Masturbation has long been observed in primates, including humans, but the evolutionary benefits of this behaviour have so far been unclear. Scientists have suggested that masturbation could be a pathology – a practise that incurs costs in terms and time energy without increasing the chances of survival – or might simply be a by-product of high arousal.
Now, however, new research from University College London indicates that masturbation in primates increases reproductive success and reduces the incidence of sexually transmitted infections (STI).
Having compiled the largest ever dataset of so-called autosexual behaviour in primates, drawing on 246 published papers and 150 questionnaires and notes from primatologists and zookeepers, a team led by anthropologist Dr Matilda Brindle tracked the history of masturbation in monkeys and apes to understand its evolutionary origins.
The team found that masturbation dates back around 40 million years, and was a behaviour practised by the common ancestor of all apes and monkeys, including humans. (It’s possible that the common ancestor of primates such as lemurs, lorises and tarsiers also masturbated but there wasn’t enough data on these groups to say for sure.)
There are couple of possible hypotheses supported by the data for why the trait may have evolved.
‘Postcopulatory selection hypothesis’ suggests that masturbation aids fertilisation, either by increasing arousal before sex, thereby enabling lower-ranking males to ejaculate sooner, before being interrupted; or because ejaculation prior to sex ensures that fresher, higher quality sperm is available for mating.
The ‘pathogen avoidance hypothesis’, on the other hand, proposes that masturbation after sex reduces the chances of contracting an STI because ejaculation clears the urethra of foreign bacteria. Both theories focus on male masturbation, with the significance of female masturbation remaining unclear and in need of further study.
Matilda Brindle says, “Our findings help shed light on a very common, but little understood, sexual behaviour and represent a significant advance in our understanding of the functions of masturbation. The fact that autosexual behaviour may serve an adaptive function, is ubiquitous throughout the primate order, and is practised by captive and wild-living members of both sexes, demonstrates that masturbation is part of a repertoire of healthy sexual behaviours.”
Main image: Bonobos are known for their hypersexuality © Anup Shah/Getty
