Thousands of animal species naturally function as both male and female, either at the same time or at different points in their lives. This trait, broadly called hermaphroditism, is far from rare or exotic. It shows up across the animal tree of life, from the earthworms in your garden to tropical reef fish, from tiny flatworms to deep-sea creatures that have never seen sunlight. The specifics vary wildly: some animals carry fully functional male and female organs simultaneously, some switch sex in response to social cues, and a few can even toggle back and forth depending on which role is needed.
How Common Is This, Really?
If you picture only mammals and birds when you think of animals, hermaphroditism seems vanishingly rare. But mammals and birds are a narrow slice of the animal kingdom. Among invertebrates, which make up the vast majority of animal species, being both male and female is extremely common. Earthworms, garden snails, most flatworms, many sea slugs, barnacles, and a large share of sponges and corals are all hermaphroditic. Even among vertebrates, hundreds of fish species change sex or function simultaneously as both.
An evolutionary reconstruction of sexual modes across the animal tree found that transitions from separate sexes to hermaphroditism have been more common throughout animal history than the reverse, meaning lineages have repeatedly evolved toward dual-sex function rather than away from it.1PubMed Central. A reconstruction of sexual modes throughout animal evolution That tells us hermaphroditism is not a dead end or a primitive holdover. It keeps being reinvented because, under the right ecological conditions, it works.
Simultaneous Hermaphrodites
The most intuitive version of “both male and female” is the simultaneous hermaphrodite: an animal that produces both eggs and sperm at the same time and can mate in either role. Earthworms are the classic backyard example. When two earthworms mate, they line up in opposite directions and each transfers sperm to the other. Research on the earthworm Eisenia andrei has shown they are surprisingly strategic about it. Worms tripled the amount of sperm they donated when mating with a partner that had already mated with someone else, and they donated even more to larger, more fertile partners.2PubMed Central. Brainless but not clueless: earthworms boost their ejaculates when they detect fecund non-virgin partners For a brainless animal, that is a remarkably fine-tuned reproductive decision.
Land snails in the helicoid and limacoid groups are simultaneous hermaphrodites with one of the strangest mating behaviors in nature. During courtship, many species stab their partner with a sharp, chalky structure called a “love dart.” The dart carries mucus from a gland that manipulates the recipient’s reproductive tract, increasing the odds that the stabber’s sperm will succeed.3Journal of Molluscan Studies. The love-darts of land snails: integrating physiology, morphology and behaviour Experiments on the garden snail confirmed that injections of dart mucus more than doubled paternity compared with saline injections.4PubMed Central. The snail’s love-dart delivers mucus to increase paternity The dart benefits the stabber, but it is genuinely harmful to the recipient: snails that received dart stabs during mating had reduced lifetime egg production and shorter lifespans.5PubMed Central. The direct cost of traumatic secretion transfer in hermaphroditic land snails: individuals stabbed with a love dart decrease lifetime fecundity This is sexual conflict playing out inside species where every individual is both male and female.
Among vertebrates, simultaneous hermaphroditism is rarest but still documented. Several species of hamlet fish (genus Hypoplectrus) on coral reefs are functional hermaphrodites that “egg trade” during spawning. Partners take turns releasing small parcels of eggs for the other to fertilize, alternating between male and female roles within a single mating session. Because reproductive success as a male depends on willingness to invest in female function, this reciprocal egg trading keeps hermaphroditism evolutionarily stable.6Animal Behaviour. The relationship between mating system and simultaneous hermaphroditism in the coral reef fish, Hypoplectrus nigricans (Serranidae)
Sequential Sex Changers
Many animals are not both sexes at once but switch from one to the other during their lifetime. This sequential hermaphroditism comes in two main flavors: female-first (protogyny) and male-first (protandry). The direction of the switch usually reflects which sex benefits more from being large.
