or they spend more time interacting with infants than is typical for males. But scientists generally don’t consider this evidence of chimpanzee gender-bending. Rodrigues told me female-like behavior by male chimps is usually interpreted as a result of low rank—it’s not that the males prefer these feminine roles, it’s that they are relegated to these positions by dominant males.

“ But,” Rodrigues said, “it may be that our existing frameworks for interpreting behavior are too focused on paternity and rank. I think one of the challenges in interpreting behavior is that our own social constructions color how we theorize and interpret data.”

( Now, you may be thinking, “What about bonobos?” Most of the evidence bearing on these questions comes from chimpanzees, who have been studied much more extensively than bonobos. That said, despite their many differences in behavior, chimpanzees and bonobos are still very closely related, and their cognitive capacities are likely very similar. If one species has something like gender, the other probably does too.)

S o far, it’s not inconceivable that chimpanzees and bonobos might have something akin to human gender. But we haven’t yet touched on the other crucial criterion for gender: an internal, mental construct. How, if at all, do nonhuman animals think about sex and social roles? Scientists get at this question using cognitive testing—specifically, by testing animals’ concepts.

I n psychology, “concepts” refer to mental categories. Round shapes vs. sharp shapes, light colors vs. dark colors, males vs. females—these are all concepts. Scientists have tried-and-true methods for getting at animals’ concepts, the most common being the match-to-sample testing paradigm: An animal is presented with a “sample” image, and then they must select the “matching” image among other options in order to receive a reward. For example, an animal might see a sample image of a female, then be rewarded for choosing a subsequently presented image of a female from alongside an image of a male. If the animal can learn to succeed at this task, it suggests that they possess a concept of “female.” This concept is, again, a mental category that allows the animal to recognize that some images depict a female and others don’t. In a few studies (like this one, this one, and this one) using this technique, monkeys have displayed concepts of male and female. In a similar study, where chimpanzees learned to match faces of individuals they knew to generic images of male and female behinds, the authors went so far as to call their findings evidence of a “gender construct.”

T hese studies are telling, but they’re not entirely conclusive. The subjects could have a full-blown, human-like concept of sex, but looking only at these tests, it’s also possible that the animals are simply learning to categorize images based on distinguishing features. Just as a sommelier learns to recognize different wines based on tannins, sweetness, and mouthfeel, subjects might be learning to recognize images of males and females based on depicted genitals, face shape, and body size rather than any social concept of the sexes.

L uckily, we don’t have to rely solely on cognitive testing we can and should interpret the results of these tests in the context of natural social behavior in which there are plenty of examples of individuals seeming to distinguish between male and female groupmates. Alone, either of these lines of evidence—social behavior or cognitive tests—would be ambiguous, but taken jointly, they strongly suggest that chimpanzees have concepts of “male” and “female,” and, like humans, categorize individuals they know according to these concepts.

T hese concepts around the sexes are certainly an important part of gender, but they don’t equal a sense of gender identity—humans take these sex concepts and go further by applying them to how they think about themselves. Do our closest relatives do this? Direct evidence on this question is lacking, but some of the cognitive abilities that chimpanzees and bonobos have shown in unrelated contexts suggest that it’s possible.

Studies in the early 2000s showed that dolphins are able to recognize themselves in mirrors. Joe Raedle /Getty Images

H ere it’s prudent to consider whether chimpanzees and bonobos have any sense of identity—or sense of self—at all. To find out, scientists have tested “mirror self-recognition”: the ability to recognize oneself in the mirror. As you might guess, chimpanzees and bonobos (along with other apes, dolphins, elephants, and some other nonhumans) show this ability, quickly realizing that the image in the mirror is a reflection of themselves and using the mirror to inspect their appearance. Scientists view this as evidence that an individual possesses an understanding of itself as an entity separate from the rest of the world. This understanding can be regarded as the foundation of a potential sense of gender identity.

A second question is: Do chimpanzees and bonobos understand that others are independent “selves” with their own internal mental lives? This understanding is really a set of abilities, collectively referred to as “theory of mind.” Chimp theory of mind is more controversial than mirror self-recognition, but the consensus view is that chimpanzees do possess this understanding, albeit probably not as fully as humans. (Again, because chimpanzees and bonobos are so closely related and have shown no major differences in cognitive abilities, we can assume the same is true of bonobos.)

S o, chimpanzees and bonobos possess a sense of self and seem to understand that others, like them, have internal mental lives. And as we saw earlier, chimps seem to hold mental concepts of “male” and “female,” and categorize acquaintances accordingly. From there, I don’t think it’s implausible that chimps might apply those concepts not only to others but to their own sense of self. If—and this is a big if—that is the case, then chimpanzees possess sex roles that are not only flexible and potentially socially determined (as we saw earlier) but also tied to mental concepts that contribute to an individual’s sense of identity. If you ask me, that sounds a lot like gender.

I t bears repeating that we lack direct evidence of an internal gender identity in chimpanzees, bonobos, and other nonhuman animals. But the question of gender in a nonhuman species has yet to be tackled in a comprehensive way, so perhaps a license to speculate a bit is warranted. If nothing else, it seems clear that gender in other species is entirely possible.

T he more closely related two species are, the more likely it is that they share cognitive processes . And since chimpanzees and bonobos are our closest evolutionary cousins, the most scientifically sound approach may actually be to interpret ambiguous data as supporting, rather than challenging, the idea of human-like gender in our closest relatives. History has seen plenty of human-exceptionalist claims refuted. Much more research needs to be done, but in time, gender may turn out to be just one in a long list of attributes once thought to make humans unique.

Men and Women Are the Same Species!

OK, we all know that men and women do not always see eye to eye. We can have different goals, desires, ideas and actions … sometimes. Other times, we are very much in synch. If you stop and think a bit about biology, it turns out that men and women are a lot more similar than most of us realize. In this post, I am going to suggest that sometimes focusing on the similarities (or better put, the "overlaps") between males and females can help us towards a better understanding of where behavioral differences actually come from.

First, let's acknowledge the core differences in biology between males and females. These are evolutionarily, and practically, important and they do matter. Females have babies (gestate and give birth) and lactate, and males do not. Males are, on average, about 10 to 15 percent larger than females and tend to have greater upper body strength. Males’ brains grow for a bit longer and are a bit larger than females. But remember, as long as it is a healthy human brain (anywhere between 1,000 and 2,000 cubic centimeters) size does not relate to function. There are also some skeletal differences between men and women due to childbirth (wider pelvis) and male size/musculature (more rugged developments on male bones). Most of you reading this already know these differences … but do you know about the similarities?

Our hormones are the same. They function the same ways and we all have the same hormones … there are no “male” or “female” hormones. There is some important variation in hormone levels and patterns, and there are some differences in how the hormones interact with male and female bodies. On average, men tend to have higher resting levels of some androgens (like testosterone), and females may have higher levels of certain reproductive hormones like Follicle Stimulating Hormone or Estradiol at certain times in their menstrual cycles. However, these same reproductive hormones also work in men and are involved in the process of sperm production. There is substantial overlap in the process and patterns of our entire endocrine system.

Our brains are the same. Aside from the slight size differences and the possibility of some differences in an area called the straight gyrus, there are no reliably and repeatedly demonstrated morphological brain differences between the sexes. Now, this is not to say that there is not a great deal of variation in brains across our species or that in some cases adult males’ and females’ brains can react differently to stimulus there is a lot of variation in neurological structure and probably some in function … but it is primarily across individuals, not sexes.

Genitals? Most people think that male and female genitals are about as different as can be: penis = male and vagina = female. But even this basic dichotomy is not really correct: the genitals emerge from the same mass of embryonic tissue. For the first six weeks of development, the tissue masses develop identically. At about six to seven weeks, depending on whether the fetus has XX or XY chromosomes (usually), the tissues start to differentiate. One part of the tissues begins to form the clitoris or penis and another forms the labia or scrotum. Another area begins to form into either the testes or the ovaries. This means that physiologically, male and female genitals are made of the same stuff and work in similar ways.

What about sexual behavior? In general, humans have a lot of sex, they have it in a variety of different ways, and most importantly, males and females both have complex sexual lives. Substantive recent overviews of sexual behavior show few major differences between males and females in sexual activity: Men and women have more or less the same amount of sex in the same kinds of ways across the lifespan (remember, it does take two to tango). But there are some important differences. For example, married women report lower interest in sex with their husbands the longer they’ve been with them, and younger men report higher frequencies of masturbation and interest in visual pornography. But are these primarily biological differences, or is something else going on? We still have a lot to learn about sexuality … and as with many other areas, it looks like variation is highest between individuals, not between sexes.

There is no doubt that our evolutionary histories result in important differences between the sexes. But these same histories and biology also result in core similarities between the sexes that are equally as important in understanding our lives. Biological differences between males and females can relate to behavioral dissimilarities (such as in physical aggression and aspects of reproduction), but the majority of our biological characteristics (like our brains) reveal that males and females are much more similar than they are different.

So why do we almost always try to explain behavior by implicating biological (evolutionary) differences between the sexes? Could it be that our perceptions of what is “natural” for the sexes is biased? Why don’t we try to start some of our inquiries into human nature with a level playing field? Let’s not assume that there is a relevant sex difference until one actually emerges from the data.

Individual variation in our species is really important and the fact that the sexes overlap as much, if not more, than they differ should tell us something about how to ask questions about human nature. Misrepresentation of human biology and evolutionary patterns in males and females by focusing only on the differences while ignoring the overlaps facilitates a myopic view that inhibits good science.

Review the references for some good readings on this topic.

A. Fausto-Sterling (2012) Sex/Gender: biology in a social world. Routledge Press & 2000), Sexing the Body: Gender Politics and the Construction of Sexuality.

L. Eliot (2009) Pink brain Blue brain. Houghton Mifflin Harcourt.

Herbenick, D., Reece, M., Schick, V., Sanders, S.A., Dodge, B., Fortenberry, J.D. (2010) Sexual behavior in the united states: results from a national probability sample of men and women ages 14-94. J. Sex Med. 7 (suppl. 5):255-265.

R.M. Jordan-Young (2010) Brainstorm: the flaws in the science of sex differences. Harvard University Press.

Z. Tang-Martinez (2000), Paradigms and primates: Bateman’s principle, passive females, and perspectives from other taxa, in S. C. Strum and L. M. Fedigan, eds., Primate Encounters: Models of Science, Gender, and Society, pp. 261–74.

M. Borgerhoff-Mulder and K. Rauch (2009), Sexual conflict in humans: Variations and solutions, Evolutionary Anthropology 18: 201–14.

J. L. Wood, D. Heitmiller, N. C. Andreasen, and P. Nopoulos (2008), Morphology of the ventral frontal cortex: relationship to femininity and social cognition, Cerebral Cortex 18: 534–40.

K. Bishop and D. Wahlsten (1997), Sex differences in the human corpus callosum: Myth or reality? Neuroscience and Biobehavioral Reviews 21(5): 581–601.

J. Shibley Hyde (2005), The gender similarities hypothesis, American Psychologist 60(6): 581–92.

J. Archer (2009a), Does sexual selection explain human sex differences in aggression? Behavioral and Brain Sciences 32: 249–311.

J. Shibley Hyde (2005), The gender similarities hypothesis, American Psychologist 60(6): 581–92.

J. Archer (2009a), Does sexual selection explain human sex differences in aggression? Behavioral and Brain Sciences 32: 249–311.

Early Humans Slept Around with More than Just Neanderthals

It’s been known for some time that our modern human ancestors interbred with other early hominin groups like the Neanderthals. But it turns out they were even more promiscuous than we thought.

New DNA research has unexpectedly revealed that modern humans (Homo sapiens) mixed, mingled and mated with another archaic human species, the Denisovans, not once but twice—in two different regions of the ancient world.

All we know about the mysterious Denisovans comes from a single set of human fossils found in a cave in the Altai Mountains of Siberia. In 2008, scientists first discovered a bone from a pinky finger in the cave, and concluded it belonged to a previously unknown ancient hominin who lived between 30,000 and 50,000 years ago. They called the species the Denisovans (pronounced �-NEE-soh-vens”) after the cave where the fossilized finger was found.

