What are the biological differences between male and female bodies?
Mass media blasts us with almost daily doses of ideas like “Men are from Mars; women are from Venus.” We’re told that men and women are physically, emotionally, and cognitively different from each other. But how much of this is true, and how much is stereotypes?
There are a few things we do know. On average, male bodies are taller than female bodies. On average, male bodies are also stronger than female bodies. It is important to keep in mind, though, that these are averages. Height and muscle mass vary widely across ethnicities, so that the average height of male bodies in some southeast Asian countries is around 5 feet 4 inches, while the average height of female bodies in some western and northern european countries is around 5 feet 6 inches. Neuroimaging studies that look at structural and functional brain differences have yielded conflicting results; to date, there are no decisive, reliable, category-defining differences between the male and female brain.
Physical development begins as a fetus grows in the uterus, and those people whose bodies will later appear female undergo a different set of changes from those whose bodies will later appear male. There is also a large group of people who p. 49↵are intersex, a term used to describe those who do not fit neatly into male or female boxes.
Embryology is the study of prenatal development and covers the events that happen from fertilization until birth. Human embryos, no matter their chromosomes, all begin with the same internal structures, and these remain until approximately six to eight weeks after conception. Internal and external genital development then occur along separate pathways, making it possible for a person to have internal organs that do not match those typical of others with similar external organs.
Internally, we each begin with two sets of ducts—mesonephric ducts and paramesonephric/Mullerian ducts. In those who will have female-typical internal organs, the Mullerian ducts develop into the uterus and upper portion of the vagina. Those with a gene called sex-determining region Y (SRY) gene, typically found on the Y chromosome, develop testes, which then secrete testosterone and anti-Mullerian hormone (AMH). One of the functions of testosterone is to further the development of the mesonephric ducts, which become the epididymis and vas deferens, structures that later carry sperm from the testes out of the body. AMH signals the Mullerian ducts to recede.
While the SRY gene is usually found on the Y chromosome, there are also people who have a Y chromosome with no SRY gene or have an SRY gene on another chromosome. This can lead to differences from typical male or female development. There are also some people whose AMH receptors do not respond to AMH. They may have male-typical internal and external organs but may also retain their Mullerian ducts and have a uterus and a portion of a vagina.
Externally, all fetuses begin with similar structures, called the urogenital groove and labioscrotal folds. Around 8 to 12 weeks into development, depending on the hormones secreted, these become either a clitoris, vagina, and labia or a penis and scrotum.
p. 50↵In some people with XX chromosomes, there may be more than the typical amount of testosterone, leading to the development of external genitals that appear somewhat between average male and female genitals. There are also some people with XY chromosomes whose bodies do not produce the enzyme that converts testosterone to dihydrotestosterone (DHT), which typically “masculinizes” the genitals. These people, with five alpha-reductase deficiency, may have female-appearing external genitals as children, but as they approach puberty, their bodies change to appear more masculine.
During puberty, our bodies differentiate from each other in a variety of ways. Those with XX chromosomes typically begin puberty a year or two earlier, around age 10 or 11, and their bodies respond to a number of different hormones, including testosterone, estrogen, and progesterone. Those with XY chromosomes also have all of these hormones in their bodies, but typically have a higher testosterone surge.
Early signs of puberty for many people, regardless of sex, are oily skin and armpit/pubic hair growth. Those who develop female “secondary sex” characteristics usually grow breasts, begin to menstruate, and their bodies have a higher fat-to-muscle ratio. Those who develop male secondary sex characteristics grow more body hair, have deeper voices, and have a lower fat-to-muscle ratio.
In popular culture, estrogen and testosterone are viewed as “sex hormones,” associated with either femininity or masculinity, and linked to stereotypical female and male behaviors. In fact, everyone, regardless of sex or gender, has some level of both of these hormones in their bodies, and each hormone has multiple functions, some of which have nothing to do with typically feminine or masculine bodies or activities. Estrogen, for example, is produced, among other places, in the sertoli cells of the testes and is important for bone health. Testosterone is produced by the ovaries and adrenal glands in addition to the testes and contributes to sex drive, muscle mass, and red blood cell production in everyone.
p. 51↵We often talk about estrogen and testosterone as causing biological differences in the way men and women think and behave. However, there is not clear evidence that these hormones lead to the stereotypical differences we often associate with them. It would be difficult to design an experiment to measure hormones’ effects on our thoughts and behaviors because we would need to randomly assign certain people, starting before birth, to receive specific hormones, and others to receive different hormones. We would also have to control for social factors that likely influence the way men and women think and behave. In other words, we would have to raise those in the study in completely gender-neutral ways.