Clownfish are the most famous male-to-female changers. In a clownfish group living in a single anemone, the largest individual is the breeding female, the next-largest is the breeding male, and the rest are non-breeding males. If the female dies, the breeding male transforms into a female and the next subordinate steps up. Research on the two-band clownfish found that the aromatase gene, which helps convert androgens into estrogens, plays a central role in driving this transformation in both the brain and the gonads.7PubMed Central. Sex Change in Clownfish: Molecular Insights from Transcriptome Analysis The switch is not cosmetic. The animal restructures its reproductive organs and becomes a fully functional female capable of laying eggs.
Wrasses go in the opposite direction. In bluehead wrasses, large colorful males defend territories and mate with multiple females. When the dominant male disappears, the largest female in the group begins changing into a male, sometimes within hours behaviorally and over days to weeks physiologically. Brain research on this species found that large females already possess the neural circuitry for male courtship behavior before they begin transforming. Chemical stimulation of certain brain regions in these females triggered the color changes typical of male courtship, suggesting the transition involves activating existing circuits rather than building new ones.8PubMed Central. Brain Regions Controlling Courtship Behavior in the Bluehead Wrasse Work on the closely related saddleback wrasse found that changes in serotonin and noradrenaline activity in specific brain regions happen in the first week of sex reversal, coinciding with the behavioral switch: the animal starts acting male before the body has fully caught up.9PubMed. Monoaminergic changes associated with socially induced sex reversal in the saddleback wrasse
Animals That Can Switch Both Ways
A handful of fish species do something even more flexible: they change sex in either direction depending on circumstances. Certain gobies, including species in the genera Trimma and Priolepis, can shift from male to female or female to male, sometimes multiple times during their lives. This ability appears to have evolved under conditions of monogamy and low population density. When you live spread out across a reef and rarely encounter potential mates, being locked into one sex means you might meet another individual of the same sex and waste a mating opportunity. Bidirectional sex change solves that problem.10PubMed Central. Evolution of bidirectional sex change and gonochorism in fishes of the gobiid genera Trimma, Priolepis, and Trimmatom
The goby Lubricogobius exiguus takes this to an extreme. It lives at low densities in burrows on sandy bottoms and can shift sex in response to losing a mate, making the most of any pairing it manages to form.11PubMed. Bidirectional sex change and plasticity of gonadal phases in the goby Lubricogobius exiguus Both bidirectional sex change and simultaneous hermaphroditism tend to show up in species facing similar ecological pressures: limited mobility, sparse populations, and few mating opportunities.
Why Being Both Sexes Pays Off
The evolutionary logic behind hermaphroditism revolves around mate scarcity. When finding a partner is hard, being able to mate with any member of your species, regardless of their sex, is a massive advantage. In a population of hermaphrodites, every individual you encounter is a potential mate. In a population with separate sexes, roughly half the individuals you meet are reproductively incompatible. Theoretical work has shown that this “mating assurance” advantage is strongest in species with low population density, limited mobility, or pair-mating systems.12PubMed. Mate-search efficiency can determine the evolution of separate sexes and the stability of hermaphroditism in animals
Hermaphroditism also allows flexible resource allocation. An animal that produces both eggs and sperm can shift investment toward whichever sex function yields better returns at the moment. And as a last resort, some simultaneous hermaphrodites can self-fertilize, which is genetically costly due to inbreeding but beats producing no offspring at all.
Sexual Conflict When Everyone Is Both
You might assume that being both male and female eliminates the battle of the sexes. It does not. In fact, hermaphrodites have their own version of sexual conflict, driven by opposing reproductive interests within the same individual. Producing eggs is metabolically expensive. Donating sperm is cheap. So a simultaneous hermaphrodite often “wants” to mate in the male role and would prefer its partner to bear the cost of egg production. When both partners have the same preference, conflict arises.
The marine flatworm Pseudoceros bifurcus takes this conflict to a violent extreme. During mating, both partners attempt to stab the other with a needle-like penis and inject sperm through the body wall, a behavior sometimes called “penis fencing.” The loser, the one who gets inseminated, takes on the energetically expensive female role. Research has suggested that this kind of physically damaging sex may actually be more favored in hermaphrodites than in species with separate sexes, precisely because each individual has a personal stake in avoiding the female role.13Nature. Sex and violence in hermaphrodites
Even the snail love-dart system described earlier reflects this tension. The dart does not help reproduction in general; it specifically helps the stabber’s sperm at a cost to the recipient’s future egg production. Each individual simultaneously benefits from stabbing and suffers from being stabbed, creating an evolutionary arms race within the species.