A Neanderthal skull and some of the Mousterian tools used by the Neanderthals are shown in this display during a tour of the 𠆊ncestors’ exhibit at the American Museum of Natural History 412 (Photo by Getty)

After the genome of the finger’s owner, a young girl, was published in 2010, researchers went on to discover traces of the Denisovan ancestry in two groups of modern-day humans. Some Melanesians (who live in Papua New Guinea and other Pacific islands) were found to have around 5 percent of Denisovan ancestry, while some East and South Asians have around 0.2 percent. One particular gene mutation, which the Denisovans are thought to have passed to modern Tibetans, allows them to survive at high altitudes.

Researchers assumed the Denisovan ancestry found in Asia was due to migration from Oceania, the larger region containing Melanesia. But recently, scientists from the University of Washington in Seattle stumbled on something surprising: evidence for a second, distinct instance of humans getting hot and heavy with Denisovans.

In their analysis of more than 5,600 whole-genome sequences from individuals from Europe, Asia, the Americas and Oceania, the research team looked for ancient DNA, which stands out due to the larger number of mutations that have developed over hundreds of thousands of years. When they found the ancient genetic information, they compared with Denisovan DNA and Neanderthal DNA to determine its origin.

VIDEO: Neanderthals: Did Cro Magnons, the ancestors of early humans, cause the Neanderthal extinction?

What they found was a distinct set of Denisovan ancestry among some modern East Asians—particularly Han Chinese, Chinese Dai and Japanese𠅊ncestry not found in South Asians or Papuans. According to the study’s findings, published in the journal Cell this week, this Denisovan DNA is actually more closely related to the sample taken from the girl in the Siberian cave.

𠇊lthough the Papuans ended up with more Denisovan ancestry, it turns out to be less similar to the sequenced Denisovan,” Sharon Browning, a research professor of biostatistics at the University of Washington School of Public Health and senior author of the study, told New Scientist. “Our research demonstrates that there were at least two distinct populations of Denisovans living in Asia, probably somewhat geographically distant.”

(Credit Browning et al.Cell)

Browning and her colleagues assume that modern humans mixed with the Denisovans shortly after migrating out of Africa, around 50,000 years ago. While they’re not sure of the location, they believe the interbreeding occurred in at least two places: eastern Asia, and further south, in Indonesia or Australia.

While the new study confirms that modern humans interbred at least three times with ancient hominins—once with Neanderthals, and twice with the Denisovans—it also raises the possibility of even more extensive intermixing on the part of our ancient ancestors. As reported in New Scientist, one-quarter of the ancient DNA that the researchers found in living humans didn’t match up with either Denisovan or Neanderthal DNA, suggesting there may be other mystery mates out there to find.


Ornamentation and coloration Edit

Common and easily identified types of dimorphism consist of ornamentation and coloration, though not always apparent. A difference in coloration of sexes within a given species is called sexual dichromatism, which is commonly seen in many species of birds and reptiles. [4] Sexual selection leads to the exaggerated dimorphic traits that are used predominantly in competition over mates. The increased fitness resulting from ornamentation offsets its cost to produce or maintain suggesting complex evolutionary implications, but the costs and evolutionary implications vary from species to species. [5] [6] The costs and implications differ depending on the nature of the ornamentation (such as the colour mechanism involved).

The peafowl constitute conspicuous illustrations of the principle. The ornate plumage of peacocks, as used in the courting display, attracts peahens. At first sight one might mistake peacocks and peahens for completely different species because of the vibrant colours and the sheer size of the male's plumage the peahen being of a subdued brown coloration. [7] The plumage of the peacock increases its vulnerability to predators because it is a hindrance in flight, and it renders the bird conspicuous in general. [7] Similar examples are manifold, such as in birds of paradise and argus pheasants.

Another example of sexual dichromatism is that of the nestling blue tits. Males are chromatically more yellow than females. It is believed that this is obtained by the ingestion of green Lepidopteran larvae, which contain large amounts of the carotenoids lutein and zeaxanthin. [8] This diet also affects the sexually dimorphic colours in the human-invisible ultraviolet spectrum. [9] [10] Hence, the male birds, although appearing yellow to humans actually have a violet-tinted plumage that is seen by females. This plumage is thought to be an indicator of male parental abilities. [11] Perhaps this is a good indicator for females because it shows that they are good at obtaining a food supply from which the carotenoid is obtained. There is a positive correlation between the chromas of the tail and breast feathers and body condition. [12] Carotenoids play an important role in immune function for many animals, so carotenoid dependent signals might indicate health. [13]

Frogs constitute another conspicuous illustration of the principle. There are two types of dichromatism for frog species: ontogenetic and dynamic. Ontogenetic frogs are more common and have permanent color changes in males or females. Ranoidea lesueuri is an example of a dynamic frog that has temporary color changes in males during breeding season. [14] Hyperolius ocellatus is an ontogenetic frog with dramatic differences in both color and pattern between the sexes. At sexual maturity, the males display a bright green with white dorsolateral lines. [15] In contrast, the females are rusty red to silver with small spots. The bright coloration in the male population serves to attract females and as an aposematic sign to potential predators.

Females often show a preference for exaggerated male secondary sexual characteristics in mate selection. [16] The sexy son hypothesis explains that females prefer more elaborate males and select against males that are dull in color, independent of the species' vision. [17]

Similar sexual dimorphism and mating choice are also observed in many fish species. For example, male guppies have colorful spots and ornamentations while females are generally grey in color. Female guppies prefer brightly colored males to duller males. [18]

In redlip blennies, only the male fish develops an organ at the anal-urogenital region that produces antimicrobial substances. During parental care, males rub their anal-urogenital regions over their nests' internal surfaces, thereby protecting their eggs from microbial infections, one of the most common causes for mortality in young fish. [19]

Most flowering plants are hermaphroditic but approximately 6% of species have separate males and females (dioecy). [20] Males and females in insect-pollinated species generally look similar to one another because plants provide rewards (e.g. nectar) that encourage pollinators to visit another similar flower, completing pollination. Catasetum orchids are one interesting exception to this rule. Male Catasetum orchids violently attach pollinia to euglossine bee pollinators. The bees will then avoid other male flowers but may visit the female, which look different from the males. [21]

Various other dioecious exceptions, such as Loxostylis alata have visibly different genders, with the effect of eliciting the most efficient behaviour from pollinators, who then use the most efficient strategy in visiting each gender of flower instead of searching say, for pollen in a nectar-bearing female flower.

Some plants, such as some species of Geranium have what amounts to serial sexual dimorphism. The flowers of such species might for example present their anthers on opening, then shed the exhausted anthers after a day or two and perhaps change their colours as well while the pistil matures specialist pollinators are very much inclined to concentrate on the exact appearance of the flowers they serve, which saves their time and effort and serves the interests of the plant accordingly. Some such plants go even further and change their appearance again once they have been fertilised, thereby discouraging further visits from pollinators. This is advantageous to both parties because it avoids damage to the developing fruit and avoids wasting the pollinator's effort on unrewarding visits. In effect the strategy ensures that the pollinators can expect a reward every time they visit an appropriately advertising flower.

Females of the aquatic plant Vallisneria americana have floating flowers attached by a long flower stalk that are fertilized if they contact one of the thousands of free floating flowers released by a male. [22] Sexual dimorphism is most often associated with wind-pollination in plants due to selection for efficient pollen dispersal in males vs pollen capture in females, e.g. Leucadendron rubrum. [23]

Sexual dimorphism in plants can also be dependent on reproductive development. This can be seen in Cannabis sativa, a type of hemp, which have higher photosynthesis rates in males while growing but higher rates in females once the plants become sexually mature. [24]

Every sexually reproducing extant species of vascular plant actually has an alternation of generations the plants we see about us generally are diploid sporophytes, but their offspring really are not the seeds that people commonly recognise as the new generation. The seed actually is the offspring of the haploid generation of microgametophytes (pollen) and megagametophytes (the embryo sacs in the ovules). Each pollen grain accordingly may be seen as a male plant in its own right it produces a sperm cell and is dramatically different from the female plant, the megagametophyte that produces the female gamete.

Insects display a wide variety of sexual dimorphism between taxa including size, ornamentation and coloration. [25] The female-biased sexual size dimorphism observed in many taxa evolved despite intense male–male competition for mates. [26] In Osmia rufa, for example, the female is larger/broader than males, with males being 8–10 mm in size and females being 10–12 mm in size. [27] In the hackberry emperor females are similarly larger than males. [28] The reason for the sexual dimorphism is due to provision size mass, in which females consume more pollen than males. [29]

In some species, there is evidence of male dimorphism, but it appears to be for the purpose of distinctions of roles. This is seen in the bee species Macrotera portalis in which there is a small-headed morph, capable of flight, and large-headed morph, incapable of flight, for males. [30] Anthidium manicatum also displays male-biased sexual dimorphism. The selection for larger size in males rather than females in this species may have resulted due to their aggressive territorial behavior and subsequent differential mating success. [31] Another example is Lasioglossum hemichalceum, which is a species of sweat bee that shows drastic physical dimorphisms between male offspring. [32] Not all dimorphism has to have a drastic difference between the sexes. Andrena agilissima is a mining bee where the females only have a slightly larger head than the males. [33]

Weaponry leads to increased fitness by increasing success in male-male competition in many insect species. [34] The beetle horns in Onthophagus taurus are enlarged growths of the head or thorax expressed only in the males. Copris ochus also has distinct sexual and male dimorphism in head horns. [35] These structures are impressive because of the exaggerated sizes. [36] There is a direct correlation between male horn lengths and body size and higher access to mates and fitness. [36] In other beetle species, both males and females may have ornamentation such as horns. [35] Generally, insect sexual size dimorphism (SSD) within species increases with body size. [37]

Sexual dimorphism within insects is also displayed by dichromatism. In butterfly genera Bicyclus and Junonia, dimorphic wing patterns evolved due to sex-limited expression, which mediates the intralocus sexual conflict and leads to increased fitness in males. [38] The sexual dichromatic nature of Bicyclus anynana is reflected by female selection on the basis of dorsal UV-reflective eyespot pupils. [39] The common brimstone also displays sexual dichromatism males have yellow and iridescent wings, while female wings are white and non-iridescent. [40] Naturally selected deviation in protective female coloration is displayed in mimetic butterflies. [41]

Many arachnid groups exhibit sexual dimorphism, [42] but it is most widely studied in the spiders. In the orb-weaving spider Zygiella x-notata, for example, adult females have a larger body size than adult males. [43] Size dimorphism shows a correlation with sexual cannibalism, [44] which is prominent in spiders (it is also found in insects such as praying mantises). In the size dimorphic wolf spider Tigrosa helluo, food-limited females cannibalize more frequently. [45] Therefore, there is a high risk of low fitness for males due to pre-copulatory cannibalism, which led to male selection of larger females for two reasons: higher fecundity and lower rates of cannibalism. [45] In addition, female fecundity is positively correlated with female body size and large female body size is selected for, which is seen in the family Araneidae. All Argiope species, including Argiope bruennichi, use this method. Some males evolved ornamentation [ vague ] including binding the female with silk, having proportionally longer legs, modifying the female's web, mating while the female is feeding, or providing a nuptial gift in response to sexual cannibalism. [45] Male body size is not under selection due to cannibalism in all spider species such as Nephila pilipes, but is more prominently selected for in less dimorphic species of spiders, which often selects for larger male size. [46] In the species Maratus volans, the males are known for their characteristic colorful fan which attracts the females during mating. [47]

Ray finned fish are an ancient and diverse class, with the widest degree of sexual dimorphism of any animal class. Fairbairn notes that "females are generally larger than males but males are often larger in species with male-male combat or male paternal care . [sizes range] from dwarf males to males more than 12 times heavier than females." [48]

There are cases where males are substantially larger than females. An example is Lamprologus callipterus, a type of cichlid fish. In this fish, the males are characterized as being up to 60 times larger than the females. The male's increased size is believed to be advantageous because males collect and defend empty snail shells in each of which a female breeds. [49] Males must be larger and more powerful in order to collect the largest shells. The female's body size must remain small because in order for her to breed, she must lay her eggs inside the empty shells. If she grows too large, she will not fit in the shells and will be unable to breed. The female's small body size is also likely beneficial to her chances of finding an unoccupied shell. Larger shells, although preferred by females, are often limited in availability. [50] Hence, the female is limited to the growth of the size of the shell and may actually change her growth rate according to shell size availability. [51] In other words, the male's ability to collect large shells depends on his size. The larger the male, the larger the shells he is able to collect. This then allows for females to be larger in his brooding nest which makes the difference between the sizes of the sexes less substantial. Male-male competition in this fish species also selects for large size in males. There is aggressive competition by males over territory and access to larger shells. Large males win fights and steal shells from competitors. Another example is the dragonet, in which males are considerably larger than females and possess longer fins.