For now, what we do know is that typical male and female bodies differ from each other in predictable ways, such as height, strength, and genital/reproductive capacity, but that there are many exceptions, and many people who do not fit neatly into these two boxes.
How similar or different are male and female brains?
Society would have us believe that women and men think so differently that we are almost separate species. Men are seen as rational, while women are supposed to be more emotional. We’re told that women are caring and men are practical and that men have a better sense of direction and women have a better understanding of interpersonal interactions. Where do these stereotypes come from? Do they hold any weight? If not, why do we cling to them?
One consistent difference between brains of people of different sexes is their size. Similarly to male and female bodies, male brains tend to be bigger than female brains. In the 19th century, male scientists used this fact to argue that women were not as smart as men. However, we now know that this difference in the size of our brains is simply reflective of our height and weight differences. In other words, brain size is proportional to body size. While women’s brains are, on p. 52↵average, about 10% smaller than men’s brains, they are more condensed, with more gray matter (the part of the brain that contains cell bodies).
What about IQ? IQ stands for intelligence quotient. IQ tests were first designed in the early 20th century as standardized methods of comparing intelligence across populations. There is significant controversy over the accuracy and utility of intelligence tests. One criticism is that there are many types of intelligence, and IQ tests generally cover only a few areas. Verbal and math skills, for example, are commonly tested, but creativity and social intelligence are not. Another area of contention regarding IQ tests is their biases. Black Americans are not less intelligent than white Americans, but they do, on average, have lower IQ test scores, reflecting differences in socialization and racial biases in testing.
There has also been a strange phenomenon over the last century in which IQ scores for younger generations are rising compared to older generations, and scoring has had to be adjusted to account for these changes. There are many proposed explanations for rising IQs, including better schooling, test familiarity, and nutritional status, but none have to do with changes in genetics, meaning that IQ is likely not a measure of inborn intelligence and is affected by social and educational factors.
When it comes to sex and IQ, there is also significant controversy. In 2005, Lawrence Summers, president of Harvard University, gave a speech in which he stated that men were more likely to be geniuses than women and that one explanation for this could be genetic differences in intelligence. He did not take into account differences in socialization. In countries with higher levels of gender equality, for example, gender gaps in math scores have disappeared. Some researchers argue that, while men and women may not differ in their average scores on standardized tests, men’s scores tend to fall more toward the two ends of the bell curve. That is, they are more likely to p. 53↵have extremely low or extremely high scores than women are. This is an area of ongoing investigation.
In addition to broad intelligence tests, researchers have also investigated possible differences between men and women on specific cognitive tasks. Men, for example, do better, on average, on tests of spatial ability. However, there is not clear evidence that this difference is genetic. Women’s scores on spatial tasks improve when they practice by playing video games or when they are primed to believe that women are good at these tasks, suggesting that male advantage may be at least partly due to socialization.
Can we tell anything about male and female brains by looking at them? A number of studies have found average differences between men and women in the size or shape of particular brain regions. There does seem to be an area of the hippocampus (memory center) that is, on average, larger in men than women. However, most studies of particular brain regions have been difficult to replicate, meaning that they did not show similar results when different sets of researchers looked at the same thing.
Men are more likely than women to be left-handed, and handedness can sometimes correlate with lateralization of the brain—that is, the side of the brain that we use to perform certain cognitive tasks, such as language. This suggests that men and women may, on average, have different lateralization of their brains, although most people’s brains are set up similarly.
