Most biologists reserve the word “gender” for the culturally constructed roles, identities, and expectations that human societies layer on top of biological sex, while “sex” refers to the reproductive categories shaped by chromosomes, hormones, and anatomy. By that conventional split, animals have biological sex but not gender in the human sense. The reality, though, is messier than that clean divide suggests. Animals display an extraordinary range of sex-determination systems, reproductive strategies, and social behaviors that blur the line between “just biology” and something richer, and researchers have spent decades debating how much of what we observe in animal societies is filtered through our own cultural assumptions about males and females.
Why the Distinction Between Sex and Gender Matters Here
When people ask whether animals have genders, they are usually asking one of two things. The first is a definitional question: does the concept of gender apply outside human culture? The second is an empirical one: do animals behave in ways that go beyond what their reproductive anatomy would predict? The definitional question is genuinely unsettled. The ethological literature has long linked evolutionarily adaptive behaviors to mating strategies and what researchers sometimes call a “gendered division of work,” but three persistent concerns shadow that framing: scientists may project cultural assumptions about gender onto nonhuman animals, they may project observations about animal behavior back onto complex human cultural phenomena, and they may treat sex and sexuality as binary and stable when they are neither.1Wiley Online Library. Gender, Ethology, and Evolution In other words, even calling a lioness “female” and ascribing her a predictable set of behaviors involves interpretive choices that are not purely objective.
The empirical question is easier to address with data. Animals show sex-role reversals, same-sex pair bonds, alternative mating tactics that look nothing like the “standard” male or female strategy, and even the ability to change reproductive sex mid-life. Whether you call any of that “gender” depends on your definitions. But the biology alone is far more varied than most people assume.
Biological Sex in Animals Is Not One System
Humans tend to think of sex determination as straightforward: XX chromosomes produce females, XY produces males. But that is just one mechanism among many, and it is not even universal among mammals in a simple sense. Across the animal kingdom, the mechanisms that determine sex are remarkably diverse and evolve rapidly.2PubMed Central. Sex determination: why so many ways of doing it?
In birds, the system is reversed: males carry two identical sex chromosomes (ZZ) and females carry two different ones (ZW). In fruit flies, sex depends on the ratio of X chromosomes to other chromosomes. In the insect order that includes ants, bees, and wasps, males develop from unfertilized eggs and are haploid (carrying one set of chromosomes), while females develop from fertilized eggs and are diploid. The nematode worm C. elegans has hermaphrodites and males but no “females” in the conventional sense.3PubMed Central. Molecular patterns of sex determination in the animal kingdom: a comparative study of the biology of reproduction These are not exotic edge cases. They represent the dominant reproductive strategies for vast groups of species.
Then there are animals whose sex is not genetically fixed at all. Many reptiles have no discernible sex-determining genes. Instead, the temperature of the environment during a critical window of egg incubation determines whether offspring develop as male or female.4PubMed. The ecology and evolution of temperature-dependent reaction norms for sex determination in reptiles: a mechanistic conceptual model In painted turtles, for instance, the annual offspring sex ratio is closely tied to mean July air temperature. Even a modest temperature increase of less than 2°C could drastically skew the sex ratio, and a 4°C increase would effectively eliminate the production of male offspring.5PubMed Central. Climate change and temperature-dependent sex determination in reptiles Wider temperature fluctuations of about ±6°C around normal male-producing incubation profiles can feminize all embryos, turning what would have been a clutch of males entirely female.6Scientific Reports. Extreme thermal fluctuations from climate change unexpectedly accelerate demographic collapse of vertebrates with temperature-dependent sex determination
The diversity of sex determination systems already challenges any tidy binary. If the very mechanism that makes an animal male or female can be chromosomal in one lineage, temperature-driven in another, and dependent on fertilization status in a third, “biological sex” is clearly not one thing across the animal kingdom.