Sexual dimorphism also occurs in hermaphroditic fish. These species are known as sequential hermaphrodites. In fish, reproductive histories often include the sex-change from female to male where there is a strong connection between growth, the sex of an individual, and the mating system it operates within. [52] In protogynous mating systems where males dominate mating with many females, size plays a significant role in male reproductive success. [53] Males have a propensity to be larger than females of a comparable age but it is unclear whether the size increase is due to a growth spurt at the time of the sexual transition or due to the history of faster growth in sex changing individuals. [54] Larger males are able to stifle the growth of females and control environmental resources.

Social organization plays a large role in the changing of sex by the fish. It is often seen that a fish will change its sex when there is a lack of dominant male within the social hierarchy. The females that change sex are often those who attain and preserve an initial size advantage early in life. In either case, females which change sex to males are larger and often prove to be a good example of dimorphism.

In other cases with fish, males will go through noticeable changes in body size, and females will go through morphological changes that can only be seen inside of the body. For example, in sockeye salmon, males develop larger body size at maturity, including an increase in body depth, hump height, and snout length. Females experience minor changes in snout length, but the most noticeable difference is the huge increase in gonad size, which accounts for about 25% of body mass. [55]

Sexual selection was observed for female ornamentation in Gobiusculus flavescens, known as two-spotted gobies. [56] Traditional hypotheses suggest that male-male competition drives selection. However, selection for ornamentation within this species suggests that showy female traits can be selected through either female-female competition or male mate choice. [56] Since carotenoid-based ornamentation suggests mate quality, female two-spotted guppies that develop colorful orange bellies during the breeding season are considered favorable to males. [57] The males invest heavily in offspring during the incubation, which leads to the sexual preference in colorful females due to higher egg quality. [57]

In amphibians and reptiles, the degree of sexual dimorphism varies widely among taxonomic groups. The sexual dimorphism in amphibians and reptiles may be reflected in any of the following: anatomy relative length of tail relative size of head overall size as in many species of vipers and lizards coloration as in many amphibians, snakes, and lizards, as well as in some turtles an ornament as in many newts and lizards the presence of specific sex-related behaviour is common to many lizards and vocal qualities which are frequently observed in frogs.

Anole lizards show prominent size dimorphism with males typically being significantly larger than females. For instance, the average male Anolis sagrei was 53.4 mm vs. 40 mm in females. [58] Different sizes of the heads in anoles have been explained by differences in the estrogen pathway. [59] The sexual dimorphism in lizards is generally attributed to the effects of sexual selection, but other mechanisms including ecological divergence and fecundity selection provide alternative explanations. [60] The development of color dimorphism in lizards is induced by hormonal changes at the onset of sexual maturity, as seen in Psamodromus algirus, Sceloporus gadoviae, and S. undulates erythrocheilus. [60]

Male painted dragon lizards, Ctenophorus pictus. are brightly conspicuous in their breeding coloration, but male colour declines with aging. Male coloration appears to reflect innate anti-oxidation capacity that protects against oxidative DNA damage. [61] Male breeding coloration is likely an indicator to females of the underlying level of oxidative DNA damage (a significant component of aging) in potential mates. [61]

Sexual dimorphism in birds can be manifested in size or plumage differences between the sexes. Sexual size dimorphism varies among taxa with males typically being larger, though this is not always the case, e.g. birds of prey, hummingbirds, and some species of flightless birds. [62] [63] Plumage dimorphism, in the form of ornamentation or coloration, also varies, though males are typically the more ornamented or brightly colored sex. [64] Such differences have been attributed to the unequal reproductive contributions of the sexes. [65] This difference produces a stronger female choice since they have more risk in producing offspring. In some species, the male's contribution to reproduction ends at copulation, while in other species the male becomes the main caregiver. Plumage polymorphisms have evolved to reflect these differences and other measures of reproductive fitness, such as body condition [66] or survival. [67] The male phenotype sends signals to females who then choose the 'fittest' available male.

Sexual dimorphism is a product of both genetics and environmental factors. An example of sexual polymorphism determined by environmental conditions exists in the red-backed fairywren. Red-backed fairywren males can be classified into three categories during breeding season: black breeders, brown breeders, and brown auxiliaries. [66] These differences arise in response to the bird's body condition: if they are healthy they will produce more androgens thus becoming black breeders, while less healthy birds produce less androgens and become brown auxiliaries. [66] The reproductive success of the male is thus determined by his success during each year's non-breeding season, causing reproductive success to vary with each year's environmental conditions.

Migratory patterns and behaviors also influence sexual dimorphisms. This aspect also stems back to the size dimorphism in species. It has been shown that the larger males are better at coping with the difficulties of migration and thus are more successful in reproducing when reaching the breeding destination. [68] When viewing this in an evolutionary standpoint many theories and explanations come to consideration. If these are the result for every migration and breeding season the expected results should be a shift towards a larger male population through sexual selection. Sexual selection is strong when the factor of environmental selection is also introduced. The environmental selection may support a smaller chick size if those chicks were born in an area that allowed them to grow to a larger size, even though under normal conditions they would not be able to reach this optimal size for migration. When the environment gives advantages and disadvantages of this sort, the strength of selection is weakened and the environmental forces are given greater morphological weight. The sexual dimorphism could also produce a change in timing of migration leading to differences in mating success within the bird population. [69] When the dimorphism produces that large of a variation between the sexes and between the members of the sexes multiple evolutionary effects can take place. This timing could even lead to a speciation phenomenon if the variation becomes strongly drastic and favorable towards two different outcomes.

Sexual dimorphism is maintained by the counteracting pressures of natural selection and sexual selection. For example, sexual dimorphism in coloration increases the vulnerability of bird species to predation by European sparrowhawks in Denmark. [70] Presumably, increased sexual dimorphism means males are brighter and more conspicuous, leading to increased predation. [70] Moreover, the production of more exaggerated ornaments in males may come at the cost of suppressed immune function. [66] So long as the reproductive benefits of the trait due to sexual selection are greater than the costs imposed by natural selection, then the trait will propagate throughout the population. Reproductive benefits arise in the form of a larger number of offspring, while natural selection imposes costs in the form of reduced survival. This means that even if the trait causes males to die earlier, the trait is still beneficial so long as males with the trait produce more offspring than males lacking the trait. This balance keeps the dimorphism alive in these species and ensures that the next generation of successful males will also display these traits that are attractive to the females.

Such differences in form and reproductive roles often cause differences in behavior. As previously stated, males and females often have different roles in reproduction. The courtship and mating behavior of males and females are regulated largely by hormones throughout a bird's lifetime. [71] Activational hormones occur during puberty and adulthood and serve to 'activate' certain behaviors when appropriate, such as territoriality during breeding season. [71] Organizational hormones occur only during a critical period early in development, either just before or just after hatching in most birds, and determine patterns of behavior for the rest of the bird's life. [71] Such behavioral differences can cause disproportionate sensitivities to anthropogenic pressures. [72] Females of the whinchat in Switzerland breed in intensely managed grasslands. [72] Earlier harvesting of the grasses during the breeding season lead to more female deaths. [72] Populations of many birds are often male-skewed and when sexual differences in behavior increase this ratio, populations decline at a more rapid rate. [72] Also not all male dimorphic traits are due to hormones like testosterone, instead they are a naturally occurring part of development, for example plumage. [73] In addition, the strong hormonal influence on phenotypic differences suggest that the genetic mechanism and genetic basis of these sexually dimorphic traits may involve transcription factors or cofactors rather than regulatory sequences. [74]

Sexual dimorphism may also influence differences in parental investment during times of food scarcity. For example, in the blue-footed booby, the female chicks grow faster than the males, resulting in booby parents producing the smaller sex, the males, during times of food shortage. This then results in the maximization of parental lifetime reproductive success. [75] In Black-tailed Godwits Limosa limosa limosa females are also the larger sex, and the growth rates of female chicks are more susceptible to limited environmental conditions. [76]

Sexual dimorphism may also only appear during mating season, some species of birds only show dimorphic traits in seasonal variation. The males of these species will molt into a less bright or less exaggerated color during the off breeding season. [74] This occurs because the species is more focused on survival than reproduction, causing a shift into a less ornate state. [ dubious – discuss ]

Consequently, sexual dimorphism has important ramifications for conservation. However, sexual dimorphism is not only found in birds and is thus important to the conservation of many animals. Such differences in form and behavior can lead to sexual segregation, defined as sex differences in space and resource use. [77] Most sexual segregation research has been done on ungulates, [77] but such research extends to bats, [78] kangaroos, [79] and birds. [80] Sex-specific conservation plans have even been suggested for species with pronounced sexual segregation. [78]

The term sesquimorphism (the Latin numeral prefix sesqui- means one-and-one-half, so halfway between mono- (one) and di- (two)) has been proposed for bird species in which "both sexes have basically the same plumage pattern, though the female is clearly distinguishable by reason of her paler or washed-out colour". [81] : 14 Examples include Cape sparrow (Passer melanurus), [81] : 67 rufous sparrow (subspecies P. motinensis motinensis), [81] : 80 and saxaul sparrow (P. ammodendri). [81] : 245

In a large proportion of mammal species, males are larger than females. [82] Both genes and hormones affect the formation of many animal brains before "birth" (or hatching), and also behaviour of adult individuals. Hormones significantly affect human brain formation, and also brain development at puberty. A 2004 review in Nature Reviews Neuroscience observed that "because it is easier to manipulate hormone levels than the expression of sex chromosome genes, the effects of hormones have been studied much more extensively, and are much better understood, than the direct actions in the brain of sex chromosome genes." It concluded that while "the differentiating effects of gonadal secretions seem to be dominant," the existing body of research "support the idea that sex differences in neural expression of X and Y genes significantly contribute to sex differences in brain functions and disease." [83]

Pinnipeds Edit

Marine mammals show some of the greatest sexual size differences of mammals, because of sexual selection and environmental factors like breeding location. [84] [85] The mating system of pinnipeds varies from polygamy to serial monogamy. Pinnipeds are known for early differential growth and maternal investment since the only nutrients for newborn pups is the milk provided by the mother. [86] For example, the males are significantly larger (about 10% heavier and 2% longer) than the females at birth in sea lion pups. [87] The pattern of differential investment can be varied principally prenatally and post-natally. [88] Mirounga leonina, the southern elephant seal, is one of the most dimorphic mammals. [89]

Sexual dimorphism in elephant seals is associated with the ability of a male to defend territories and control large groups of females, which correlates with polygynic behavior. [90] The large sexual size dimorphism is partially due to sexual selection, but also because females reach reproductive age much earlier than males. In addition the males do not provide parental care for the young and allocate more energy to growth. [91] This is supported by the secondary growth spurt in males during adolescent years. [91]

Primates Edit

Humans Edit

Top: Stylised illustration of humans on the Pioneer plaque, showing both male (left) and female (right).
Bottom: Comparison between male (left) and female (right) pelvises.