In neurological research, each study tends to be small, but meta-analyses (review papers that gather together evidence from many smaller studies) seem to show that there are average differences between men and women in the size of grey matter areas (cell bodies) and white matter areas (cell axons) in certain parts of the brain. The corpus callosum, for instance, connects the two hemispheres of the brain and appears to be larger in women than men. However, it is not clear what these differences mean and whether they are genetic or the result of socialization.
p. 54↵If we take a step back and look at the evidence related to sex differences in our brains, it becomes clear that men and women are much more similar than they are different. So what makes it so appealing to view us as having wildly different brains? Perhaps if we were to accept that we are, on average, similar in intelligence and emotionality, it would mean that we would have to reconsider our stereotypes about men and women.
What are sex chromosomes?
Until the early 20th century, there was no common understanding of how a person’s sex was determined. Scientists did not know why some people’s bodies appeared male and others female. It wasn’t until 1905 that two scientists, one of whom was a woman (Nettie Stevens), separately were credited with discovering human sex chromosomes.
What exactly are sex chromosomes? To begin, it’s important to talk about what a chromosome is. Deoxyribonucleic acid (DNA) is the genetic material that almost all living organisms use to store and pass on information. DNA is typically folded into a more condensed structure called a chromosome, which allows a large amount of DNA to fit into a small area.
Most bacteria have a single circular chromosome because they have smaller amounts of DNA than animals. Animals typically have a number of paired chromosomes. For example, dogs have 39 pairs of chromosomes and cats have 19 pairs. A greater number of chromosomes does not always signify more genes. Although humans have only 23 pairs of chromosomes, compared to dogs’ 39 chromosomes, both humans and dogs have around the same number of genes (20,000).
The majority of humans have 23 pairs of chromosomes, divided into autosomal chromosomes (numbered 1–22) and sex chromosomes (X and Y). The sex chromosomes are called X and Y because of their shapes. When an embryo is created from an egg and sperm meeting, the egg typically contributes an X chromosome and the sperm either an X or Y chromosome. p. 55↵Most people with female bodies have two X chromosomes (XX), and most people with male bodies have one X and one Y chromosome (XY).
Some people have a slight variation in their chromosomes. For example, those with Down syndrome have an extra copy of chromosome 21 (trisomy 21). When the variation is in one of the sex chromosomes (X or Y), the person is considered intersex. One type of variation involving the sex chromosomes is Klinefelter syndrome, where a person has two X chromosomes and a Y chromosome (XXY). People with Klinefelter syndrome appear male but may have less body hair, some breast growth, and infertility issues.
While sex chromosomes take their name from their contribution to sex determination, they actually play a number of roles in other parts of our lives. The X chromosome contains about 1,000 genes, and most have nothing to do with sex. They represent diverse functions spanning from color vision to blood clotting to hair growth. The Y chromosome, on the other hand, contains only about 200 genes and is much smaller than the X chromosome. Because not everyone has a Y chromosome, it does not contain essential genes. However, it is involved in immune system regulation, cancer growth, and heart disease risk.
The functions of the sex chromosomes are not limited to sex determination, and they are also not the only chromosomes that affect this process. Some autosomal chromosomes contribute to sex determination. While the SRY gene on the Y chromosome triggers the cascade that leads to male sex organs, the SOX9 gene on chromosome 17 is also essential, and without it, the body develops as female.
Some people wonder whether there are ways to change the likelihood that certain sperm (carrying either an X or Y chromosome) meet an egg for fertilization. Urban legends suggest that couples can have sex at particular times during the month to increase the likelihood of having an XX or XY baby, with some suggesting that, because Y chromosomes are smaller, p. 56↵sperm carrying them can move faster. Studies using computer-assisted sperm analysis have failed to show any differences in motility of sperm carrying X or Y chromosomes. However, there is evidence that sperm sorting, performed in a lab, may have an accuracy rate of around 75% to 85%. This sorting is typically done by treating sperm with a fluorescent dye. Because X chromosomes have more DNA than Y chromosomes, sperm can be separated based on how bright the dye appears.
Interestingly, while human sex differentiation largely rests on chromosomal variance, not all animals’ sex is determined similarly. The sex of alligators, for example, depends on the temperature of their nest.
What does testosterone do?
