Monkey Business

Sexual behaviour between animals of the same sex has been documented in a wide array of species across nearly all branches of the animal tree of life. Many sources estimate that approximately 1500 species have been observed engaging in same-sex sexual behaviour (SSB), dating back as far as the 1830s, with observations of SSB between male maybugs. While sporadic observations are widespread, data-based, scientific research is sparse. Biologists and naturalists under-report observations of SSB. A study published in 2024 found that while over 77% of polled researchers had observed SSB, only 48% published about it (although admittedly, the study had a small sample size of researchers). When reports are published, they suffer from a lack of standardized language, especially historically.

This makes the paper published last week by Imperial professor Vincent Savolainen and team all the more impressive. They collated and analysed existing reports of SSB in primates to identify statistical relationships between SSB and other characteristics. Primates are colloquially known as monkeys, and are a diverse order including lemurs, apes and tarsiers. The team analysed over 1700 reports of SSB, identifying 96 papers which they used to perform comprehensive statistical analyses of SSB in 59 species of primates.

They tested 3 hypotheses for the origin of SSB. One was that SSB occurred more often in species under pressure from their environment, like predators or a lack of resources. Generally, different-sex sexual behaviour decreases in these conditions, so it was interesting when the hypothesis was validated by SSB increasing.
Another was that more long-lived species with larger differences between male and female individuals engage in SSB more than species without these characteristics, another hypothesis that was confirmed. The authors suspect that this may be because the long lifespans of these primates living in large groups require them to form more cohesive and interconnected social structures. Finally, the tested the hypothesis that primates living in more complex social structures engage in more SSB, because SSB is a tension-reducing behaviour. They found that solitary primates engage in less SSB than those that live in large groups, although they commented that there was not enough research to create a model relating SSB to individual complex behaviours like child-rearing.

The team cautions against extrapolating conclusions about human behaviour from their investigations and “explicitly reject any use of [their] findings to condone discrimination or minimize LGBTQ+ experiences.”

Prof. Savolainen and his team highlight that this research is just “setting the stage” for further evolutionary research. Regardless, this is an exciting step in a surprisingly under-researched field.