Sexual dimorphism among humans includes differentiation among gonads, internal genitals, external genitals, breasts, muscle mass, height, the endocrine (hormonal) systems and their physiological and behavioral effects. Human sexual differentiation is effected primarily at the gene level, by the presence or absence of a Y-chromosome, which encodes biochemical modifiers for sexual development in males. [92] According to Clark Spencer Larsen, modern day Homo sapiens show a range of sexual dimorphism, with average body mass difference between the sexes being roughly equal to 15%. [93]

The average basal metabolic rate is about 6 percent higher in adolescent males than females and increases to about 10 percent higher after puberty. Females tend to convert more food into fat, while males convert more into muscle and expendable circulating energy reserves. Aggregated data of absolute strength indicates that females have, on average, 40–60% the upper body strength of males, and 70–75% the lower body strength. [94] The difference in strength relative to body mass is less pronounced in trained individuals. In Olympic weightlifting, male records vary from 5.5× body mass in the lowest weight category to 4.2× in the highest weight category, while female records vary from 4.4× to 3.8×, a weight adjusted difference of only 10–20%, and an absolute difference of about 30% (i.e. 472 kg vs 333 kg for unlimited weight classes)(see Olympic weightlifting records). A study, carried about by analyzing annual world rankings from 1980 to 1996, found that males' running times were, on average, 11% faster than females'. [95]

Females are taller, on average, than males in early adolescence, but males, on average, surpass them in height in later adolescence and adulthood. In the United States, adult males are, on average, 9% taller [96] and 16.5% heavier [97] than adult females. There is no comparative evidence of differing levels of sexual selection having produced sexual size dimorphism between human populations. [98]

Males typically have larger tracheae and branching bronchi, with about 30 percent greater lung volume per body mass. On average, males have larger hearts, 10 percent higher red blood cell count, higher hemoglobin, hence greater oxygen-carrying capacity. They also have higher circulating clotting factors (vitamin K, prothrombin and platelets). These differences lead to faster healing of wounds and higher peripheral pain tolerance. [99]

Females typically have more white blood cells (stored and circulating), more granulocytes and B and T lymphocytes. Additionally, they produce more antibodies at a faster rate than males. Hence they develop fewer infectious diseases and succumb for shorter periods. [99] Ethologists argue that females, interacting with other females and multiple offspring in social groups, have experienced such traits as a selective advantage. [100] [101] [102] [103] [104]

Considerable discussion in academic literature concerns potential evolutionary advantages associated with sexual competition (both intrasexual and intersexual) and short- and long-term sexual strategies. [105] According to Daly and Wilson, "The sexes differ more in human beings than in monogamous mammals, but much less than in extremely polygamous mammals." [106]

In the human brain, a difference between sexes was observed in the transcription of the PCDH11X/Y gene pair unique to Homo sapiens. [107] Sexual differentiation in the human brain from the undifferentiated state is triggered by testosterone from the fetal testis. Testosterone is converted to estrogen in the brain through the action of the enzyme aromatase. Testosterone acts on many brain areas, including the SDN-POA, to create the masculinized brain pattern. [108] Brains of pregnant females carrying male fetuses may be shielded from the masculinizing effects of androgen through the action of sex hormone-binding globulin. [109]

The relationship between sex differences in the brain and human behavior is a subject of controversy in psychology and society at large. [110] [111] Many females tend to have a higher ratio of gray matter in the left hemisphere of the brain in comparison to males. [112] [113] Males on average have larger brains than females however, when adjusted for total brain volume the gray matter differences between sexes is almost nonexistent. Thus, the percentage of gray matter appears to be more related to brain size than it is to sex. [114] [115] Differences in brain physiology between sexes do not necessarily relate to differences in intellect. Haier et al. found in a 2004 study that "men and women apparently achieve similar IQ results with different brain regions, suggesting that there is no singular underlying neuroanatomical structure to general intelligence and that different types of brain designs may manifest equivalent intellectual performance". [116] (See the sex and intelligence article for more on this subject.) Strict graph-theoretical analysis of the human brain connections revealed [117] that in numerous graph-theoretical parameters (e.g., minimum bipartition width, edge number, the expander graph property, minimum vertex cover), the structural connectome of women are significantly "better" connected than the connectome of men. It was shown [118] that the graph-theoretical differences are due to the sex and not to the differences in the cerebral volume, by analyzing the data of 36 females and 36 males, where the brain volume of each man in the group was smaller than the brain volume of each woman in the group.

Sexual dimorphism was also described in the gene level and shown to extend from the sex chromosomes. Overall, about 6500 genes have been found to have sex-differential expression in at least one tissue. Many of these genes are not directly associated with reproduction, but rather linked to more general biological features. In addition, it has been shown that genes with sex specific expression undergo reduced selection efficiency, which lead to higher population frequencies of deleterious mutations and contributing to the prevalence of several human diseases. [119] [120]

Sexual dimorphism in immune function is a common pattern in vertebrates and also in a number of invertebrates. Most often, females are more ‘immunocompetent’ than males. The underlying causes are explained by either the role of immunosuppressive substances, such as testosterone, or by fundamental differences in male and female life histories. It has been shown that female mammals tend to have higher white blood cell counts (WBC), with further associations between cell counts and longevity in females. There is also a positive covariance between sexual dimorphism in immunity, as measured by a subset of WBC, and dimorphism in the duration of effective breeding. This is consistent with the application of ‘Bateman’s principle’ to immunity, with females maximizing fitness by lengthening lifespan through greater investment in immune defences. [121]

Phenotypic differences between sexes are evident even in cultured cells from tissues. [122] For example, female muscle-derived stem cells have a better muscle regeneration efficiency than male ones. [123] There are reports of several metabolic differences between male and female cells [124] and they also respond to stress differently. [125]

In theory, larger females are favored by competition for mates, especially in polygamous species. Larger females offer an advantage in fertility, since the physiological demands of reproduction are limiting in females. Hence there is a theoretical expectation that females tend to be larger in species that are monogamous. Females are larger in many species of insects, many spiders, many fish, many reptiles, owls, birds of prey and certain mammals such as the spotted hyena, and baleen whales such as blue whale. As an example, in some species, females are sedentary, and so males must search for them. Fritz Vollrath and Geoff Parker argue that this difference in behaviour leads to radically different selection pressures on the two sexes, evidently favouring smaller males. [126] Cases where the male is larger than the female have been studied as well, [126] and require alternative explanations.

One example of this type of sexual size dimorphism is the bat Myotis nigricans, (black myotis bat) where females are substantially larger than males in terms of body weight, skull measurement, and forearm length. [127] The interaction between the sexes and the energy needed to produce viable offspring make it favorable for females to be larger in this species. Females bear the energetic cost of producing eggs, which is much greater than the cost of making sperm by the males. The fecundity advantage hypothesis states that a larger female is able to produce more offspring and give them more favorable conditions to ensure their survival this is true for most ectotherms. A larger female can provide parental care for a longer time while the offspring matures. The gestation and lactation periods are fairly long in M. nigricans, the females suckling their offspring until they reach nearly adult size. [128] They would not be able to fly and catch prey if they did not compensate for the additional mass of the offspring during this time. Smaller male size may be an adaptation to increase maneuverability and agility, allowing males to compete better with females for food and other resources.

Some species of anglerfish also display extreme sexual dimorphism. Females are more typical in appearance to other fish, whereas the males are tiny rudimentary creatures with stunted digestive systems. A male must find a female and fuse with her: he then lives parasitically, becoming little more than a sperm-producing body in what amounts to an effectively hermaphrodite composite organism. A similar situation is found in the Zeus water bug Phoreticovelia disparata where the female has a glandular area on her back that can serve to feed a male, which clings to her (note that although males can survive away from females, they generally are not free-living). [129] This is taken to the logical extreme in the Rhizocephala crustaceans, like the Sacculina, where the male injects itself into the female's body and becomes nothing more than sperm producing cells, to the point that the superorder used to be mistaken for hermaphroditic. [130]

Some plant species also exhibit dimorphism in which the females are significantly larger than the males, such as in the moss Dicranum [131] and the liverwort Sphaerocarpos. [132] There is some evidence that, in these genera, the dimorphism may be tied to a sex chromosome, [132] [133] or to chemical signalling from females. [134]

Another complicated example of sexual dimorphism is in Vespula squamosa, the southern yellowjacket. In this wasp species, the female workers are the smallest, the male workers are slightly larger, and the female queens are significantly larger than her female worker and male counterparts. [ citation needed ]

Sexual dimorphism by size is evident in some extinct species such as the velociraptor. In the case of velociraptors the sexual size dimorphism may have been caused by two factors: male competition for hunting ground to attract mates, and/or female competition for nesting locations and mates, males being a scarce breeding resource. [136]

In 1871, Charles Darwin advanced the theory of sexual selection, which related sexual dimorphism with sexual selection.

It has been proposed that the earliest sexual dimorphism is the size differentiation of sperm and eggs (anisogamy), but the evolutionary significance of sexual dimorphism is more complex than that would suggest. [137] Anisogamy and the usually large number of small male gametes relative to the larger female gametes usually lies in the development of strong sperm competition, [138] [139] because small sperm enable organisms to produce a large number of sperm, and make males (or male function of hermaphrodites [140] ) more redundant. This intensifies male competition for mates and promotes the evolution of other sexual dimorphism in many species, especially in vertebrates including mammals. However, in some species, the females can be larger than males, irrespective of gametes, and in some species females (usually of species in which males invest a lot in rearing offspring and thus no longer considered as so redundant) compete for mates in ways more usually associated with males.

In many non-monogamous species, the benefit to a male's reproductive fitness of mating with multiple females is large, whereas the benefit to a female's reproductive fitness of mating with multiple males is small or nonexistent. [141] In these species, there is a selection pressure for whatever traits enable a male to have more matings. The male may therefore come to have different traits from the female.

These traits could be ones that allow him to fight off other males for control of territory or a harem, such as large size or weapons [142] or they could be traits that females, for whatever reason, prefer in mates. [143] Male-male competition poses no deep theoretical questions [144] but mate choice does.

Females may choose males that appear strong and healthy, thus likely to possess "good alleles" and give rise to healthy offspring. [145] In some species, however, females seem to choose males with traits that do not improve offspring survival rates, and even traits that reduce it (potentially leading to traits like the peacock's tail). [144] Two hypotheses for explaining this fact are the sexy son hypothesis and the handicap principle.

The sexy son hypothesis states that females may initially choose a trait because it improves the survival of their young, but once this preference has become widespread, females must continue to choose the trait, even if it becomes harmful. Those that do not will have sons that are unattractive to most females (since the preference is widespread) and so receive few matings. [146]

The handicap principle states that a male who survives despite possessing some sort of handicap thus proves that the rest of his genes are "good alleles". If males with "bad alleles" could not survive the handicap, females may evolve to choose males with this sort of handicap the trait is acting as a hard-to-fake signal of fitness. [147]


The natural history of malaria involves cyclical infection of humans and female Anopheles mosquitoes. In humans, the parasites grow and multiply first in the liver cells and then in the red cells of the blood. In the blood, successive broods of parasites grow inside the red cells and destroy them, releasing daughter parasites (&ldquomerozoites&rdquo) that continue the cycle by invading other red cells.

The blood stage parasites are those that cause the symptoms of malaria. When certain forms of blood stage parasites (gametocytes, which occur in male and female forms) are ingested during blood feeding by a female Anopheles mosquito, they mate in the gut of the mosquito and begin a cycle of growth and multiplication in the mosquito. After 10-18 days, a form of the parasite called a sporozoite migrates to the mosquito&rsquos salivary glands. When the Anopheles mosquito takes a blood meal on another human, anticoagulant saliva is injected together with the sporozoites, which migrate to the liver, thereby beginning a new cycle.

Thus the infected mosquito carries the disease from one human to another (acting as a &ldquovector&rdquo), while infected humans transmit the parasite to the mosquito, In contrast to the human host, the mosquito vector does not suffer from the presence of the parasites.