The word testosterone evokes images of weightlifters, warriors, and other hypermasculinized figures. We typically associate testosterone with men and masculinity. However, both testosterone and estrogen, which is often considered a female hormone, are present in all of our bodies. These hormones are close enough in structure that a single enzyme, called aromatase, can convert testosterone to estrogen. In addition, each hormone has multiple functions in our bodies outside of sexuality and reproduction. Testosterone, for example, increases the production of red blood cells and contributes to bone density.
Testosterone plays different roles over the course of the lifespan. As early as fetal development, hormones can affect the appearance of our bodies. Those with XY chromosomes typically have a gene on the Y chromosome that signals the growing fetus to create testicles. The testicles then produce testosterone, which leads to the development of external male genitals (penis and scrotum). Some argue that testosterone exposure as a fetus can lead not only to masculinization of the body but also the brain.
In a famous experiment in 1959, researchers found that by castrating male guinea pigs or giving testosterone shots to p. 57↵female guinea pigs early in life, they could alter their sexual behavior. Female guinea pigs given testosterone attempted to mount other guinea pigs, a behavior typical of males, and castrated male guinea pigs positioned themselves to be mounted, a behavior typical of females. It is difficult to say what this change in behavior means exactly. Does it have to do with sexuality, gender, or something else? We have no way to know. What we do know is that early hormone exposure appears to be capable of affecting our brains and not just our bodies.
Despite exaggerated claims to the contrary, research demonstrates that men’s and women’s brains are much more similar than they are different. In fact, it is often impossible to tell the difference between a male and a female brain by inspecting it or by viewing the results of magnetic resonance images or computer tomography scans. However, many researchers believe that there are subtle, average differences between brains of different genders. On average, for example, men tend to perform better on spatial tasks than women do, although one particular woman may perform better than one particular man. Some researchers argue that this average difference is due to early hormone exposure. Autism, described as a disorder by some and a difference in cognition by others, is much more prevalent in males (although it may be underdiagnosed in females). It has been suggested that the increased rate of autism in males is also related to early hormone exposure.
Testosterone’s first role is to act on the developing fetus to guide its physical development and likely also its cognitive development in ways that are still unclear at this time. The next stage at which testosterone acts is puberty. While each individual begins puberty at a slightly different time and proceeds at their own rate, there are some generalities that apply to most people.
In those with testes, testosterone is produced by the testes and also by the adrenal glands, which are found just above the kidneys and manufacture a variety of hormones. The adrenals are best known for producing the stress hormone cortisol. p. 58↵It may be surprising to learn that in those with ovaries, testosterone is produced in the adrenal glands, but also in the ovaries, alongside estrogen. Adult testosterone levels in those with testes typically range from 250 to 900 ng/dL and in those with ovaries, from 15 to 70 ng/dL.
During puberty, testosterone has a number of effects on different areas of the body, including muscle mass, facial structure, body and facial hair, and phallus size. Trans men who take testosterone often experience changes similar to cisgender boys, although effects may be slightly different due to age. One of the first changes most people notice with an increase in testosterone level is acne. Testosterone stimulates oil glands, leading to clogged follicles. For most people, acne is manageable, but for some, it can be serious and leave scarring.
Testosterone also leads to body and facial hair growth, causing thinner, finer hair to develop into coarser, thicker hair. A person’s ability to grow a beard is largely dependent on genetics. For many people going through puberty, facial and body hair growth are wanted changes, but for others, they are not. Hair changes induced by testosterone are permanent. Even with a drastic reduction of testosterone, be it from age or medical intervention, most people whose hormones played a part in their hair growth will keep these changes throughout their lifetimes. Electrolysis can be effective in removing unwanted typical “male-pattern” facial and body hair, a process some transgender women pursue if accessible.
Receding hairlines are also linked to testosterone. Specifically, testosterone is converted to another hormone called DHT, which can lead to hair loss. The way that each person’s hair responds to DHT has to do with their genetics. Trans men who start testosterone in their late 20s or 30s can sometimes begin to have balding soon into their transition, at the age when others in their family began to go bald. In addition, this kind of hair loss is irreversible, so trans women who have already begun balding prior to physically transitioning cannot regain hair that has been lost.
p. 59↵Another irreversible effect of testosterone is deepening of the voice. Testosterone leads to thickening and lengthening of the vocal cords, which do not return to their original state if testosterone is stopped. The Adam’s apple common in cisgender men is cartilage growth induced by testosterone to protect the vocal cords. Trans women whose voices have already dropped often utilize speech therapy to adjust their pitch. Trans men who take testosterone undergo a deepening of their voices but may also use voice training to sound more masculine, as gendered speech does not depend solely on pitch, but also involves aspects such as word choice, intonation, and volume.