The Energetic Price of Dual Function
Maintaining two complete reproductive systems is not free. Work on the great pond snail found that individuals mating in both roles simultaneously experienced a roughly 40 percent drop in egg production compared with snails that did not mate at all. That fecundity cost applied whether the snail mated as a sperm donor, a sperm recipient, or both.14PubMed Central. Costs of receipt and donation of ejaculates in a simultaneous hermaphrodite This trade-off helps explain why hermaphroditism is not universal. Species that have easy access to mates and benefit from large-scale egg production often do better with separate sexes, each optimizing one function.
Gynandromorphs Are Something Different
Occasionally, an animal appears that is literally split down the middle: one half male, the other half female. These gynandromorphs look striking (a cardinal that is bright red on one side and dull brown on the other, for instance) but they are fundamentally different from hermaphrodites. Gynandromorphism is typically a developmental accident, not an evolved reproductive strategy. It arises when sex-determining events go differently in different cells during early development.
A famous gynandromorphic zebra finch, with genetically male cells on the right side of its brain and genetically female cells on the left, taught researchers something unexpected. The male-side song circuit was more developed than the female side, even though both halves were bathed in the same hormones from the bird’s single set of gonads. The implication: the genetic sex of individual brain cells contributes to sexual differentiation independently of circulating hormones.15PubMed Central. Neural, not gonadal, origin of brain sex differences in a gynandromorphic finch Gynandromorphs are not “both sexes” in the reproductive sense. They are genetic mosaics, fascinating for what they reveal about sex determination but not examples of functional hermaphroditism.
When Parasites Decide Your Sex
Wolbachia bacteria, which infect an enormous range of arthropods, can override their host’s genetic sex. In terrestrial isopods like the common pill bug (Armadillidium vulgare), Wolbachia feminizes genetic males, turning them into functional “neo-females” that can reproduce as females despite carrying male chromosomes.16PubMed Central. A cost of Wolbachia-induced sex reversal and female-biased sex ratios: decrease in female fertility after sperm depletion in a terrestrial isopod Since Wolbachia are passed from mother to offspring, skewing the population toward females maximizes the bacterium’s own transmission. Surveys have found Wolbachia infection and its feminizing effects widespread across isopod crustaceans.17PubMed Central. Evidence for widespread Wolbachia infection in isopod crustaceans: molecular identification and host feminization
Interestingly, male pill bugs can tell the difference. In mate-choice experiments, males preferred genetic females over Wolbachia-feminized neo-females, suggesting the sex reversal is not quite perfect from the male’s perspective.18Journal of Evolutionary Biology. Sexual selection in an isopod with Wolbachia-induced sex reversal: males prefer real females This is not hermaphroditism in the traditional sense; the animal ends up functioning as one sex rather than both. But it blurs the boundary between male and female in ways that matter for the population’s sex ratio and evolutionary trajectory.
Temperature-Driven Sex Reversal in Reptiles
Central bearded dragons offer another twist on the question. In this Australian lizard, sex is normally determined by chromosomes, but high incubation temperatures can override genetic sex, transforming chromosomally male embryos into functional females. These sex-reversed females can lay eggs and reproduce. But behaviorally and morphologically, they are not quite the same as genetically female dragons: they tend to be bolder, more active, and have longer tails relative to body size, resembling genetic males in everything except reproductive anatomy.19PubMed Central. The behavioural consequences of sex reversal in dragons These individuals are not both male and female simultaneously, but they carry a mix of male genetics and female reproductive function that complicates any simple definition of sex in this species.