Animals That Change Sex
Some fish do not just have a flexible sex-determination system at conception; they change their functional reproductive sex as adults. Clownfish are the best-known example. They are protandrous hermaphrodites, meaning they begin life as males and can transition to female. In a clownfish group, the dominant individual is female. If she dies or is removed, the largest male undergoes a complete sex change, restructuring his gonads and altering his behavior to take over the female role.7PubMed Central. Sex Change in Clownfish: Molecular Insights from Transcriptome Analysis This transformation is triggered by social cues rather than any genetic switch, and recent research shows it involves extensive reorganization of the forebrain, not just the reproductive organs.8PubMed Central. Adult sex change leads to extensive forebrain reorganization in clownfish
Other fish species go the opposite direction, starting female and becoming male (protogyny), while some can change sex in either direction depending on social circumstances. These are not rare curiosities. Sequential hermaphroditism is a widespread reproductive strategy among reef-dwelling fish.7PubMed Central. Sex Change in Clownfish: Molecular Insights from Transcriptome Analysis The fact that a socially triggered cue can cause an adult animal to completely overhaul its reproductive biology and brain structure blurs any strict boundary between “social role” and “biological sex.”
Sex-Role Reversals
Even in species with stable male and female categories, the behavioral roles associated with each sex are not always what textbooks predict. The classic model in sexual selection holds that females are choosy and invest heavily in parental care, while males are competitive and promiscuous. This stereotype persists in the research literature despite decades of evidence that it is frequently wrong.9Animal Behaviour. Let’s talk about sex roles: what affects perceptions of sex differences in animal behaviour?
In several bird and fish species, the conventional script is flipped. Female coucals, for example, compete intensely for access to males, and males provide most or all of the parental care. In the classically polyandrous black coucal, a female maintains a harem of several males.10Ethology. The evolution of reversed sex roles and classical polyandry: Insights from coucals and other animals Research on two sympatric coucal species in East Africa found that females of both species had higher variation in reproductive success than males did, and that sexual selection pressure was stronger on females than on males. For female black coucals, a strongly male-biased adult sex ratio combined with male-only parental care allowed them to boost their reproductive success by mating with multiple partners.11Proceedings of the Royal Society B: Biological Sciences. Sex-role reversal and the Bateman gradient in coucals—females benefit from mating with multiple partners
These reversals are not anomalies explained away by special circumstances. They show that the behavioral patterns we associate with being “male” or “female” are shaped by ecological pressures like sex ratio, predation risk, and the costs of parental care, not locked in by sex itself.
Alternative Male Strategies and Female Mimicry
Within a single species, males sometimes adopt entirely different reproductive strategies that are so divergent they barely resemble one another. Bluegill sunfish offer a vivid example. The species has three distinct male tactics: “parentals” delay maturation and eventually court females and guard nests; “sneakers” mature early and dart in to release sperm during a parental male’s spawning; and “satellites” mature early and mimic females in appearance and behavior to get close to spawning pairs and steal fertilizations.12Behavioral Ecology. Sperm investment and alternative mating tactics in bluegill sunfish (Lepomis macrochirus)
The satellite male is especially interesting for this discussion: it is genetically and reproductively male, but it looks and acts like a female to fool other males. Similar sneaker-versus-dominant dynamics show up in cephalopods and other fish, and modeling work suggests that the relative success of each strategy shifts with ecological conditions like predation pressure.13bioRxiv. A Theoretical Comparison of Alternative Male Mating Strategies in Cephalopods and Fishes In practical terms, a male bluegill satellite inhabits a behavioral and morphological “role” that humans might casually describe as feminine, even though its biological sex is unambiguously male. Whether that counts as a gendered behavior is a philosophical question, but the biology is undeniable.
Same-Sex Pair Bonds and Sexual Behavior
Same-sex sexual behavior has been documented in hundreds of animal species, and in some cases it involves long-term pair bonding with cooperative offspring-rearing. On the Hawaiian island of Oahu, about 31% of Laysan albatross pairs were found to be female-female, a pattern driven by a female-biased sex ratio of roughly 59% females. These female pairs fledged fewer chicks than male-female pairs but performed significantly better than the alternative of not breeding at all. In most female-female pairs that raised chicks over multiple years, at least one offspring was genetically related to each female, meaning both had managed to reproduce.14PubMed Central. Successful same-sex pairing in Laysan albatross
Follow-up research showed that females in same-sex pairs produced about 80% fewer chicks and had lower survival than those in male-female pairs, but same-sex pairing was still a better strategy than not breeding. Interestingly, females in same-sex pairs that successfully raised a chick sometimes acquired a male mate the following year, while those in failed same-sex pairs never did, hinting that males may assess female quality and that lower-quality females end up in same-sex pairings as an adaptive fallback.15PubMed Central. Adaptive value of same-sex pairing in Laysan albatross
In bonobos, one of our closest living relatives, same-sex sexual behavior among females is not a fallback but a central feature of social life. About 65% of observed sexual events in one wild bonobo community consisted of female-female genito-genital rubbing. Females showed greater increases in urinary oxytocin after these interactions than after copulations with males, and the frequency of same-sex sexual contact positively predicted how often unrelated females supported each other in coalitions.16PubMed. The cooperative sex: Sexual interactions among female bonobos are linked to increases in oxytocin, proximity and coalitions Same-sex sexual behavior among bonobo females appears to have become an engine for cooperation outside of kinship and pair bonds. Calling it “just biological sex” feels reductive when the behavior is socially mediated, hormonally reinforced, and functionally tied to alliance-building.