While there are clear physical changes with testosterone during puberty or when taken as an adult, there is debate about testosterone’s effects on cognition and behavior. There does seem to be clear evidence that testosterone can increase sex drive, so much so that it is prescribed to cisgender women for low libido. However, testosterone itself does not wholly determine sex drive. Cisgender men with depression who have testosterone levels within normal range can still have very low sex drives due to their mental health symptoms. Humans are complicated, and so is their expression of their sexuality.
Another complex human behavior is aggression, which is often linked in our minds to testosterone. However, studies attempting to link testosterone and aggression have produced extremely varied results. What we do know is that it is not simply a matter of increased testosterone levels leading to increased aggression. Like sexuality, aggression is situation-dependent and revolves largely around social circumstances and interactions.
Recently, magazines and TV channels directed at cisgender men have begun to advertise treatment for “low T,” or low testosterone, also known as hypogonadism. These commercials suggest that vague symptoms like fatigue and weight gain may be related to insufficient testosterone levels. While p. 60↵there are men who experience this phenomenon, the majority of cases involving these kinds of symptoms turn out to be something else, such as depression. For those with diagnosed hypogonadism, testosterone supplementation can be beneficial. However, there are risks to taking extra testosterone if it is not needed, including possible heart problems. While there is a natural decrease in testosterone levels with aging, most men do not report significant issues and do not require supplementation.
What does estrogen do?
Like testosterone, estrogen has been dubbed a “sex hormone.” It is thought of as being relevant only for female bodies but is present in everyone and has multiple functions outside of sexuality and reproduction. Among other things, estrogen contributes to bone health and also increases “good” (high-density lipoprotein) cholesterol and decreases “bad” (low-density lipoprotein) cholesterol.
Unlike the testes, which produce testosterone in utero, the ovaries do not manufacture estrogen while the fetus is developing. Instead, they remain quiet until puberty. After puberty, in those with ovaries, estrogen is mainly produced there. However, in both those with and without ovaries, estrogen is also synthesized in many other areas of the body, including the liver, pancreas, bone, and adrenal glands. Additionally, the body can convert testosterone to estrogen through the actions of the enzyme aromatase.
Average estrogen levels in cisgender men are typically between 10 to 40 pg/mL. Levels in cisgender women vary significantly throughout the menstrual cycle, but range from approximately 25 to 450 pg/mL. After menopause, levels can drop to less than 40 pg/mL.
Estrogen is the primary hormone driving pubertal changes in cisgender women. Transgender women may also take estrogen, sometimes with a testosterone blocker (often p. 61↵spironolactone). As estrogen levels rise, a number of changes occur, including breast development and body fat redistribution to a more traditionally feminine body shape.
Trans women who take estrogen as adults after undergoing male puberty typically experience similar changes in body fat and breast growth. They may also notice decreased muscle mass and softer, less oily skin. However, estrogen does not reverse all of the effects that testosterone had earlier in life. Irreversible changes from testosterone include deepening of the voice, body and facial hair growth, and changes in facial structure. Estrogen also does not cause regrowth of hair that was lost to male pattern baldness.
Menopause is often a time when the topic of estrogen comes up. In their late 40s or early 50s, people with ovaries may begin to experience changes in the patterns of their periods, hot flashes, mood swings, vaginal dryness, and other symptoms. The eggs in the ovaries age and decrease in number, which leads to fluctuations in hormone levels as the feedback loops between the ovaries and the body’s hormones are thrown out of balance. Menopause is not a straightforward process of gradual decreases in estrogen levels. In fact, for a few years, estrogen levels may intermittently be 20% to 30% higher than they were previously. Because estrogen contributes to bone health, once through menopause, there is a higher risk of developing osteoporosis and bone fractures. Heart disease also increases, although there is debate about the role of estrogen in this process.
