Same-sex sexual behavior has been documented in over 1,500 animal species. It occurs across virtually all major groups — from insects and worms to reptiles, birds, and mammals. While it’s not the norm, it’s also not particularly rare. Around 4% of all mammalian species exhibit this behavior, and that’s likely a conservative estimate. It often goes unnoticed or is hard to confirm in the wild.

Interestingly, this behavior appears in both wild and semi-wild populations and shows up in both male and female mammals at nearly equal rates.

Things get even more interesting when we look at our closest animal relatives.

“Same-sex sexual behaviour (SSSB) is particularly prevalent in nonhuman primates, where it has been observed in at least 51 species from lemurs to apes,” write the authors of a recent study on homosexuality in animals. The behavior seems to be common in species that are smarter and more social. For instance, bonobos use SSSB to reconcile after conflicts, and dolphins form alliances through such behaviors.

Same-sex sexual behavior is also common in humans. Historical and anthropological evidence suggests it likely existed throughout human history and across a wide range of societies and cultures. From ancient Greece to Indigenous communities in North America and the Pacific Islands, such behaviors were often accepted or even ritualized. Although in many parts of the world, same-sex behavior has also been heavily stigmatized, criminalized, or suppressed, same-sex behavior is a natural part of human sexuality.

Same-sex behaviors evolved multiple times

This all begs the question ‘why’? Why would have this type of behavior evolve in nature? Researchers from Granada University in Spain found some interesting clues.

To get some answers, a team of researchers led by José M. Gómez turned to evolutionary biology. Using phylogenetic tools — methods that reconstruct the evolutionary relationships between species — they traced the origins of same-sex behavior across mammals. They found that this trait likely evolved multiple times independently, rather than being inherited from a single common ancestor.

In other words, it’s a classic case of convergent evolution, where unrelated species develop similar traits under similar selective pressures.

But this makes it all the more intriguing because it suggests that there are some very good reasons for same-sex behaviors to emerge not once, but multiple times.

Researchers propose two main reasons.

The first one is that SSSB may serve to establish and maintain social bonds. This is especially important in species that rely on group living for survival. For example, in bottlenose dolphins, SSSB strengthens alliances that are critical for navigating their complex social structures. The second is that SSSB might help to reduce aggression and establish dominance hierarchies. In species where males compete fiercely for mates, such as in certain primates and bison, SSSB could defuse tension, preventing lethal conflicts.

These theories are not mutually exclusive; they could both be playing a role. And there could also be other factors involved. The current research can’t tell with certainty what the case is.

Phylogenetic analyses also revealed that nodes (ancestral species) displaying SSSB were evolutionarily younger than those without. This implies that SSSB is a relatively recent adaptation in many lineages, particularly in primates and social mammals.

What about our lineage?

The findings also offer insights into the evolutionary roots of human SSSB. The study suggests that SSSB was likely present in the common ancestor of great apes, which were social species with complex group dynamics. This aligns with evidence from extant primates, such as bonobos and chimpanzees, which use SSSB for social bonding. The study shows that SSSB is widespread and often evolves in social species, especially those with complex social structures. Humans, as highly social primates, share these evolutionary pressures.

However, the researchers distinguish between SSSB and sexual orientation. While SSSB involves occasional same-sex interactions, sexual orientation encompasses consistent patterns of attraction and identity, particularly prominent in humans.

While SSSB in animals supports the naturalness of such behaviors, human experiences of sexuality include layers of identity, culture, and personal meaning that go beyond biological explanations. Homosexuality in humans often involves stable sexual orientations and relationships, distinct from the transient or context-dependent SSSB observed in some animal species.

Ultimately, the widespread occurrence of SSSB in mammals, especially primates, strongly suggests that such behaviors are natural and adaptive. Normalizing same-sex behavior as a part of this spectrum aligns with both biological evidence and a broader understanding of human social and emotional complexity.

The study “The evolution of same-sex sexual behaviour in mammals” was published Nature Communications.