The malaria parasite life cycle involves two hosts. During a blood meal, a malaria-infected female Anopheles mosquito inoculates sporozoites into the human host . Sporozoites infect liver cells and mature into schizonts , which rupture and release merozoites . (Of note, in P. vivax and P. ovale a dormant stage [hypnozoites] can persist in the liver (if untreated) and cause relapses by invading the bloodstream weeks, or even years later.) After this initial replication in the liver (exo-erythrocytic schizogony ), the parasites undergo asexual multiplication in the erythrocytes (erythrocytic schizogony ). Merozoites infect red blood cells . The ring stage trophozoites mature into schizonts, which rupture releasing merozoites . Some parasites differentiate into sexual erythrocytic stages (gametocytes) . Blood stage parasites are responsible for the clinical manifestations of the disease. The gametocytes, male (microgametocytes) and female (macrogametocytes), are ingested by an Anopheles mosquito during a blood meal . The parasites&rsquo multiplication in the mosquito is known as the sporogonic cycle . While in the mosquito&rsquos stomach, the microgametes penetrate the macrogametes generating zygotes . The zygotes in turn become motile and elongated (ookinetes) which invade the midgut wall of the mosquito where they develop into oocysts . The oocysts grow, rupture, and release sporozoites, which make their way to the mosquito&rsquos salivary glands. Inoculation of the sporozoites into a new human host perpetuates the malaria life cycle.

Human Factors And Malaria

Biologic characteristics and behavioral traits can influence an individual&rsquos risk of developing malaria and, on a larger scale, the intensity of transmission in a population.

Where does malaria transmission occur?

For malaria transmission to occur, conditions must be such so that all three components of the malaria life cycle are present:

  • Anopheles mosquitoes, which able to feed on humans humans, and in which the parasites can complete the &ldquoinvertebrate host&rdquo half of their life cycle
  • Humans. who can be bitten by Anopheles mosquitoes, and in whom the parasites can complete the &ldquovertebrate host&rdquo half of their life cycle
  • Malaria parasites.

In rare cases malaria parasites can be transmitted from one person to another without requiring passage through a mosquito (from mother to child in "congenital malaria" or through transfusion, organ transplantation, or shared needles.)


Climate is a key determinant of both the geographic distribution and the seasonality of malaria. Without sufficient rainfall, mosquitoes cannot survive, and if not sufficiently warm, parasites cannot survive in the mosquito.

Anopheles lay their eggs in a variety of fresh or brackish bodies of water, with different species having different preferences. Eggs hatch within a few days, with resulting larvae spending 9-12 days to develop into adults in tropical areas. If larval habitats dry up before the process is completed, the larvae die if rains are excessive, they may be flushed and destroyed. Life is precarious for mosquito larvae, with most perishing before becoming adults.

Life is usually short for adult mosquitoes as well, with temperature and humidity affecting longevity. Only older females can transmit malaria, as they must live long enough for sporozoites to develop and move to the salivary glands. This process takes a minimum of nine days when temperatures are warm (30°C or 86°F) and will take much longer at cooler temperatures. If temperatures are too cool (15°C or 59°F for Plasmodium vivax, 20°C or 68°F for P. falciparum), development cannot be completed and malaria cannot be transmitted. Thus, malaria transmission is much more intense in warm and humid areas, with transmission possible in temperate areas only during summer months.

In warm climates people are more likely to sleep unprotected outdoors, thereby increasing exposure to night-biting Anopheles mosquitoes. During harvest seasons, agricultural workers might sleep in the fields or nearby locales, without protection against mosquito bites.

Anopheles Mosquitoes

The types (species) of Anopheles present in an area at a given time will influence the intensity of malaria transmission. Not all Anopheles are equally efficient vectors for transmitting malaria from one person to another. Those species that are most prone to bite humans are the most dangerous, as bites inflicted on animals that cannot be infected with human malaria break the chain of transmission. If the mosquito regularly bites humans, the chain of transmission is unbroken and more people will become infected. Some species are biologically unable to sustain development of human malaria parasites, while others are readily infected and produce large numbers of sporozoites (the parasite stage that is infective to humans).

Many of the most dangerous species bite human indoors. For these species insecticide treated mosquito nets and indoor residual spray (whereby the inner walls of dwellings are coated with a long-lasting insecticide) are effective interventions. Both of these interventions require attention to insecticide resistance, which will evolve if the same insecticide is used continuously in the same area.


Biologic characteristics (inborn and acquired) and behavioral traits can influence an individual&rsquos malaria risk and, on a larger scale, the overall malaria ecology.


Characteristics of the malaria parasite can influence the occurrence of malaria and its impact on human populations, for example:

  • Areas where P. falciparum predominates (such as Africa south of the Sahara) will suffer more disease and death than areas where other species, which tend to cause less severe manifestations, predominate
  • P. vivax and P. ovale have stages (&ldquohypnozoites&rdquo) that can remain dormant in the liver cells for extended periods of time (months to years) before reactivating and invading the blood. Such relapses can result in resumption of transmission after apparently successful control efforts, or can introduce malaria in an area that was malaria-free
  • P. falciparum (and to a lesser extent P. vivax) have developed strains that are resistant to antimalarial drugs. Such strains are not uniformly distributed. Constant monitoring of the susceptibility of these two parasite species to drugs used locally is critical to ensure effective treatment and successful control efforts. Travelers to malaria-risk areas should use for prevention only those drugs that will be protective in the areas to be visited.

Plasmodium falciparum predominates in Africa south of the Sahara, one reason why malaria is so severe in that area.

Animal Reservoirs

A certain species of malaria called P. knowlesi has recently been recognized to be a cause of significant numbers of human infections. P. knowlesi is a species that naturally infects macaques living in Southeast Asia. Humans living in close proximity to populations of these macaques may be at risk of infection with this zoonotic parasite.

Areas Where Malaria Is No Longer Endemic

Malaria transmission has been eliminated in many countries of the world, including the United States. However, in many of these countries (including the United States) Anopheles mosquitoes are still present. Also, cases of malaria still occur in non-endemic countries, mostly in returning travelers or immigrants (&ldquoimported malaria&rdquo). Thus the potential for reintroduction of active transmission of malaria exists in many non-endemic parts of the world. All patients must be diagnosed and treated promptly for their own benefit but also to prevent the reintroduction of malaria.

Genetic Factors

Biologic characteristics present from birth can protect against certain types of malaria. Two genetic factors, both associated with human red blood cells, have been shown to be epidemiologically important. Persons who have the sickle cell trait (heterozygotes for the abnormal hemoglobin gene HbS) are relatively protected against P. falciparum malaria and thus enjoy a biologic advantage. Because P. falciparum malaria has been a leading cause of death in Africa since remote times, the sickle cell trait is now more frequently found in Africa and in persons of African ancestry than in other population groups. In general, the prevalence of hemoglobin-related disorders and other blood cell dyscrasias, such as Hemoglobin C, the thalassemias and G6PD deficiency, are more prevalent in malaria endemic areas and are thought to provide protection from malarial disease.

Persons who are negative for the Duffy blood group have red blood cells that are resistant to infection by P. vivax. Since the majority of Africans are Duffy negative, P. vivax is rare in Africa south of the Sahara, especially West Africa. In that area, the niche of P. vivax has been taken over by P. ovale, a very similar parasite that does infect Duffy-negative persons.

Other genetic factors related to red blood cells also influence malaria, but to a lesser extent. Various genetic determinants (such as the &ldquoHLA complex,&rdquo which plays a role in control of immune responses) may equally influence an individual&rsquos risk of developing severe malaria.

Acquired Immunity

Acquired immunity greatly influences how malaria affects an individual and a community. After repeated attacks of malaria a person may develop a partially protective immunity. Such &ldquosemi-immune&rdquo persons often can still be infected by malaria parasites but may not develop severe disease, and, in fact, frequently lack any typical malaria symptoms.

In areas with high P. falciparum transmission (most of Africa south of the Sahara), newborns will be protected during the first few months of life presumably by maternal antibodies transferred to them through the placenta. As these antibodies decrease with time, these young children become vulnerable to disease and death by malaria. If they survive repeated infections to an older age (2-5 years) they will have reached a protective semi-immune status. Thus in high transmission areas, young children are a major risk group and are targeted preferentially by malaria control interventions.

In areas with lower transmission (such as Asia and Latin America), infections are less frequent and a larger proportion of the older children and adults have no protective immunity. In such areas, malaria disease can be found in all age groups, and epidemics can occur.

Anemia in young children in Asembo Bay, a highly endemic area in western Kenya. Anemia occurs most between the ages of 6 and 24 months. After 24 months, it decreases because the children have built up their acquired immunity against malaria (and its consequence, anemia).

Mother and her newborn in Jabalpur Hospital, State of Madhya Pradesh, India. The mother had malaria, with infection of the placenta.

Pregnancy and Malaria

Pregnancy decreases immunity against many infectious diseases. Women who have developed protective immunity against P. falciparum tend to lose this protection when they become pregnant (especially during the first and second pregnancies). Malaria during pregnancy is harmful not only to the mothers but also to the unborn children. The latter are at greater risk of being delivered prematurely or with low birth weight, with consequently decreased chances of survival during the early months of life. For this reason pregnant women are also targeted (in addition to young children) for protection by malaria control programs in endemic countries.

Behavioral Factors

Human behavior, often dictated by social and economic reasons, can influence the risk of malaria for individuals and communities. For example:

  • Poor rural populations in malaria-endemic areas often cannot afford the housing and bed nets that would protect them from exposure to mosquitoes. These persons often lack the knowledge to recognize malaria and to treat it promptly and correctly. Often, cultural beliefs result in use of traditional, ineffective methods of treatment.
  • Travelers from non-endemic areas may choose not to use insect repellent or medicines to prevent malaria. Reasons may include cost, inconvenience, or a lack of knowledge.
  • Human activities can create breeding sites for larvae (standing water in irrigation ditches, burrow pits)
  • Agricultural work such as harvesting (also influenced by climate) may force increased nighttime exposure to mosquito bites
  • Raising domestic animals near the household may provide alternate sources of blood meals for Anopheles mosquitoes and thus decrease human exposure
  • War, migrations (voluntary or forced) and tourism may expose non-immune individuals to an environment with high malaria transmission.

Human behavior in endemic countries also determines in part how successful malaria control activities will be in their efforts to decrease transmission. The governments of malaria-endemic countries often lack financial resources. As a consequence, health workers in the public sector are often underpaid and overworked. They lack equipment, drugs, training, and supervision. The local populations are aware of such situations when they occur, and cease relying on the public sector health facilities. Conversely, the private sector suffers from its own problems. Regulatory measures often do not exist or are not enforced. This encourages private consultations by unlicensed, costly health providers, and the anarchic prescription and sale of drugs (some of which are counterfeit products). Correcting this situation is a tremendous challenge that must be addressed if malaria control and ultimately elimination is to be successful.

Protective Effect of Sickle Cell Trait Against Malaria

The sickle cell gene is caused by a single amino acid mutation (valine instead of glutamate at the 6th position) in the beta chain of the hemoglobin gene. Inheritance of this mutated gene from both parents leads to sickle cell disease and people with this disease have shorter life expectancy. On the contrary, individuals who are carriers for the sickle cell disease (with one sickle gene and one normal hemoglobin gene, also known as sickle cell trait) have some protective advantage against malaria. As a result, the frequencies of sickle cell carriers are high in malaria-endemic areas.

CDC&rsquos birth cohort studies (Asembo Bay Cohort Project in western Kenya) conducted in collaboration with the Kenya Medical Research Institute allowed an investigation into this issue. It was found that that the sickle cell trait provides 60% protection against overall mortality. Most of this protection occurs between 2-16 months of life, before the onset of clinical immunity in areas with intense transmission of malaria.

Graph of survival curves (&ldquosurvival function estimates&rdquo) of children without any sickle cell genes (HbAA), children with sickle cell trait (HbAS), and children with sickle cell disease (HbSS). Those who had the sickle cell trait (HbAS) had a slight survival advantage over those without any sickle cell genes (HbAA), with children with sickle cell disease (HbSS) faring the worst.

Reference: Protective Effects of the Sickle Cell Gene Against Malaria Morbidity and Mortality. Aidoo M, Terlouw DJ, Kolczak MS, McElroy PD, ter Kuile FO, Kariuki S, Nahlen BL, Lal AA, Udhayakumar V. Lancet 2002 359:1311-1312.

