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Same-sex sexual behaviour has been reported in more than 1,500 animal species, but that is a count of species with published reports—not a percentage of animals or a measure of how often the behaviour occurs. Scientists do not have a reliable, standardized global prevalence figure. The evidence is uneven, and a reported behaviour does not by itself establish an animal’s exclusive sexual preference or identity.
What does “common” mean in animal research?
Researchers generally use the term same-sex sexual behaviour (SSB) for observed sexual activity between same-sex individuals. A 2023 mammal study defined it as “any attempted sexual activity between members of the same sex.” The term describes behaviour; it does not assume that an individual has a lasting preference or an identity equivalent to a human sexual identity. Gómez, González-Megías and Verdú, Nature Communications (2023)
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A species count answers whether researchers have published a report for a species. It does not tell us what fraction of that species’ individuals engage in SSB, how frequently they do so, or whether particular individuals engage only with same-sex partners. Those questions require different observations and denominators.
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What do the available counts measure?
| Evidence | Reported figure | What the figure represents |
|---|---|---|
| Non-human primates, comparative study | 491 species assessed; SSB documented in 59 | Species in the study’s comparative dataset, not the share of individual primates that engage in the behaviour. Coxshall et al., Nature Ecology & Evolution (published January 12, 2026) |
| Invertebrates, systematic synthesis | Approximately 207 species | Species with published evidence in the review; the authors note that research effort is concentrated in certain groups and study types. Insects (2026) |
| Mammal researchers, online expert survey | 77% had observed SSB; 48% had collected data; 19% had published findings | Responses from surveyed researchers about their study species—not percentages of mammals displaying SSB. Anderson et al., PLOS ONE (2024) |
These figures cannot be combined into a single prevalence estimate: they describe different taxonomic groups and evidence types, and their denominators are species in datasets or researchers who answered a survey—not a representative sample of individual animals across the world.
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Why is there no reliable percentage for all animals?
There is no consistent worldwide protocol that has measured the same behaviour, in the same way, across representative samples of animal species and individuals. Studies can differ in how they define SSB, how intensively they observe animals, and whether they record an anecdotal report, systematic observations, or other evidence. Species that are little studied may have no published record even if the behaviour occurs.
- Research effort is uneven. The invertebrate synthesis found that published work is concentrated in insects and particular study types, so the visible pattern partly reflects where researchers have looked.
- Non-reporting creates gaps. The 2024 survey results show a difference between researchers who said they had observed SSB and those who had collected data or published findings. A lack of publications is not proof that a behaviour is absent.
- Species occurrence and individual frequency are different measures. A species can have a documented report without researchers knowing how many individuals engage in the behaviour or how often.
The 2023 mammal analysis also identifies missing information and possible false negatives as limits on interpreting species records. Consequently, “not documented” should not be read as “does not happen.” Gómez, González-Megías and Verdú (2023)
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What does the 2026 primate study suggest?
In its comparison of non-human primates, Coxshall and colleagues examined associations between SSB and 15 environmental, life-history and social traits. They reported greater occurrence in species associated with drier environments, greater food scarcity and predation pressure, greater size dimorphism, longer lifespans, and more complex social structures and hierarchies. Their structural equation model indicated a direct association between social complexity and occurrence, while environmental and life-history traits were mainly associated indirectly. Coxshall et al. (2026)
This is a comparative result about patterns among primate species. It does not show that any one factor causes SSB in every species, or that the behaviour serves the same function in each one. The authors describe it as context-dependent, shaped by interactions among ecological, life-history and social factors.
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What can scientists conclude about its evolutionary role?
Evidence that a behaviour occurs is not, by itself, an explanation for why it evolved or what effects it has. A 2009 review discussed both adaptive and non-adaptive explanations and called for theoretical, comparative and experimental approaches to clarify the behaviour’s evolution and consequences. No single evolutionary explanation is established across animal groups. Bailey and Zuk, Trends in Ecology & Evolution (2009)
When comparing claims about SSB, check what was actually measured: the taxonomic group; wild or captive observations; the study’s operational definition; whether evidence came from reports, systematic observation, a survey or comparative analysis; and whether the denominator was species, individuals or researchers. Also check whether a figure describes occurrence, frequency or publication history. These distinctions determine what a result can—and cannot—say.
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