A Lab Worm That Mostly Does It Alone
The nematode Caenorhabditis elegans, one of the most studied organisms in biology, reproduces primarily as a self-fertilizing hermaphrodite. Each hermaphrodite produces both sperm and eggs internally and can generate offspring without mating. Males exist but at low frequency, typically below five percent of wild populations.20PubMed Central. The evolutionary role of males in C. elegans Why bother keeping males around at all? Part of the answer is that males arise spontaneously through chromosome errors during cell division, and part is that occasional mating with males introduces genetic diversity that helps populations adapt to changing environments. Lab populations subjected to new temperature or rearing conditions maintained males at frequencies anywhere from five to 40 percent, far above the baseline rate in wild populations.21Journal of Heredity. Outcrossing and the Maintenance of Males within C. elegans Populations The C. elegans system shows that self-fertilizing hermaphroditism and sexual reproduction are not mutually exclusive. A species can lean heavily on self-fertilization in stable times and fall back on outcrossing when conditions shift.
Why Mammals Almost Never Do This
If hermaphroditism is so common elsewhere, why is it essentially absent in mammals? The short answer is that mammalian development locks sex in early and irreversibly. The SRY gene on the Y chromosome triggers a cascade that commits the embryo to male development, and hormonal signals from the developing testes further masculinize the body. Reversing that process partway through life, the way a wrasse or clownfish does, is not feasible with mammalian biology. Mammals also have genomic imprinting, where certain genes behave differently depending on whether they were inherited from the mother or the father, which adds another layer of developmental inflexibility around sex.
Birds face a similar constraint. Their sex chromosomes (ZW in females, ZZ in males) and the cell-autonomous sex identity demonstrated by the gynandromorphic finch both work against easy sex switching. The result is that natural functional hermaphroditism in warm-blooded vertebrates is essentially unheard of.
Anglerfish and the Extreme End of Male-Female Fusion
Deep-sea anglerfish have taken the relationship between males and females to a place that borders on science fiction. In several anglerfish species, the tiny male bites into the much larger female and fuses permanently to her body, eventually sharing her bloodstream and losing his eyes, most internal organs, and independent existence. He becomes, in effect, a pair of gonads attached to the female, releasing sperm when she releases eggs. To make this permanent tissue fusion possible, these species have lost key components of their adaptive immune system, the very system that would normally reject foreign tissue. Some species lack functional genes for antibody maturation; others have gone further and lost the genes responsible for generating diverse immune receptors altogether.22PubMed. The immunogenetics of sexual parasitism 23PubMed Central. Histocompatibility and Reproduction: Lessons from the Anglerfish
The fused pair is not technically hermaphroditic: the female remains female and the male remains male. But the resulting organism is a single body carrying both male and female reproductive tissue, which is about as close to “both male and female” as you can get through a completely different evolutionary route. It is a vivid reminder that nature’s solutions to the problem of finding a mate in a vast, dark ocean can be radically different from anything on land.
Pollution and Unintended Intersex
Not all cases of animals being “both sexes” are natural adaptations. Endocrine-disrupting chemicals in waterways can cause intersex conditions in fish that are normally single-sexed. Intersex in this context means the appearance of female germ cells within a male gonad, or vice versa. Surveys of sport fish in river systems have linked intersex to contamination by endocrine-active compounds from agricultural runoff, pharmaceuticals, and industrial waste.24PubMed. Relation of contaminants to fish intersex in riverine sport fishes Studies of tilapia in Nigerian rivers have found intersex prevalence as high as 24 percent in some populations, with both male and female fish developing gonadal tissue of the opposite sex.25PubMed. Gonado-histopathological changes, intersex and endocrine disruptor responses in relation to contaminant burden in Tilapia species from Ogun River, Nigeria
Unlike natural hermaphroditism, pollution-driven intersex typically impairs rather than enhances reproduction. These fish are not gaining a reproductive advantage from having both types of tissue; they are suffering developmental disruption that can reduce fertility and serve as a warning sign that the ecosystem is under chemical stress. The distinction matters. Calling a pollution-affected fish “hermaphroditic” conflates a pathological response with a finely tuned evolutionary strategy, and the two could not be more different in their causes or consequences.