Are There Intersex Animals? The Science of Sex Variation

Sex variation is widespread across the animal kingdom, from insects split visually down the middle into male and female halves to fish that change sex partway through life to mammals born with a mixture of male and female reproductive anatomy. The phenomenon goes by different names depending on the species and the mechanism involved, but the short answer is that intersex conditions, broadly defined, have been documented in virtually every major animal group. What makes the topic scientifically rich is the sheer variety of ways it happens and the very different consequences it carries for individual animals and their populations.

What Counts as Intersex in Animals

In humans, “intersex” refers to a range of conditions where reproductive anatomy, chromosomes, or hormones don’t fit neatly into the standard male or female categories. The term is used in animal biology too, but scientists also use more specific labels depending on what they observe. An animal with distinct male tissue on one side of its body and female tissue on the other is called a gynandromorph. An animal with blended or intermediate sexual traits, like a gonad containing both egg-producing and sperm-producing tissue, is more typically called intersex. In practice, these categories blur. A 2023 review in the journal Theory in Biosciences argued that the distinction between gynandromorphism and intersexuality, while useful for description, “is not always possible or sensible,” and that terminology based on what researchers actually observe in the animal’s body is more reliable than terminology tied to any single assumed cause.

1PubMed Central. Descriptive versus causal morphology: gynandromorphism and intersexuality

This matters because sex determination itself works differently across species. In mammals, sex is usually set by chromosomes (XX or XY). In birds, it runs on a different chromosome system (ZZ for males, ZW for females). In many reptiles and some fish, the temperature at which eggs develop can override genetic signals entirely. And in various invertebrates, parasitic infections can flip an animal’s sex after birth. With so many roads leading to sex, there are correspondingly many ways the process can produce something other than a straightforward male or female.

Gynandromorph Chickens and the Bird System

One of the most visually striking examples of sex variation comes from birds. Gynandromorphic chickens are split roughly in half: one side of the body looks male, with larger muscle mass and a rooster’s plumage, while the other side looks female, with smaller features and hen-like feathering. Unlike mammals, where hormones circulating through the bloodstream largely dictate sexual traits across the entire body, bird cells appear to respond to their own chromosomal identity to a significant degree. That is why you can get a bird that is literally half-and-half.

A detailed study of one such chicken found that the male-appearing right side was almost entirely ZZ (the male chromosome complement, at about 96%), while the female-appearing left side contained a mixture of ZZ (77%) and ZW (23%) cells. The right gonad was a functional testis with seminiferous tubules and spermatozoa, while the left gonad was mostly testicular but included a small region of ovarian-like tissue making up less than 10% of its area. The bird’s hormone profile skewed heavily male, with testosterone levels roughly ten times higher than those of a normal control male.

2Oxford Academic. Gonadal and Endocrine Analysis of a Gynandromorphic Chicken

These cases are rare in any given flock, but they have been invaluable for researchers trying to understand how sex works at the cellular level. The chicken gynandromorph essentially showed that in birds, individual cells “know” their own chromosomal sex and develop accordingly, rather than just following hormonal orders from the gonads. That is a fundamentally different arrangement from how things work in most mammals.

Freemartins and Mammalian Intersex

In cattle, the most well-known form of intersex is the freemartin. When a cow carries twins of different sexes, the placentas of the two calves typically fuse early in development, creating shared blood vessels. This allows hormones and cells from the male twin to reach the female twin, disrupting her reproductive development. The result is a genetically female calf (XX) that carries a mixture of XX and XY cell lines and develops masculinized or nonfunctional reproductive organs.

3PubMed Central. Early Diagnostics of Freemartinism in Polish Holstein-Friesian Female Calves

Most freemartin heifers are infertile, and their gonads can show a remarkable degree of sex reversal. Research has documented freemartin gonads developing seminiferous tubule-like structures and typical testicular cell types, despite the animal being chromosomally female.

4Journal of Reproduction and Development. Evidence for Almost Complete Sex-reversal in Bovine Freemartin Gonads: Formation of Seminiferous Tubule-like Structures and Transdifferentiation into Typical Testicular Cell Types

Freemartinism is a big deal in the dairy and beef industries. Farmers with mixed-sex twin pregnancies need to know early whether the heifer will be fertile, because raising an infertile animal is a significant economic loss. Diagnostic approaches include chromosome testing and, more recently, analysis of hormonal markers like anti-Müllerian hormone.

5PubMed. Age-specific analysis of anti-müllerian hormone and liver biochemical parameters in freemartin holstein calves and heifers: A pilot study

Outside of cattle, intersex conditions have been documented in sheep, horses, pigs, dogs, and other domestic mammals. A case study of a hermaphroditic sheep found the animal was chromosomally male (XY) but developed both male and female reproductive structures. The cause was traced to a single nucleotide change in the promoter region of the SRY gene, the gene that normally triggers male development.