Estrogen can sometimes come up in conversation related to breast cancer. There are certain types of breast cancer that are considered estrogen receptor–positive. In these cases, the cancer cells have estrogen receptors that allow them to grow in response to estrogen. Treatment for estrogen receptor–positive breast cancers may include a selective estrogen receptor modulator such as tamoxifen.
p. 62Why do women live longer than men?
On average, across the world, women live about three years longer than men. Somewhat counterintuitively, women outlive men despite social inequality, unequal pay, higher rates of abuse, and increased levels of depression and anxiety. This gender gap in longevity varies from less than 1 year in India, to 6 years in the United States, to a staggering 12 years in Russia. However, it is consistent and well documented. And it is not completely clear what drives it.
The fact that the gap is wider in certain countries suggests that there are social and cultural influences involved. In Russia, for instance, alcohol and tobacco use are thought to have a significant impact on men’s health and longevity. Throughout the world, substance abuse is more common in men. Men are also less likely to be adherent with medical treatments and more likely to die from accidents and homicide. It is more socially acceptable in most cultures for men to take risks; risk-taking is built into gender roles and ideas about masculinity. Although women are more likely to attempt suicide, men are more likely to die by suicide, as they tend to choose more lethal means, such as firearms.
Research has also focused on possible biological influences on the gender gap in longevity. Those with XY chromosomes appear to have a disadvantage from the very beginning. XY infants have an over 25% greater chance of dying in the first year of life than XX infants. Some have argued that the structure of our chromosomes affects our survival. While most women have two copies of the X chromosome, which is large (approximately 1,000 identified genes) and performs many different functions important to the body, most men have one X chromosome and one Y chromosome, a smaller piece of DNA with only around 200 identified genes. Having two copies of the X chromosome may be beneficial for women because there is a “backup” copy available, providing extra protection against harmful mutations.
p. 63↵Scientists have also suggested that the hormones estrogen and testosterone may affect longevity. Testosterone has been shown to increase “bad” cholesterol and decrease “good” cholesterol, while estrogen seems to do the opposite. Men are more likely to die early from heart disease, which is linked to both biological and lifestyle factors. There is evidence that eunuchs—those people who, in certain cultures, had their testicles removed—lived longer than other men. Animal studies also appear to show that female animals that have their ovaries removed do not live as long as other female animals.
Finally, there are simpler explanations that have been proposed for the difference between male and female longevity. Taller, bigger people (who more often tend to be men) have more cells in their bodies, making them more susceptible to cell mutations, which can lead to cancer. Bigger people also use more energy and burn more calories, putting more wear and tear on their bodies.
A recent global study of 101 mammal species found that, on average, females lived 18.6% longer than males from the same species, a difference much larger than that between human men and women, which hovers around 8%. Researchers propose that this disparity is likely due to environmental conditions, sex-specific genetic variations, and the way in which different sexes utilize their resources. For instance, in the case of bighorn sheep, when natural resources were consistently available, there was little difference in lifespan between males and females. However, in areas with extreme weather conditions, males were found to live much shorter lives, likely because they depleted their resources through sexual competition and the growth of larger body masses.
Whatever the reason for the gap in life expectancy for women and men, it is clear from the wide variation across the world that lifestyle matters. Which means that all of us, no matter our sex or gender, have the possibility of living longer if we lead healthier lives.
p. 64What do we know about sex and gender in nonhuman species?
Have you ever gone to a pond and watched the ducks? What is most striking about them? The mallard is one of the most common species of duck in the world and is found throughout Asia, Europe, and North America. Looking out onto a pond of mallards, half of the ducks (the males) have beautiful green heads, while the other half (the females) are various shades of dull brown.
What about us? How different are men’s and women’s bodies? Men are generally taller, stronger, and more hairy. Women have larger chests and men have larger external genitals. But there are men who are frequently mistaken for women and vice versa.
The amount of difference between males and females in a species is called sexual dimorphism, and many species are much more sexually dimorphic than we are. One of the best-known examples of sexual dimorphism is the peacock and peahen. While the peahen is certainly beautiful, the peacock’s elaborate plumage can be almost breathtaking.