Anopheles Mosquitoes

Malaria is transmitted to humans by female mosquitoes of the genus Anopheles. Female mosquitoes take blood meals for egg production, and these blood meals are the link between the human and the mosquito hosts in the parasite life cycle. The successful development of the malaria parasite in the mosquito (from the &ldquogametocyte&rdquo stage to the &ldquosporozoite&rdquo stage) depends on several factors. The most important is ambient temperature and humidity (higher temperatures accelerate the parasite growth in the mosquito) and whether the Anopheles survives long enough to allow the parasite to complete its cycle in the mosquito host (&ldquosporogonic&rdquo or &ldquoextrinsic&rdquo cycle, duration 9 to 18 days). In contrast to the human host, the mosquito host does not suffer noticeably from the presence of the parasites.

Diagram of Adult Female Mosquito

Map of the world showing the distribution of predominant malaria vectors

Anopheles freeborni mosquito pumping blood
Larger Picture

General Information

There are approximately 3,500 species of mosquitoes grouped into 41 genera. Human malaria is transmitted only by females of the genus Anopheles. Of the approximately 430 Anopheles species, only 30-40 transmit malaria (i.e., are &ldquovectors&rdquo) in nature. The rest either bite humans infrequently or cannot sustain development of malaria parasites.

Geographic Distribution

Anophelines are found worldwide except Antarctica. Malaria is transmitted by different Anopheles species in different geographic regions. Within geographic regions, different environments support a different species.

Anophelines that can transmit malaria are found not only in malaria-endemic areas, but also in areas where malaria has been eliminated. These areas are thus at risk of re-introduction of the disease.

Life Stages

Like all mosquitoes, anopheles mosquitoes go through four stages in their life cycle: egg, larva, pupa, and adult. The first three stages are aquatic and last 7-14 days, depending on the species and the ambient temperature. The biting female Anopheles mosquito may carry malaria. Male mosquitoes do not bite so cannot transmit malaria or other diseases. The adult females are generally short-lived, with only a small proportion living long enough (more than 10 days in tropical regions) to transmit malaria.

Adult females lay 50-200 eggs per oviposition. Eggs are laid singly directly on water and are unique in having floats on either side. Eggs are not resistant to drying and hatch within 2-3 days, although hatching may take up to 2-3 weeks in colder climates.


Mosquito larvae have a well-developed head with mouth brushes used for feeding, a large thorax, and a segmented abdomen. They have no legs. In contrast to other mosquitoes, Anopheles larvae lack a respiratory siphon and for this reason position themselves so that their body is parallel to the surface of the water.

Top: Anopheles Egg note the lateral floats.
Bottom: Anopheles eggs are laid singly.

Larvae breathe through spiracles located on the 8th abdominal segment and therefore must come to the surface frequently.

The larvae spend most of their time feeding on algae, bacteria, and other microorganisms in the surface microlayer. They do so by rotating their head 180 degrees and feeding from below the microlayer. Larvae dive below the surface only when disturbed. Larvae swim either by jerky movements of the entire body or through propulsion with the mouth brushes.

Larvae develop through 4 stages, or instars, after which they metamorphose into pupae. At the end of each instar, the larvae molt, shedding their exoskeleton, or skin, to allow for further growth.

Anopheles Larva. Note the position, parallel to the water surface.

The larvae occur in a wide range of habitats but most species prefer clean, unpolluted water. Larvae of Anopheles mosquitoes have been found in fresh- or salt-water marshes, mangrove swamps, rice fields, grassy ditches, the edges of streams and rivers, and small, temporary rain pools. Many species prefer habitats with vegetation. Others prefer habitats that have none. Some breed in open, sun-lit pools while others are found only in shaded breeding sites in forests. A few species breed in tree holes or the leaf axils of some plants.


The pupa is comma-shaped when viewed from the side. This is a transitional stage between larva and adult. The pupae does not feed, but undergoes radical metamorphosis. The head and thorax are merged into a cephalothorax with the abdomen curving around underneath. As with the larvae, pupae must come to the surface frequently to breathe, which they do through a pair of respiratory trumpets on the cephalothorax. After a few days as a pupa, the dorsal surface of the cephalothorax splits and the adult mosquito emerges onto the surface of the water.

The duration from egg to adult varies considerably among species and is strongly influenced by ambient temperature. Mosquitoes can develop from egg to adult in as little as 7 days but usually take 10-14 days in tropical conditions.

Anopheles Adults. Note (bottom row) the typical resting position.


Like all mosquitoes, adult anopheles have slender bodies with 3 sections: head, thorax and abdomen.

The head is specialized for acquiring sensory information and for feeding. The head contains the eyes and a pair of long, many-segmented antennae. The antennae are important for detecting host odors as well as odors of aquatic larval habitats where females lay eggs. The head also has an elongate, forward-projecting proboscis used for feeding, and two sensory palps.

The thorax is specialized for locomotion. Three pairs of legs and a single pair of wings are attached to the thorax.

The abdomen is specialized for food digestion and egg development. This segmented body part expands considerably when a female takes a blood meal. The blood is digested over time serving as a source of protein for the production of eggs, which gradually fill the abdomen.

Anopheles mosquitoes can be distinguished from other mosquitoes by the palps, which are as long as the proboscis, and by the presence of discrete blocks of black and white scales on the wings. Adult Anopheles can also be identified by their typical resting position: males and females rest with their abdomens sticking up in the air rather than parallel to the surface on which they are resting.

Adult mosquitoes usually mate within a few days after emerging from the pupal stage. In some species, the males form large swarms, usually around dusk, and the females fly into the swarms to mate. The mating habitats of many species remain unknown.

Males live for about a week, feeding on nectar and other sources of sugar. Females will also feed on sugar sources for energy but usually require a blood meal for the development of eggs. After obtaining a full blood meal, the female will rest for a few days while the blood is digested and eggs are developed. This process depends on the temperature but usually takes 2-3 days in tropical conditions. Once the eggs are fully developed, the female lays them then seeks blood to sustain another batch of eggs.

The cycle repeats itself until the female dies. Females can survive up to a month (or longer in captivity) but most do not live longer than 1-2 weeks in nature. Their chances of survival depend on temperature and humidity, but also upon their ability to successfully obtain a blood meal while avoiding host defenses.

Female Anopheles dirus feeding

Factors Involved in Malaria Transmission and Malaria Control

Understanding the biology and behavior of Anopheles mosquitoes can aid in designing appropriate control strategies. Factors that affect a mosquito&rsquos ability to transmit malaria include its innate susceptibility to Plasmodium, its host choice, and its longevity. Long-lived species that prefer human blood and support parasite development are the most dangerous. Factors that should be taken into consideration when designing a control program include the susceptibility of malaria mosquitoes to insecticides and the preferred feeding and resting location of adult mosquitoes.

Preferred Sources for Blood Meals

One important behavioral factor is the degree to which an Anopheles species prefers to feed on humans (anthropophily) or animals such as cattle (zoophily). Anthrophilic Anopheles are more likely to transmit the malaria parasites from one person to another. Most Anopheles mosquitoes are not exclusively anthropophilic or zoophilic many are opportunistic and feed upon whatever host is available. However, the primary malaria vectors in Africa, An. gambiae and An. funestus, are strongly anthropophilic and, consequently, are two of the most efficient malaria vectors in the world.

Life Span

Once ingested by a mosquito, malaria parasites must undergo development within the mosquito before they are infectious to humans. The time required for development in the mosquito (the extrinsic incubation period) takes 9 days or longer, depending on the parasite species and the temperature. If a mosquito does not survive longer than the extrinsic incubation period, then she will not be able to transmit any malaria parasites.

It is not possible to measure directly the life span of mosquitoes in nature, but many studies have indirectly measured longevity by examination of their reproductive status or via marking, releasing, and recapturing adult mosquitoes. The majority of mosquitoes do not live long enough to transmit malaria, but some may live as long as three weeks in nature. Though evidence suggests that mortality rate increases with age, most workers estimate longevity in terms of the probability that a mosquito will live one day. Usually these estimates range from a low of 0.7 to a high of 0.9. If survivorship is 90% daily, then a substantial proportion of the population would live longer than 2 weeks and would be capable of transmitting malaria. Any control measure that reduces the average lifespan of the mosquito population will reduce transmission potential. Insecticides thus need not kill the mosquitoes outright, but may be effective by limiting their lifespan.

Patterns of Feeding and Resting

Most Anopheles mosquitoes are crepuscular (active at dusk or dawn) or nocturnal (active at night). Some Anopheles mosquitoes feed indoors (endophagic) while others feed outdoors (exophagic). After blood feeding, some Anopheles mosquitoes prefer to rest indoors (endophilic) while others prefer to rest outdoors (exophilic). Biting by nocturnal, endophagic Anopheles mosquitoes can be markedly reduced through the use of insecticide-treated bed nets (ITNs) or through improved housing construction to prevent mosquito entry (e.g., window screens). Endophilic mosquitoes are readily controlled by indoor spraying of residual insecticides. In contrast, exophagic/exophilic vectors are best controlled through source reduction (destruction of larval habitats).

Insecticide Resistance

Insecticide-based control measures (e.g., indoor spraying with insecticides, ITNs) are the principal way to kill mosquitoes that bite indoors. However, after prolonged exposure to an insecticide over several generations, mosquitoes, like other insects, may develop resistance, a capacity to survive contact with an insecticide. Since mosquitoes can have many generations per year, high levels of resistance can arise very quickly. Resistance of mosquitoes to some insecticides has been documented within a few years after the insecticides were introduced. There are over 125 mosquito species with documented resistance to one or more insecticides. The development of resistance to insecticides used for indoor residual spraying was a major impediment during the Global Malaria Eradication Campaign. Judicious use of insecticides for mosquito control can limit the development and spread of resistance, particularly via rotation of different classes of insecticides used for control. Monitoring of resistance is essential to alert control programs to switch to more effective insecticides.


Some Anopheles species are poor vectors of malaria, as the parasites do not develop well (or at all) within them. There is also variation within species. In the laboratory, it has been possible to select for strains of An. gambiae that are refractory to infection by malaria parasites. These refractory strains have an immune response that encapsulates and kills the parasites after they have invaded the mosquito&rsquos stomach wall. Scientists are studying the genetic mechanism for this response. It is hoped that some day, genetically modified mosquitoes that are refractory to malaria can replace wild mosquitoes, thereby limiting or eliminating malaria transmission.

Malaria Parasites

Malaria parasites are micro-organisms that belong to the genus Plasmodium. There are more than 100 species of Plasmodium, which can infect many animal species such as reptiles, birds, and various mammals. Four species of Plasmodium have long been recognized to infect humans in nature. In addition there is one species that naturally infects macaques which has recently been recognized to be a cause of zoonotic malaria in humans. (There are some additional species which can, exceptionally or under experimental conditions, infect humans.)

Ring-form trophozoites of P. falciparum in a thin blood smear.

Ring-form trophozoites of P. vivax in a thin blood smear.

Trophozoites of P. ovale in a thin blood smear.

Band-form trophozoites of P. malariae in a thin blood smear.

Schizont and ring-form trophozoite of P. knowlesi in a thin blood smear.

This Worm Can Express Three Sexes Simultaneously

It might look simple, but a nematode species’ chromosomal gymnastics could shine a light for humans, too.

Neel V. Patel

Sinclair Stammers/Science Photo Library

Hermaphrodites are a wild concept for most people to understand, but for scientists, they’re sort of passé. Case in point: nematodes, who have had us beat for millions of years. In a new paper published in Current Biology, Diane Shakes, a biologist at the University of William & Mary, and her colleagues describe a species of nematode that has three sexes—male, female, and hermaphrodite—and how they’ve managed to hack into a unique mechanism of reproductive biology that’s unlike anything scientists have ever seen before.

Many animal species utilize hermaphroditism as a way to enable sexual reproduction more widely across the species and make all individuals potential partners. Some hermaphrodites are actually self-fertilizing, and confer what Shakes likes to call a “boom in a bust” situation. “If things are really bad,” she told The Daily Beast, “a single worm can go off and find a new food source, and establish a whole new population.”

In nematode species, hermaphrodites are certainly not new. “The model nematode that everyone has been studying since the middle of the ’70s, Caenorhabditis elegans, has hermaphrodites and males,” said Shakes.