6Veterinary and Animal Science. The study of a hermaphroditic sheep caused by a mutation in the promoter of SRY gene

In horses, intersex conditions can be particularly difficult to diagnose without internal examination. Laparoscopy, combined with karyotyping and tissue analysis, has been used to confirm suspected cases.

7Journal of Equine Veterinary Science. The use of laparoscopy to diagnose a case of equine intersex: A case report

Species Where Sex Blurring Is the Norm

Not all sex variation in animals is anomalous. In some species, traits that look intersex by mammalian standards are perfectly normal and even adaptive. Female European moles, for instance, all possess ovotestes: gonads that contain both ovarian and testicular tissue. The ovarian portion is fully functional and allows normal reproduction, while the testicular portion is sterile but produces high levels of testosterone.

8PubMed. The evolution of female mole ovotestes evidences high plasticity of mammalian gonad development

This testosterone production follows a seasonal cycle. In autumn, when the ovarian tissue regresses, the testicular portion of the ovotestis enlarges and testosterone levels climb. Research on Talpa europaea found that female plasma testosterone in autumn was significantly higher than in spring, roughly two and a half times the concentration seen in pregnant or postpartum females.

9PubMed. Testis-like steroidogenesis in the ovotestis of the European mole, Talpa europaea

The elevated testosterone is thought to give female moles a competitive edge in digging and defending tunnel territories outside the breeding season. It is a case where what might look like a disorder in another mammal is, in moles, a specialized adaptation that the entire female population shares.

Spotted hyenas offer another example where female anatomy defies typical mammalian patterns. Female spotted hyenas develop a penile clitoris and fused scrotal-like tissue, making them the only female mammals that lack an external vaginal opening. Research has shown that the formation of this unusual anatomy appears to be largely independent of androgens, meaning it is not simply the result of exposure to male hormones during development, despite the fact that the hyena placenta readily converts maternal hormones to testosterone.

10Cell Press (Trends in Endocrinology & Metabolism). Female spotted hyenas (Crocuta crocuta) are the only female mammals that lack an external vaginal opening

Fish That Change Sex on Purpose

Many fish species take sex variation a step further: they change sex entirely as a normal part of their life cycle. This is called sequential hermaphroditism, and it is especially common on coral reefs. Clownfish are the most famous example. All clownfish are born male, and the dominant individual in a group becomes female. When the female dies, the next-ranking male transitions into a fully functional female, complete with ovarian tissue capable of producing eggs.

11PubMed Central. Sex Change in Clownfish: Molecular Insights from Transcriptome Analysis

The transition is not automatic, though. Removing the dominant female is necessary but not sufficient to trigger the change. A recent study modeling the decision process found that sex change in clownfish requires the convergence of multiple factors: the animal’s structural readiness, its hormonal state, social conditions (such as whether there is an open “slot” for a female), and the right developmental timing. Experimental evidence shows that hormonal manipulation can block the transition even when social conditions are permissive, and stress can delay or prevent the process.

12PubMed. Sex change in clownfish as an ARCH-governed biological decision

Other reef fish go in the opposite direction: species like certain wrasses start female and become male. Some species, like various groupers, are simultaneous hermaphrodites, carrying functional male and female tissue at the same time. For fish, sex is often less of a fixed identity and more of a life-stage strategy shaped by ecology.

Temperature, Reptiles, and Sex Reversal

Many reptile species use temperature during egg incubation rather than chromosomes to determine sex. In some turtles, alligators, and lizards, warmer or cooler nest temperatures push developing embryos toward one sex or the other. When temperatures fall in an intermediate zone, embryos can develop intersex gonads with features of both sexes, and the degree of intersexuality correlates with subtle differences in aromatase activity, an enzyme involved in estrogen production.

13PubMed Central. Temperature-dependent sex determination and gonadal differentiation in reptiles

Even in species that do have sex chromosomes, temperature can override genetics. The Central Bearded Dragon (Pogona vitticeps) normally uses a ZZ/ZW chromosomal system, but high incubation temperatures can transform genetically male (ZZ) embryos into functional females. These sex-reversed females are not just cosmetically female; they reproduce successfully. Research has found that these reversed females can display male-like or novel behavioral traits that may enhance their fitness, potentially allowing a temperature-based sex determination system to gradually replace the genetic one within a population.

14PubMed Central. The behavioural consequences of sex reversal in dragons

In a viviparous skink (Carenascincus ocellatus), researchers identified sex-reversed XX males occurring in both high- and low-elevation populations. These XX males appeared more frequently when thermal opportunities were reduced, and they were found in the same litters as XX females and XY males, confirming that the reversal was environmentally driven rather than genetic.