Other examples of sexual dimorphism include species in which males and females differ significantly in size. Mandrills, primates living in tropical rainforests in Africa, can be distinguished by both their size and coloring. Males typically weigh between 40 and 80 pounds, and females, between 20 and 30 pounds. On the most extreme end, the female triplewart seadevil fish is approximately 25 times as big as the male, measuring 12 inches, while the male is less than half an inch.
Sexual dimorphism can develop due to a number of factors. Females of a species may be larger because this allows them to produce more offspring and provide better parental care. In species where there is strong competition by males for females, males often develop characteristics that distinguish themselves as attractive or healthy and, over time through evolution, can come to look very different from females of the same species.
p. 65↵There are also species in which the two sexes are more similar than humans in certain ways. The female spotted hyena, for example, has a clitoris that is as long as the male’s penis. For intercourse to occur, the male must enter the female through her retracted clitoris. Female spotted hyenas also urinate and give birth through this same organ.
In some species, there is no division between male and female, and instead, all members of the species have both male and female genitals. Banana slugs, named for their bright yellow color, are hermaphroditic. They each have a penis on their heads, and when mating they simultaneously enter each other to deliver sperm, then find a hiding spot and lay their eggs.
Some animals do have a sex, but this sex can change over the course of their lifetimes. In the female-dominated world of the clownfish, each clownfish is born with the ability to become male or female and starts off male. Schools of all-male clownfish are run by a female leader, along with her submissive male partner. When the female lays eggs, her male partner guards them until they hatch. When she dies, her male partner physically transforms into female and takes over as leader of the school. Another type of fish, called wrasses, experiences almost the opposite. When the male leader of a school dies, a female takes over, growing male sex organs.
Sex changes in fish species reveal that there is a social hierarchy and specific sex roles. Humans are no different. We have expectations based on sex that determine how someone should behave, what kinds of roles they can play in families and work worlds, and who they should have sex with.
When humans act in stereotypical ways, such as men being promiscuous, we often point to nonhuman species, arguing that we can’t be blamed for behaviors that are part of our “nature.” Similarly, we make judgments about behaviors we find sinful or offensive, such as homosexuality, and use animals’ lack of that behavior to contend that it is “unnatural.” However, a quick survey of the animal kingdom reveals that p. 66↵animals have diverse and interesting gender roles and sexual appetites.
Although males are typically thought of as more aggressive and therefore better at hunting, female lions (lionesses) catch more prey than male lions. While we think of it as “normal” for animal males to physically fight over females, certain species of female antelopes lock horns in battle over their desired males.
Male members of certain animal groups have also been known to take on what we think of as traditionally female activities. Male penguins stay at home and sit on their eggs while female penguins hunt for fish to feed their offspring. During this time, the male penguins fast, and if their chicks are born before the mother returns, the father’s body produces and regurgitates a curdlike substance to feed the baby. In some species, fathers are even the ones to become pregnant. When seahorse males mate with seahorse females, the females insert their eggs through a tube into the males’ abdominal pouches, where sperm and egg meet. The males then carry the embryos until they are ready to enter the world and care for themselves.
When it comes to sexuality, it would be very difficult for us to take our cues from animals. While there are certainly many species where heterosexuality is the norm, there are also many that have unique ways of interacting sexually. Bonobos, known for their extremely sexual lives, are bisexual, and most of their sex is not for reproduction. Homosexuality is found in numerous species, including Japanese macaques and bottlenose dolphins. A small percentage of domestic sheep have been found to prefer homosexual relationships over heterosexual relationships, even when members of the other sex are available for mating.
Sexuality is complicated and does not simply reflect the sex of an animal’s partners. Another dimension of sexuality is the propensity to mate with one partner (monogamy) versus multiple partners (polygamy). Monogamous animals include beavers, otters, wolves, barn owls, bald eagles, and more. Polygamy is much more common than monogamy. In most p. 67↵polygamous species, the males are more promiscuous than the females, but this is not always true. Honey bees and mole rats both have social systems where a female queen has liaisons with males, but most of the rest of the colony is made up of workers who do not participate in sexual activity at all. Like other animals, humans are unique and possess our own distinctive ways of interacting sexually.