But trisex species found in nature are extremely rare. In 2004, a researcher named Marie-Anne Felix published a paper outlining a free-living nematode called SB347 (later named Auanema rhodensis) that produced not one, not two, but three sexes: males, females, and hermaphrodites. The females only produce eggs and can only reproduce by mating with males, while hermaphrodites produce eggs and sperm, and are capable of mating with males or self-fertilizing their own eggs with their own sperm. The males only produce sperm and are capable of mating with both other sexes.

How exactly this “trioecious” species operated, however, was a mystery. Shakes and her colleagues have been trying to investigate this among other questions, and the result was entirely unexpected.

“The actual paper, to tell you the truth, is more of a ‘gee-whiz, genetics!’ paper,” Shakes said. “We’re breaking the laws of Gregor Mendel and our fundamental assumptions of how genetics is supposed to work.” Mendelian genetics generally dictates that a sexually-reproducing species will be a roughly even split of males and females. In humans, this is determined by the fact that females possess two X chromosomes (XX), and males are XY. Sex cells work by contributing a chromosome each upon fertilization, so all eggs produced by females carry a single X, while male sperm are generated with a 50:50 mix of X or Y. As a result, each pregnancy has an equal chance of being male or female, and so the population as a whole skews to an even gender ratio.

A. rhodensis upends these rules not just by having three sexes, but by using only one sex chromosome (X) to determine that sex. Unlike in humans, these nematodes don’t have a Y sex chromosome. Females and hermaphrodites are XX, while males are single X.

Even with just X chromosomes to play around with, classical genetics would presume half the nematode sperm (in males and hermaphrodites) would have an X chromosome and half would not. It would also presume all female and hermaphrodite eggs have one X chromosome. Mating in any sense would produce half XX progeny (females and hermaphrodites) and half X progeny (males). But previous studies show that A. rhodensis males only produce single X sperm. So when males breed with females, they only produce female offspring.

That already warps things, but this new paper takes things to a more absurd level: Shakes and her team found that, while the females are making single X eggs, most hermaphrodites eggs have no X chromosomes, and almost all the sperm are XX. Thus, the hermaphrodites, when they self-fertilize, produce mostly XX offspring (females and hermaphrodites), although still a few males. When they breed with males, the result is mostly male (single X) offspring.

“This was a huge surprise,” said Shakes. “We really didn’t expect this.” But it actually makes sense from an evolutionary standpoint. Cross-mating is more ideal because it produces more genetic diversity, but self-fertilization gives the species an opportunity to venture off as individuals and sire offspring elsewhere. Hermaphrodites, Shakes explained, are “explorers” able to seek new food patches and survive tougher conditions.

In addition, shirking the skew towards 50:50 male:female representation is also advantageous. A population in any species actually grows faster with fewer males mating with more females, so a distribution like this with three different sexes actually helps ensure there are more egg-producing members and less sperm-producing ones. Ultimately, three sexes gives A. rhodensis a remarkable ability to adapt to changing conditions and funnel resources to population growth more efficiently than other species.

But what determines whether XX progeny will be female or hermaphrodite? Shakes’ co-author, Andre Pires-da Silva from the University of Warwick in the U.K., has found that larval exposure to a particular pheromone (and another undetermined factor controlled by the mother) can affect what sex the nematode grows up to be.

This is quite a far ways away from the standard XX and XY distribution we see in humans, although there is a bit of shared experience between nematodes and the human species. Chromosomal abnormalities in humans sometimes results in an individual possessing an irregular number of sex chromosomes. Triple X syndrome (also known as trisomy) is when a female individual has three X chromosomes symptoms include learning difficulties, decreased muscle tone, and some issues with the kidneys. Klinefelter syndrome in males is the result of an XXY distribution, and primarily results in sterility and small testicles, with other less pronounced symptoms sometimes manifesting. Turner syndrome occurs in females with only one X chromosome, and confers a shorter and webbed neck, shorter stature, and swollen hands and feet.

The main difference, obviously, is that irregular sex chromosome segregation is a purposeful, evolutionary-advantageous phenomena in nematodes. “This species is manipulating the X in a way that’s actually helpful for the worm, and it’s doing so with accuracy and precision,” Shakes said. That’s certainly not the case in humans, although modern medicine has certainly made great strides into making those afflicted by genetic disorders live easier, more normal lives.

That doesn’t mean we can’t apply what we learn from A. rhodensis to human genetics. “This is a very extreme system of chromosomal segregation,” she said. “If you’re an examining an extreme segregation, you can start to learn why it is that only the X chromosome behaves in a certain way as opposed to other chromosomes. If you start to understand how and why this happening, that could apply to what’s going on in abnormal situations in humans where the chromosomes aren’t separating properly.”

Another biomedical benefit goes back to dealing with parasitic nematode species, some which affect humans, and many which are devastating to agricultural plants grown by farmers. “One question we have is whether this extreme trisexual mechanism is happening out in the wild,” Shakes pointed out. Understanding those dynamics in depth means we could learn something about how to better combat those pest species and develop interventions that could keep people and crops safer. Rather than, say, genetically engineering a nematode-resistant crop, we could develop a poison that targets nematode reproductive mechanisms directly, with its male, female, or hermaphrodite.

Those implications, however, won’t augur into something practical until much later on. For now, Shakes and her colleagues still have a ton of outstanding questions they’d like to answer. She’s excited they finally have evidence that this crazy reproductive behavior is really happening, but she and her team also want to know more about the molecular factors that allow for this occur, as well as determine whether related species are exhibiting the same or similar mechanisms in their own biology.

The same but different

This is where a remarkable group of songbirds is helping us. The southern capuchino seedeaters (genus: Sporophila) is a group of at least ten songbird species that radiated very rapidly within the past million years despite living and breeding in the same geographic location. (In contrast, the evolutionary split between humans and chimpanzees is estimated to have occurred somewhere between five and six million years ago.) The southern capuchino seedeaters are one of the most rapid avian radiations on the planet and consequently, these bird species have almost imperceptible differences in both their ecology and genomes.

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Despite their genetic uniformity, a 2017 study did uncover a few tiny genomic regions that are involved the regulation of plumage color and patterning for different capuchino seedeater species (more here). Although they are small, these genomic differences have enormous effects: Adult males of different species have dramatically different plumage colors and patterns whilst adult females and juveniles are visually indistinguishable. In addition to their striking genomic similarities, this group of birds also has remarkably low levels of ecological divergence living alongside each other in the same area, and eating the same foods.

The wet grasslands of Iberá National Park in Corrientes, Argentina. (Credit: Sheela Turbek)

Seven of the ten (or so) known capuchino seedeater species breed within the vast swampy grasslands of the Iberá National Park, a newly created national park in northeastern Argentina. These finch-like songbirds are smaller than a canary and have short, stout bills that are adapted for opening seeds. Most capuchino seedeater species are strongly sexually dimorphic with males showing dramatically different plumage colors and patterns from females and juveniles, which all appear identical to human eyes, regardless of species (figure 1, right panel).

“The capuchino seedeaters of South America are what we call a ‘species flock’,” the senior author of the study, evolutionary biologist Leonardo Campagna, a research associate at the Cornell Lab of Ornithology, said in a statement. “This group is branching out rapidly and each of its dozen species is in a very early stage of evolution.”

When a new species of seedeater, the Iberá seedeater, Sporophila iberaensis, was first observed in 2001, and scientifically described in 2016, it was noted that this species breeds side-by-side with its very close relative, the tawny-bellied seedeater, Sporophila hypoxantha. Although it is increasing in local abundance, the Iberá seedeater has a very small and restricted breeding range whereas the tawny-bellied seedeater is the most abundant and widespread capuchino species in Iberá National Park.

F I G U R E 1 : Adult male Iberá seedeater (Sporophila iberaensis left), and adult male . [+] tawny-bellied seedeater (Sporophila hypoxantha center). Adult females of both species (right) cannot be visually distinguished. (doi:10.1126/science.abc0256)

Adult males of these two species are easy to distinguish at a glance: Adult male Iberá seedeaters have black cheeks and throat, pale grey underparts, darker grey wings and back (figure 1, left panel). In contrast, adult male tawny-bellied seedeaters have rust-colored cheeks, throat and underparts with dark grey wings and back (figure 1, center panel). Both species have dark eyes, bills, legs and tails. How did the Iberá seedeater appear so quickly and how does it remain distinct from its closest relative, the tawny-bellied seedeater?

We have the wrong idea about males, females and sex

Once upon a time, animal courtship was thought to run something like a Barbara Cartland novel. The rakish males battle it out for a chaste female, who sits around choosing the prince charming to father her young. While her mate may sow his wild oats far and wide, she patiently tends her brood.

Notwithstanding a few counterexamples, these roles were thought to be largely the same across the animal kingdom: males were thought to be promiscuous, dominant and aggressive and the females chaste and passive. For many people, it was just the natural order of the world.

But have we been blinkered by our own cultural prejudices, casting animals in the kinds of roles we saw in the society around us? That is the view of a small but growing number of biologists. "It's almost like they are using this locker-room logic &ndash counting which males 'score' the most," says Joan Roughgarden at the Hawai'i Institute of Marine Biology.

The dividing line between male and female is frequently blurred or easily crossed

Researchers such as Roughgarden argue that it was a classic case of "confirmation bias". Many biologists were seeing what they wanted to believe, and then using the results to justify prevailing cultural norms. "You get this back-and-forth: science is reinforcing societal mores, and the mores are reinforcing what the science is saying," says Zuleyma Tang-Martinez at the University of Missouri &ndash St Louis.

The result, Tang-Martinez and Roughgarden believe, is that scientists have often failed to recognise astonishingly diverse sexual behaviours across the animal kingdom. There are now myriad examples of animals that break the rules entirely &ndash from intersex kangaroo to a fish with four separate "genders".

If they are right, we should rethink many of our assumptions about sex differences. As with humans, the dividing line between male and female is frequently blurred or easily crossed.

Much of our modern understanding of sex differences came from Charles Darwin's struggles to explain the peacock's tale. How could such a cumbersome and extravagant display ever contribute to the animal's survival? "The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!" he wrote in an 1860 letter to his colleague Asa Gray.

Darwin saw the same patterns &ndash males being "passionate", females "coy" &ndash across the animal kingdom

Darwin's solution was "sexual selection": a form of evolution that comes directly from the challenges of reproduction.

When many males compete for a single female, each male has to show off his worth in some way either through direct combat, or in a showy display that proves he would be the healthiest father for her young. The resulting arms race led to the evolution of ever more excessive traits in the males of certain species: hence the peacock's tale, which helps it to advertise its good health to the peahen.

Darwin saw the same patterns &ndash males being "passionate", females "coy" &ndash across the animal kingdom. Later, the evolutionary biologist Angus John Bateman argued that this could be explained through basic economics.

Eggs, Bateman said, are huge and packed full of nutrients, making them costly to produce. By contrast, sperm are so small they can be produced in their millions.

The bottom line is that males have evolved to be promiscuous and females have evolved to be choosy

This means the stakes of the mating game are much higher for a female, and so she needs to choose her gamble carefully. Meanwhile, the male has sperm to spare, letting him take a gamble wherever he chooses.

The female's investment is even greater if she has to spend time gestating and rearing the young, so she needs to make sure she chooses a mate who will give her young the best genes and the best chances of survival.

"The bottom line is that males have evolved to be promiscuous and females have evolved to be choosy &ndash they should only mate with the best male," says Tang-Martinez.

Some of the first evidence came from an experiment Bateman conducted on fruit flies in 1948. He found that males had a better chance of passing on their genes if they mated with many different partners, whereas the females did not produce any more offspring after their initial mating.

Just like peacocks, female pipefish have evolved bright, colourful markings as a result of sexual selection

The same kind of logic has since explained the behaviours of many different species, from dragonflies and grouse to baboons and elephant seals. Indeed, a seminal 1972 paper on the subject by Robert Trivers has now been cited more than 11,000 times, making it one of the most influential ideas in evolutionary biology.

True, there were always some exceptions. For instance, in certain species of pipefish the female actively courts the male, before "gluing" her eggs to her chosen mate. While she can swim off to find another partner, he spends time nourishing the growing young.