15Proceedings of the Royal Society B: Biological Sciences. Sex reversal explains some, but not all, climate-mediated sex ratio variation within a viviparous reptile

Climate change makes this especially relevant. As global temperatures shift, nest temperatures for wild reptile populations are changing too, which could push sex ratios in skewed directions. If a warming climate produces mostly females in a turtle species, for example, the population could eventually struggle to find mates.

When Pollution Creates Intersex

Some of the most alarming cases of intersex in wildlife are caused not by natural mechanisms but by chemical pollution. Endocrine-disrupting compounds, substances that interfere with hormonal signaling, have been linked to intersex conditions in fish and amphibians around the world.

16PubMed. Intersex in fishes and amphibians: population implications, prevalence, mechanisms and molecular biomarkers

In fish, the hallmark finding is testicular oocytes: egg cells growing inside what should be an entirely male gonad. This was first widely reported in smallmouth bass from the Potomac River drainage in the eastern United States, where health assessments in 2003 revealed immature oocytes within testes during investigations of fish kills and skin lesions.

17PubMed. Intersex (testicular oocytes) in smallmouth bass from the Potomac River and selected nearby drainages

Since then, testicular oocytes have been documented in many freshwater species globally, and the condition is strongly tied to exposure to endocrine-disrupting compounds in aquatic ecosystems.

18Journal of Fish and Wildlife Management. Survey of Intersex (Testicular Oocytes) in Black Crappie Collected from the Illinois River Waterway

Researchers studying sport fish in rivers across North and South Carolina measured a suite of endocrine-active compounds and assessed their relationship to intersex occurrence and severity, confirming that these pollutants represent a widespread threat to aquatic ecosystems.

19PubMed. Relation of contaminants to fish intersex in riverine sport fishes

Amphibians may be even more vulnerable. The herbicide atrazine, one of the most widely used pesticides in North America, has been shown to cause hermaphroditism in leopard frogs at concentrations as low as 0.1 parts per billion, both in the lab and at contaminated field sites across the United States.

20Environmental Health Perspectives. Atrazine-induced hermaphroditism at 01 ppb in American leopard frogs (Rana pipiens): Laboratory and field evidence

In African clawed frogs, atrazine exposure produced even more dramatic results: about 10% of genetically male frogs developed into functional females that mated with unexposed males and produced viable eggs. The remaining exposed males showed suppressed testosterone, reduced sperm production, and decreased fertility.

21PubMed Central. Atrazine induces complete feminization and chemical castration in male African clawed frogs (Xenopus laevis)

Parasites That Rewrite Their Host’s Sex

In invertebrates, sex can be hijacked by parasites. The bacterium Wolbachia, which infects a huge range of arthropods, is one of the most widespread sex manipulators in nature. It is passed from mother to offspring and can cause several reproductive alterations, including feminization, where genetically male hosts develop as females.

22PubMed Central. Evidence for widespread Wolbachia infection in isopod crustaceans: molecular identification and host feminization

In butterflies, Wolbachia infection has been shown to disrupt sex chromosome inheritance, producing intersexual offspring that express both male and female versions of key sex-determination genes.

23PubMed Central. Feminizing Wolbachia endosymbiont disrupts maternal sex chromosome inheritance in a butterfly species

Parasitic barnacles (Sacculinidae) take a different approach in crabs. These parasites infiltrate the host’s body and interfere with its endocrine system, effectively reshaping both male and female crabs toward the morphology and behavior of egg-carrying females. For the parasite, this is strategic: it uses the host’s egg-brooding behavior to care for the parasite’s own offspring.

24Zoological Journal of the Linnean Society. The effects of sacculinid parasites on two Jamaican crabs

What Intersex Means for Wild Populations

When intersex conditions occur naturally and at low rates, they typically have little impact on population health. But when pollution or other environmental pressures push intersex rates higher, the consequences can be serious. Wild intersex roach, a common European freshwater fish, have been shown to produce lower-quality gametes, achieve lower fertilization rates, and generate fewer viable offspring compared to normal males.

25Biology of Reproduction. Wild Intersex Roach (Rutilus rutilus) Have Reduced Fertility

Population modeling paints a starker picture. One study found that if intersex females replaced normal females at just 5% of the population, the modeled population collapsed in about six years. At 10%, collapse came in roughly two years. The dynamics were more complicated when intersex females replaced males instead: female-biased sex ratios actually boosted population numbers up to a threshold, because the total number of egg-producing individuals increased. But once intersex females reached about half the number of normal females, the reduced reproductive efficiency overwhelmed any numerical advantage, and the population headed toward extinction within a decade.

26PubMed. Population level effects of intersexuality in the marine environment

These modeling results underscore why pollution-driven intersex is more than a curiosity. For species already under pressure from habitat loss or overharvesting, even a modest increase in intersex prevalence could tip the balance. Monitoring wild fish and amphibian populations for testicular oocytes and other intersex markers has become a routine part of environmental health assessment in many countries, functioning as an early warning system for endocrine disruption in waterways.