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Gender Differences of Animals

Sex differences are real, biologically based, and here to stay.

In an era of increasing gender fluidity, it is worth asking whether gender differences in psychology had been overstated in the past. One way of assessing this is in terms of gender differences of other species that lack our political agendas.

Size, Strength, and Morphology

Among mammals, males are generally larger, and more aggressive than females. This pattern is related to greater production of testosterone that is also associated with increased aggression.

There are intriguing exceptions including hyenas, and chinchillas, where the females are larger and more socially dominant.

Such exceptions are sometimes related to increased testosterone production by females. This involves activity in the adrenal gland that is the primary source of female testosterone.

Hyenas are particularly interesting because, in addition to being larger than males, females have a pseudo penis superficially indistinguishable from that of males that plays a role in social greeting and dominance relationships.

Size affects behavior and larger individuals have an advantage in establishing social dominance. This means that females of most species defer to males.

In addition to being larger, males generally are the ones who develop bodily signals designed to facilitate reproductive success by attracting females and intimidating rivals. Examples include the antlers of deer and the beards of humans.

Human females are unusual in carrying most of the sexually attractive signaling from youthful facial features to permanently enlarged breasts. This may be because men invest far more in offspring than most other male mammals do so that they are in greater demand as mates. Females thus bear the burden of advertisement.

If males are generally larger, there are other behavioral traits that generally distinguish the sexes.

Aggression and Risk-Taking

Among humans, men are responsible for about nine-tenths of serious crimes of violence, although women are more verbally aggressive and more willing to pick a fight.

In addition to being less involved in dangerous physical aggression, women are generally more risk-averse. They avoid situations that are threatening to life and limb. For example, there are few societies where women participate in warfare.

If aggression can be considered a masculine adaptation for mating competition, risk aversion of females makes sense given their greater investment in children. They may be particularly risk-averse if they are mothers of young children given that their injury, or possible death, would have adverse consequences for their children in terms of survival and social success. These patterns likely reflect the psychological differences of other mammals.

Parental behavior itself is another conspicuous sex difference among mammals although different species vary greatly in the extent of paternal investment in offspring.

Parental Behavior

Sex differences in parental behavior are generally broad-ranging, early-developing, and consequential. As the gender with the largest investment in offspring, females are more strongly disposed to make that investment. Beginning with a larger gamete, the egg, females bear the huge additional costs of gestation and lactation.

Among mammals, as a group, females are more strongly inclined to care for young than males. Indeed, males of many species are hostile to young and may kill them on sight as a way of bringing the mother into reproductive condition earlier. This occurs among several primates, and even in human indigenous societies such as the Ache of Paraguay (1).

There are some intriguing exceptions, such as lion tamarins where males do most of the care of young.

Of course, many men are now the primary caregivers for children. Interestingly, new fathers experience a decline in testosterone levels as they adjust to a nurturing way of life.

Another arena where the reality of gender differences in biology strikes home is in health.


Women and men are each more vulnerable to specific diseases than the other. For example, women are more vulnerable to rheumatoid arthritis whereas men have higher rates of heart disease. To the extent that these differences are a feature of biological differences, they are presumably present in other species.

Such differences are sometimes explainable in terms of varied hormone profiles. For example, testosterone suppresses immune function, which could explain why women are more vulnerable to autoimmune disorders like rheumatoid arthritis.

Women enjoy a significant health advantage over men in developed countries and enjoy life expectancies that are about four to five years longer.

This health advantage is complex and involves more than hormones. Genes are expressed differently in males and females, which can have health consequences.

More important is the fact that women have better health behavior tending to take fewer health risks and take better care of their health. They are also better at avoiding violence and are less likely to die through accidents.

Another intriguing mechanism is the fact that caregivers generally live longer with primate males living as long as females if they care for offspring. When men take the primary role in caring for children, their testosterone level falls, suggesting one mechanism through which the influence of caring on health may be expressed.

Males are generally socially dominant over female mammals but there are exceptions.

Status and Leadership

In general, if females are larger than males, they tend to be socially dominant, as is true of chinchillas and hyenas, for example. This phenomenon is related to social leadership among primates where a single dominant male is a common pattern.

Some mammalian societies are matriarchies, however. This is true of elephants where senior females lead groups across long migrations to food and water. In this case, acquired knowledge is more important than size, strength, or aggression.

This is clearly true of modern societies where women are slowly acquiring an equal position in every area of leadership.


1. Hill, K., and Hurtado, M. (1996). Ache life history. New York: Aldine de Gruyter.

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