In this case the male invests more in the young than the female does. But such cases of "sex role reversal" were generally considered to be rare

They were also thought to be exceptions that proved the rule. Just like peacocks, female pipefish have evolved bright, colourful markings as a result of sexual selection. These females are also larger than the males, and form hierarchies of dominance determining who can access the "harem".

Still, in the vast majority of species, males were assumed to play the jock while the females waited patiently on the sidelines. This assumption is now under attack by some biologists, who wonder whether it has been shaped by prevailing cultural preferences.

The arguments are particularly troubling when sexual selection theory is used to explain human behaviours.

Even that very first study of fruit flies has come under scrutiny

For instance, some researchers had argued that men are naturally funnier than women, with humour acting as a sexual display akin to bright, colourful plumage &ndash even though any apparent sex differences could easily be the result of sexist stereotyping rather than evolutionary history.

Perhaps biologists just have not looked hard enough to truly understand the complex ways that males and females may interact.

"We haven't really asked any questions about how sexual selection may be acting on females," says Patricia Gowaty at the University of California, Los Angeles. "We know barely anything about what's going on in competitive arenas of females&hellip and the people that have asked seem to think the only way it might act is the same way it does on males."

Even that very first study of fruit flies &ndash the cornerstone of parental investment theory &ndash has come under scrutiny. When Gowaty tried to replicate the results in 2012, she failed to find convincing evidence that the males benefited from being more promiscuous than the females.

In a paper published in April 2016, Tang-Martinez describes many examples in which females do not play by the rules laid down by sexual selection theory.

Female lionesses may mate 100 times a day with a string of different partners

For instance, the females of many bird species had been thought to be exclusively monogamous, with the female faithfully sticking with her chosen partner.

In fact, this could not be further from the truth. Female birds often have dalliances even when in a stable partnership. Among the fairy wren, for instance, just 5% of the clutches will have been fathered by a single mate.

As further evidence, Tang-Martinez points out that female lionesses may mate 100 times a day with a string of different partners. The same seemingly-indiscriminate lust can be seen in many species of primates: not just the famously sexually-active bonobos, but langurs, lemurs and capuchin monkeys. That's not to mention countless studies of beetles, crickets, salamanders, snakes, geckos and house mice.

In all these cases, the females simply do not sit around waiting for Prince Charming, as Bateman had proposed. But the idea that this overthrows Bateman's ideas is rather controversial.

While Trivers says he was surprised by some of the findings in songbirds, he argues that the balance of evidence still hangs in favour of parental investment theory. "There's no question about it, the general theory is alive and well," he says.

It is dangerous to come up with simple explanations for all species

A study published in February 2016 compared the behaviours of 60 different species, and it supports Trivers. "As far as our data go, it's true for [the] vast majority of species," says co-author Nils Anthes of the University of Tübingen in Germany &ndash although he agrees there are many exceptions.

But even in this comprehensive study, Tang-Martinez points out that the overall differences between the sexes were rather weak: according to one measure, they were not even statistically significant.

Furthermore, the number of species studied was still relatively small, she says. The study also did not fully account for the fact that sex differences may change depending on circumstances &ndash like the ratio of males and females within the population, which could influence how the individuals pair up.

In any case, Tang-Martinez is not suggesting that we should throw out the whole theory. Clearly, it is true for some animals. Instead, she thinks it is time to drop the more sweeping generalisations about male and female behaviour. "It is dangerous to come up with simple explanations for all species," she says.

Joan Roughgarden would firmly agree. Formerly known as Jonathan, she began thinking about the evolution of gender at a Gay Pride march in San Francisco, shortly before her gender transition.

When I got into it I was astonished by just how much variation there is

How, Roughgarden wondered, does biology account for such a huge population, normally considered an unfortunate footnote in scientific theory? "When scientific theory says something is wrong with so many people, perhaps the theory is wrong, not the people," she concluded.

The result was her 2004 book Evolution's Rainbow, which examined the multitudinous ways that sex is expressed in nature. It goes far beyond our black-and-white definitions of "male" and "female".

"As a biologist, you think there may be a couple &ndash maybe as many as a dozen &ndash of cases that depart from heteronormative binary," says Roughgarden. "But when I got into it I was astonished by just how much variation there is."

Scientists generally assume that sex is determined by the presence or absence of certain chromosomes. In humans, it is the X and Y chromosomes.

An intersex female bear actually mates and gives birth through the tip of her 'penis'

However, the relevant genes can still be expressed in different ways. The result is that, within any species, many individuals will show characteristics of two sexes.

There are plenty of examples of hermaphrodite invertebrates: leopard slugs are one of many. But Roughgarden has also found that intersex individuals are common among mammals, including red kangaroos, tammar wallabies, Vanuatu pigs, and America's black and brown bears.

According to a 1988 study, between 10 to 20% of female bears have a penis-like structure in place of a vagina. "An intersex female bear actually mates and gives birth through the tip of her 'penis'," says Roughgarden.

These are extreme cases. But many other animals cannot be classified simply as "males" and "females", as if members of each sex will look and act according to the same template.

In Evolution's Rainbow, Roughgarden cites many species that could be considered to have three, four or five separate "genders": that is, animals that belong to the same biological sex but that have distinct appearances and sexual behaviours.

Among white-throated sparrows there appear to be two kinds of males and two kinds of females

For instance, the bluegill sunfish has three male genders, each of which reproduces in a different way. The largest, most aggressive males show off a flashy orange breast, and actively court females to lay eggs in their territory. In contrast, the smallest males are duller in colour and have no territory of their own, but will dart into one of the dominant male's territory's to fertilise some of his mate's eggs.

It is the medium-sized males who are the most surprising. They appear to actively court the larger males with a dance in the water. If the big male accepts their advances, they may then form a ménage a trois with an approaching female, with both males both fertilising her eggs.

Why would the larger male team up with the weaker partner in this way? One possibility is that his presence helps to reassure the female that the larger male is not too aggressive. For this reason, Roughgarden describes these medium-sized males as marriage brokers.

In other species, a range of "genders" may offer a greater variety of parenting styles.

For instance, among white-throated sparrows there appear to be two kinds of males and two kinds of females. Each is defined by the colour of the stripes in their feathers, their relative dominance or aggression, and the amount of parental care they offer the young. Noticeable differences can also be found in their brain structure.

A "cross-dressing" male with more feminine features is often described as "deceptive"

The result is a number of different possible couplings, each dividing the responsibilities of parenting &ndash such as feeding and defending the young &ndash in a different way.

Roughgarden's book offers many similar examples among hummingbirds, wrasse and tree lizards, each showing a spectrum of genders.

There is also a growing list of species that engage in homosexual behaviours. With such variation, it begins to make less sense to discuss "male" and "female" behaviour as if it means the same thing for all species, or even all individuals within a species.

Roughgarden says evolutionary theory has not done justice to this broad spectrum.

Even when biologists have noted these exceptions, they tend to describe them in pejorative terms, she says. For instance, a "cross-dressing" male with more feminine features is often described as "deceptive".

The living world is made of rainbows within rainbows within rainbows

"Again, it's this locker-room story &ndash you go to a bar, see this cute-looking girl, and it turns out to be a guy, so you feel fooled and taken advantage of," she says.

Still, Roughgarden thinks attitudes are changing, albeit slowly. Today, homosexual behaviour in animals is attracting more research, and she hopes the same will be true of sex and gender roles more generally. "There's just beginning to be a discussion about non-binary gender variation," she says.

By ignoring this variation, we simplify the story of evolution and neglect some of nature's most astonishing adaptations. As Roughgarden puts it: "The living world is made of rainbows within rainbows within rainbows."

This story is part of our Sexual Revolutions series on our evolving understanding of sex and gender.

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What makes a species different?

New Rochester research has shown there are more factors at play in the genetic incompatibility of different species—specifically the presence of “selfish genes,” whose flow among species may dictate whether two species converge or diverge. (Getty Images photo)

Most evolutionary biologists distinguish one species from another based on reproductivity: members of different species either won’t or can’t mate with one another, or, if they do, the resulting offspring are often sterile, unviable, or suffer some other sort of reduced fitness.

For most of the 20 th century, scientists believed that this reproductive incompatibility evolved gradually between species as a by-product of adapting to different ecological circumstances: if two species were geographically isolated, they would adapt differences based on their environment. New research conducted at the University of Rochester, in collaboration with the University of Nebraska, shows, however, that there are more factors at play—specifically the presence of selfish genes called meiotic drive elements, whose flow among species may dictate whether two species converge or diverge. In a new paper published in the journal eLife, the researchers show that sex chromosomes evolve to be genetically incompatible between species faster than the rest of the genetic chromosomes and reveal the factors at play in this incompatibility.

When members of a species mate and exchange genetic material, this is known as gene flow. But, when there are genetic incompatibilities between species, gene flow is reduced. “Genes from one species can’t always talk to genes from the other species,” says Daven Presgraves, a Dean’s Professor of Biology at Rochester. Though genes may work fine in their own genetic background, they can have negative effects when they are moved into the genetic background of another species. “All of the gene copies in you and me function well in the human genome. But if we take a gene from us and stick it into a chimpanzee, it may not function properly— it hasn’t seen this genome before, and it might not work together with the chimpanzee’s genes. That could compromise some aspect of development or fertility.”

This is what happened when Presgraves and members of his lab crossed two different species of fruit flies, one from Madagascar and the other from the island of Mauritius. When the two species were crossed, their hybrid female offspring were fertile, but their hybrid male offspring were completely sterile. “One of the steps during the gradual evolutionary build up of complete reproductive incompatibility is that the XY sex becomes sterile first,” Presgraves says.

The researchers use genetic markers to track segments of the X chromosome that they move from one species of Drosophila (fruit fly) into a different species in order to find X-linked genes that cause male sterility. Genetic markers that affect eye color are located on the X chromosome, so the researchers start with Drosophila mauritiana that have two genetic markers—giving them dark red eyes, left—and cross them to white-eyed Drosophila simulans. (University of Nebraska photos / Rodolfo Villegas)

Chromosomes are divided into two types: sex chromosomes and autosomes. Sex chromosomes are the XY chromosomes that denote a male in both fruit flies and humans and the XX that denote a female. When the researchers mapped the factors that cause hybrid males to become sterile, they found that there were many more incompatibility factors on the X sex chromosome compared to on the autosomes. This means that sex chromosomes become functionally different between species much faster than non-sex chromosomes, Presgraves says.

But what is it that makes sex chromosomes evolve genetic incompatibilities faster than the rest of the genome?

The researchers found that a class of “selfish genes” called meiotic drive elements are partly responsible for making sex chromosomes genetically incompatible at a faster rate. In general, selfish genes are parasites of the genome—they propagate themselves at the expense of other genes. Meiotic drive elements in particular subvert the rules of typical inheritance: in normal Mendelian inheritance, a gene is transmitted to half of the offspring. Meiotic drive elements, however, manipulate reproduction so they can transmit themselves to greater than 50 percent—more than their fair share. In male fruit flies, meiotic drive elements usually kill sperm that don’t carry them, leaving only (or mostly) sperm that do.

“When multiple meiotic drive elements from both parental species are unsuppressed in hybrids, their combined action can cause sterility,” says Colin Meiklejohn, an assistant professor of biology at the University of Nebraska and former postdoctoral researcher in Presgraves’s lab.

In a twist, however, the researchers also found that if meiotic drive elements have the opportunity for gene flow between species, they can also help bring species together. During the early stages of speciation, when two different species are just beginning to break away from one another, reproductive incompatibility can be incomplete and “leaky”—some parts of the genome may still be compatible and exchangeable.

“If two populations are leaky and there is opportunity for gene flow, a selfish gene can migrate from one species to another and spread there,” Presgraves says. If the selfish gene is functional in the other species, instead of becoming incompatible, “that part of the genome will become perfectly exchangeable. In some cases, then, a selfish gene can erase the buildup of incompatibilities for a part of the genome.”

That is, meiotic drive elements can cause incompatibilities between species if they do not experience gene flow, or they can cause a convergence of the species, if the species does experience gene flow. A major factor in determining whether or not a species is compatible hinges on how much gene flow occurs between the species, Presgraves says. “Species— even ones that are geographically isolated—are leakier than we thought.”

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