Gender Differences in the Animal Kingdom

POV Travel
Jul 27
9 min read

Updated: Jul 28

There is a fish on a coral reef living in a small group. The largest individual is female. When she dies, the largest male changes sex, becomes female, and takes her place. This happens routinely. It is not an aberration. It is how the species works.

Somewhere else, a female seahorse deposits eggs into a pouch on the male's abdomen, and he carries them, nourishes them and gives birth to live young.

In a hyena clan, every female outranks every male. In an elephant herd, an old matriarch holds the knowledge on which the survival of the group depends. Among orcas, females live decades beyond their reproductive years, and their sons follow them.

And in a great many species, there is no meaningful difference at all.

Almost everything most people assume about males and females in nature turns out to be a description of a few familiar mammals, generalised far beyond the evidence. The reality is stranger, more various, and considerably more interesting.

Gender differences in the animal kingdom

Quick Answer

Biologists generally use the word sex for the biological trait and reserve gender for the social dimension in humans, so what follows is about sex differences in animals.

Those differences are enormous but they are not uniform. How sex is determined varies wildly across species, by chromosomes, by temperature, by social circumstance. Some animals change sex during their lives. Some have both sets of organs.

The familiar pattern of the large competitive male and the caring female is one strategy among many. Sex roles are frequently reversed, and many species show almost no difference between sexes at all. Nature offers no single template.

Sex is not even determined the same way twice

Start with something most people never learn, which quietly dismantles the assumption that any of this is fixed.

In mammals, including us, sex is generally determined by chromosomes, with males carrying two different ones and females two of a kind. It feels fundamental. It is not even the only chromosomal system.

In birds, the arrangement is inverted. It is the female that carries two different sex chromosomes and the male that carries a matched pair. Whatever biological fundamentals people believe they are appealing to, birds do it the other way around.

In many reptiles, including crocodiles and most turtles, chromosomes have nothing to do with it. Sex is determined by the temperature at which the egg incubates. A clutch of turtle eggs in warm sand and a clutch in cool sand produce different sexes. There is no genetic difference between them. The nest decides.

Some fish determine sex socially, according to the composition of the group they live in. Some invertebrates have systems stranger still, where sex depends on whether an egg was fertilised at all.

So the very first thing to understand is that there is no universal mechanism. Sex is a biological outcome that evolution has arrived at through many independent and incompatible routes. What looks like the deep structure of life turns out to be a family of local solutions.

The animals that change

This is the part that reliably astonishes people, and it is entirely ordinary in the sea.

Sequential hermaphroditism means changing sex during a lifetime. In some fish, individuals begin as males and become female, usually when they grow large enough that being female pays better, because a bigger body produces far more eggs. In others, the change runs the other way, males becoming female or females becoming male depending on the social structure of the group.

In the anemonefish, the arrangement is neat and famous. A breeding pair lives with smaller non breeding fish. The female is the largest. If she dies, the breeding male changes sex and becomes the new female, and the largest of the remaining fish matures into the breeding male. The hierarchy simply shifts up a place.

Some species are simultaneous hermaphrodites, carrying functional male and female organs at the same time. Many snails, slugs, earthworms and flatworms live this way, and it makes sense for slow moving creatures that may not meet many partners. Any two individuals can mate.

None of this is rare or marginal. It is a widespread, successful, unremarkable feature of life on this planet, and it has been for a very long time.

When the roles reverse

The familiar story runs that males compete and display while females choose and care for young. It is a real pattern with a real explanation, and it is far from universal.

In seahorses and pipefish, the male becomes pregnant. He receives eggs, carries them, provisions them and delivers live young.

In several wading birds, including the jacanas, the female is larger, more brightly coloured and more aggressive. She holds a territory containing several males, competes with other females for access to them, lays clutches for each, and departs. The males incubate the eggs and raise the chicks alone.

Among phalaropes, small elegant shorebirds, the female is the colourful one, and it is the male who sits on the nest, drab and inconspicuous, because the parent who stays is the parent who must not be seen.

The underlying logic, which biologists have worked out with some care, is that ornament and competition tend to appear in whichever sex invests less in offspring and therefore competes for access to the sex that invests more. In most species that means males compete. Where males take on the caring role, the pattern flips, exactly as the theory predicts. Nature is not being whimsical. It is being consistent, in a way that produces radically different appearances.

Where females rule

The assumption that males dominate is a generalisation drawn from a handful of species, and it does not travel well.

Spotted hyena clans are led by females, who are larger than males and outrank all of them. The lowest ranking adult female is socially above the highest ranking male.

Elephant societies are matriarchal. Herds are led by an old female whose memory of water sources, routes and dangers, accumulated over decades, materially determines whether the group survives a drought. When she dies, the knowledge dies with her, which is one reason poaching old individuals is more damaging than the number of deaths suggests.

Orca pods are organised around females, and mothers remain associated with their sons for life, with evidence that a son's survival depends on his mother's presence. Killer whales are among the very few animals besides humans in which females live long past the end of reproduction, and the leading explanation is that an old female's knowledge and support raise the survival of her descendants.