When Female Animals Look Male
Sometimes an animal’s outward appearance diverges sharply from what its sex would predict. Female spotted hyenas are famously “masculinized”: they are larger than males, socially dominant, and possess an elongated clitoris that closely resembles a male penis. Research suggests that exposure to sex steroids during fetal development contributes to this unusual anatomy and social dominance, producing overt physical traits not shared by other hyena species.17Endocrinology. Phylogenetic Comparisons Implicate Sex Hormone-Binding Globulin in “Masculinization” of the Female Spotted Hyena (Crocuta crocuta)
In lions, cases of phenotypic masculinization are rarer but have been documented. A study of six captive lionesses in South Africa found that these animals were phenotypically female throughout most of their lives until, as older adults, they began developing manes, enlarged clitorises, and increased restlessness and aggression. Pathological examination pointed to ovarian changes as the likely cause. Similar transformations have been reported very rarely in free-ranging lionesses.18Theriogenology Wild. A case for ovarian transdifferentiation in six adult captive masculinized lionesses (Panthera leo) in South Africa: Pathological evidence These cases show that even in mammals with well-established chromosomal sex determination, the expression of sex-linked traits can shift during an animal’s lifetime.
Gynandromorphs and Mosaics
Some animals are not neatly male or female in any part of their body. Gynandromorphs are organisms in which different body sections are genetically different sexes. The most striking examples are bilateral gynandromorphs, which are literally half male and half female, split down the midline. This phenomenon has been documented in butterflies, birds, crustaceans, and other groups. Developmental network theory explains the embryogenesis of these variations through the way sex-determination pathways interact during early cell divisions.19arXiv. A Developmental Network Theory of Gynandromorphs, Sexual Dimorphism and Species Formation
Gynandromorphs are not common, but they are important because they demonstrate that sex can vary at the level of individual cells and tissues within a single body. A bilateral gynandromorph cardinal, for instance, has the bright red plumage of a male on one side and the muted brown of a female on the other. Its two halves may even have different chromosomal complements. This is sex as a mosaic property, not a whole-organism binary.
Caste, Reproduction, and Social Roles in Insects
Eusocial insects like ants, bees, and termites add yet another layer to the question. In these species, most colony members share the same genetic background but occupy profoundly different social roles. A queen and a worker honeybee may be genetically identical sisters, yet the queen is a reproductively active individual with a fully developed ovary, while the worker is effectively sterile and spends her life foraging, building, or defending the nest. The difference between them is driven not by distinct genes but by transcriptional differences, shaped heavily by nutrition, pheromones, and epigenetic modifications during development.20PubMed Central. (Epi)Genetic Mechanisms Underlying the Evolutionary Success of Eusocial Insects In some species, individuals can even switch castes in response to environmental cues, moving from a non-reproductive to a reproductive state.
Calling these social roles “genders” would stretch the term beyond its conventional use. But the underlying point stands: social role, reproductive function, and behavior can diverge sharply from genetic sex in the animal kingdom, and the divergence is sometimes governed by environmental and social context rather than chromosomes.
The Anthropomorphism Problem
One of the most persistent criticisms in animal behavior research is that scientists unconsciously project human gender norms onto the animals they study. The “sex roles” framework in sexual selection, which casts females as choosy caregivers and males as competitive seekers, has faced scrutiny for decades but remains pervasive. Research has examined whether the demographic traits and personal experiences of scientists themselves influence how they perceive sex differences in animals, raising the possibility that stereotyped sex roles persist in the literature partly because of the human observers documenting them.9Animal Behaviour. Let’s talk about sex roles: what affects perceptions of sex differences in animal behaviour?
