2023-06-10 12:37:37
This practice surrounded by many taboos can leave people perplexed as to its evolutionary usefulness. So why spend time and energy on this non-reproductive activity? However, it is widely documented in the animal kingdom, so is it really useless?
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In the animal world, taboos do not exist, especially when it comes to sexual activity. Whether performing apparently reproductive functions or not, a wide variety of practices have been documented, including masturbation. This self-stimulating sexual activity is found in a large number of species, in particular mammals including horses, dogs, cats, elephants, walruses, dolphins, bats, but also in many birds, and, not surprisingly, in primates. If it is so widespread, does that mean that it is of evolutionary interest? This is according to a study published in the journal Proceedings of The Royal Society B.
What could masturbation be used for?
In the past, some authors have considered autosexuality as pathological, resulting from a life in captivity, or as having no particular advantage and simply resulting from too much sexual arousal. But this behavior has since been regularly observed in wild animals, in individuals surrounded by very willing potential partners, as well as just before or after copulation. Although this does not entirely invalidate these previous hypotheses, it was necessary to examine other explanations, and some authors argue that masturbation may indeed serve a beneficial function.
Two main explanations have been put forward. The first, the post-copulatory selection hypothesis (PSH) proposes that masturbation increases the chances of fertilization, because it would allow, for example, in males to expel semen of lower quality to make room for new spermatozoa, and , in females, to increase their vaginal pH, thus making it more hospitable for the latter. The second hypothesis is that of prevention against pathogens (PAH), according to which an ejection of semen after copulation would help prevent the spread of sexually transmitted infections (STIs) by expelling infectious agents with it.
The search for a phylogenetic signal
Despite the almost universal character of autosexuality in primates, few phylogenetic analyzes have really focused on the distribution of this behavior within the order. The one led by Dr. Matilda Brindle, from the Department of Anthropology of the University College London, has made it possible to establish that the practice is very old, probably present in the common ancestor of the taxon which includes modern apes, including the great apes and humans. The fact that this behavior is so common in our clade, when a wide variety of reproductive strategies (monogamy, polygamy, etc.) and many different living environments are observed, is fortunate because it allowed researchers to test the two main hypotheses.
The first of these, PSH, predicts that the practice would increase fertility, and would be all the more useful in schemes where several males can mate with the same female, since the pressure for the sperm of an individual are more competitive than those of others is stronger. As for the second, PAH, which predicts that masturbation reduces the prevalence of infections, it should therefore be more widespread among taxa whose living conditions contribute to a high presence of pathogens, tropical climates and with more precipitation than example.
The researchers actually observed a statistical prevalence of masturbation in primate taxa where a reproductive strategy with several males had developed, and also in those for which environmental conditions favor infections. This strongly supports both hypotheses! The study sheds light on the question of masturbation in male primates, but there is still a lot of investigation to be done to understand this common but less reported behavior in females.
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