Bonobos, among our closest relatives, live in societies where females form coalitions and hold considerable social power, in marked contrast to chimpanzees, our equally close relatives, who do not.

Two species, both about equally related to us, organised in opposite ways. Whatever we are inclined to conclude about nature and hierarchy, we can find a primate to agree with us. That should make anyone cautious.

And often there is no difference at all

The dramatic cases attract attention, but the quiet fact is that in a very large number of species the sexes look and behave alike.

In many birds, particularly those where both parents feed the chicks and neither competes for multiple partners, males and females are nearly indistinguishable to the human eye. In many fish, reptiles, insects and mammals, the differences are minor or invisible.

Where strong sexual dimorphism does exist, it is generally a signal that something specific is going on: intense competition between one sex, or mate choice by the other, or a division of parental labour. Its absence tells you something too.

The lesson is not that difference does not exist. It plainly does, and sometimes spectacularly, in antlers and manes and tail feathers and enormous size disparities. The lesson is that the differences are consequences of particular ecological and reproductive circumstances, not expressions of some universal principle that runs through all of life.

The mistake worth avoiding

Here is where we should be careful, because this subject is regularly conscripted into arguments it cannot settle.

There is a persistent habit of reaching into the animal kingdom for evidence about how human beings ought to live. It is done from every direction. Point at lions for one conclusion, at hyenas for the opposite. Point at chimpanzees, or at bonobos, and reach contradictory answers with equal confidence.

The reason this always works is that the animal kingdom contains millions of species with wildly different arrangements, and you can find an example supporting almost any position you already hold. That is not evidence. That is a mirror.

Biology describes what is. It does not prescribe what ought to be, and this holds regardless of which conclusion someone is reaching for. That a seahorse gestates does not obligate anybody. That a stag fights does not excuse anybody. The is and the ought are separate questions, and philosophers have been pointing this out for a very long time without much success.

What the diversity of animal sex does establish, firmly, is a negative claim, and it is the useful one. Nobody can honestly say that nature mandates a single arrangement, because nature demonstrably does not. Nature has tried nearly everything, repeatedly, and much of it works.

Why we find this wonderful

Our whole approach to wildlife is built on seeking the creatures that do not fit the story. The crocodile in the desert. The elephant that mines caves in the dark. The dolphin hundreds of miles from any sea.

Sex in the animal kingdom is the same discovery in a different register. You arrive expecting a simple binary rule inherited from a few mammals, and you find temperature dependent turtles, pregnant male seahorses, sex changing reef fish, matriarchal elephants, hyena clans where the smallest female outranks the largest male, and a hundred million years of evolutionary experimentation that never settled on one answer.

Every one of those creatures is an argument against the tidiness of our assumptions. That is precisely why they are worth travelling to see, and precisely why they are worth thinking about carefully rather than recruiting into a position.

The natural world is not a moral instruction manual. It is far better than that. It is a demonstration of how many ways there are to be alive.

How POV Travel approaches this

We take small groups to watch wild animals behave as they actually behave, which is rarely how the documentaries have led people to expect.

When we track the Iberian lynx in Andalucía, or watch pink river dolphins in the Amazon, we talk about the biology honestly, including the parts that complicate the picture. We think travellers deserve the real science, with its uncertainties intact, rather than a story simplified until it stops being true.

We also think a wildlife encounter should change how you think, not confirm what you already thought. The animal in front of you is not a symbol, a lesson or an argument. It is a creature with its own way of being alive, arrived at over millions of years, entirely indifferent to what we would like it to represent.

We question. We teach. We give you the evidence and we leave the conclusions to you, on this subject perhaps more carefully than on any other, because it is the one where people most want the animals to agree with them.

The animals are not listening. They are busy being far stranger than we imagined.

Frequently Asked Questions

What is the difference between sex and gender in animals?

Biologists generally use sex for the biological trait, determined by chromosomes, temperature or social circumstance depending on the species. Gender is usually reserved for social and cultural dimensions in humans, so discussion of animals concerns sex.

How is sex determined in animals?

In many ways. Mammals use chromosomes, with males carrying two different ones. In birds this is reversed. In many reptiles, incubation temperature decides. In some fish, social circumstances determine sex.

Which animals change sex?

Many fish, including anemonefish and various wrasses, change sex during their lives depending on size or social structure. Snails, slugs and earthworms are often simultaneous hermaphrodites, carrying both sets of organs at once.

Do males always dominate in nature?

No. Spotted hyena clans are led by females who outrank all males. Elephant herds are matriarchal. Orca pods are organised around long lived females. In jacanas and phalaropes, females compete and males raise the young.

Does animal behaviour tell us how humans should live?

No. The animal kingdom contains such diversity that almost any human position can find an example to support it, which makes it useless as evidence. Biology describes what exists, it does not prescribe what people ought to do.

See the survivors for yourself

Some animals cling on against every odd. The Iberian lynx, the rarest cat in the world, hunting again in Andalucía after coming within a whisker of vanishing forever. The cave mining elephants of Mount Elgon, feeling their way through the dark. To seek them is to witness the stubbornness of life itself, in small groups, treading lightly, on the animals' terms.

Explore the expeditions: Rare Wildlife Encounters →