This matters because when we ask “do animals have gender,” we are simultaneously asking whether the concept of gender is useful for describing animal behavior and whether we have been inadvertently applying it all along. The answer to the second question is almost certainly yes. Descriptions of animal behavior have historically borrowed heavily from human social categories, and disentangling biological observation from cultural projection is an ongoing challenge in ethology.1Wiley Online Library. Gender, Ethology, and Evolution
Social Learning Along Sex Lines
Some of the most intriguing evidence for sex-differentiated behavior that goes beyond pure reproduction comes from social learning studies. In capuchin monkeys, immature individuals preferentially watch foragers who are eating rare or hard-to-process foods, and young foragers, especially females, tend to adopt the same foraging techniques as their close associates.21PubMed Central. Social traditions and social learning in capuchin monkeys (Cebus) This is not the same as human gender socialization, where children learn elaborate cultural scripts about masculinity and femininity. But it shows that behavioral traditions can be transmitted along sex-differentiated social networks in at least some nonhuman primates. Females learn from females not because of any genetic mandate but because of who they spend time with. The parallel to human gender learning is loose but suggestive.
Environmental Threats That Scramble Sex
The question of whether animals “have gender” takes on a practical urgency when you consider how environmental contaminants can disrupt sex determination and reproductive behavior. Endocrine-disrupting chemicals in the environment have affected numerous wildlife species, impairing reproduction, brain function, and immune systems. Conservation organizations have flagged behavioral effects as deserving more consideration in environmental risk assessments, since altered behavior, including reproductive and parental behavior, can cascade to population-level consequences.22PubMed Central. Viewpoint: Policy requirements for protecting wildlife from endocrine disruptors
Fish living downstream of wastewater treatment plants have been found with intersex gonads, containing both egg and sperm tissue. Alligators in contaminated Florida lakes showed altered genital development. In species with temperature-dependent sex determination, climate change itself becomes a sex disruptor, skewing entire populations toward one sex as average temperatures shift. When the environment can rewire an animal’s reproductive anatomy and behavior this easily, the line between “biological sex” and “everything else” starts to look thinner than the neat conceptual division implies.
Parthenogenesis and Reproduction Without Males
Some vertebrate species have abandoned males entirely. Several species of lizards, fish, and even sharks can reproduce through parthenogenesis, in which females produce offspring without fertilization by a male. Studying these lineages expands our understanding of the evolution and ecology of sex itself, since they represent natural experiments in what happens when one sex category disappears from a population.23Annual Review of Ecology, Evolution, and Systematics. Evolutionary Dynamics and Consequences of Parthenogenesis in Vertebrates In an all-female species of whiptail lizard, individuals still perform courtship behaviors typically associated with the male role, alternating who plays “male” and who plays “female” during mating season. Calling this “gender” would be a stretch, but the behavioral repertoire of “male” and “female” roles persists even in the total absence of males, suggesting these behavioral programs have a degree of independence from the sex that normally performs them.
Brains, Hormones, and the Mosaic
One reason the sex-versus-gender debate gets complicated in animals is that brains are not as cleanly sexed as gonads. In humans, research on brain structure has shown that most brains are mosaics of features that are statistically more common in one sex or the other, rather than being uniformly “male-type” or “female-type.” Sex category explains only a small part of the variability in brain structure, and sex effects on the brain can even reverse direction under different conditions. These findings have led to a non-binary framework for understanding brain sex, in which mosaic brains exist in a multi-dimensional space that cannot be meaningfully reduced to a male-female binary.
In nonhuman animals, the organizational-activational framework has long been the dominant model for understanding how hormones shape the brain. The original hypothesis, proposed in 1959, held that steroid hormones during early development permanently organize brain circuits into male or female patterns, and then hormones in adulthood activate those circuits.24PubMed Central. Back to the future: The organizational-activational hypothesis adapted to puberty and adolescence But experimental findings over the decades have complicated this picture. Permanent brain effects can occur in adulthood, and the neat division between early “organization” and later “activation” does not always hold.25PubMed. Organizational and activational effects of sex steroids on brain and behavior: a reanalysis Clownfish undergoing social sex change, for instance, restructure their forebrains during adulthood, not during some fixed developmental window. If the brain is a mosaic and its sex-typed features can shift under the right conditions, then even at the neural level, sex is less of a fixed binary than it might seem.