Homosexual behaviour between animals may have evolved to ‘keep the peace’ in species where social conflict can lead to death.
Sex between same-sex couples has been reported in more than 1,500 animal species, including all the main groups of vertebrates, such as fish, amphibians, reptiles, birds and mammals.
It has also been seen in the main invertebrate groups, including insects, spiders, echinoderms – such as starfish – and nematodes, which are tiny parasitic worms.
Same-sex mating is particularly common in nonhuman primates, including lemurs and apes. In total, it has been observed in at least 51 species.
Over the years a number of theories have been proposed as to why animals engage in same-sex sexual behaviour – referring only to mating, not monogamous same-sex relationships, which also occur. These include cases of mistaken identity, limited availability of partners of the opposite sex and sexual frustration.
However, a review of studies by researchers based in Spain suggests the behaviour has much more deep-rooted and important origins, arising independently many times over.
Tracing the evolution of same-sex sexual behaviour across mammals, the team, led by José Gómez, found it was more likely to evolve in social species, including bonobos, chimpanzees, bighorn sheep, lions and wolves.
This led them to suggest the behaviour helps establish and maintain positive social relationships.
Same-sex love in the animal world
It’s not just same-sex sexual behaviour seen in animals, but also same-sex relationships.
Alongside penguins and albatrosses, two species famed for engaging in same-sex relationships – and even raising chicks – other animals that court their own sex include elephants, giraffes, ostriches and flamingos.
The team also found male same-sex sexual behaviour was more likely to evolve in species where males often kill each other as adults, known as adulticide.
In this scenario, same-sex sex may be used to diffuse tension in groups and reduce the risk of violent conflict between males.
However, given the prevalence of same-sex sexual behaviour in both males and females across a wide range of animals, the authors noted their study does not exclude other theories behind its evolution.
Same-sex behaviour has only been studied in a minority of mammal species, and so further research is required to better understand its evolution and benefits.
In addition, the authors stress their findings are unrelated to the evolution of sexual orientation in one particular primate species – humans. This is because the study only considers the short-term physical interactions involved in same-sex sexual behaviour, not permanent sexual preference.
The study is published in Nature Communications.
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