Brindle, Matilda-Jane; (2022) Autosexual Behaviour in Primates: Form, Phylogeny and Function. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Autosexual behaviour, or masturbation, occurs throughout the animal kingdom, but appears to be particularly prevalent in the primates. However, the fitness benefits of this solitary behaviour are unclear and its evolutionary significance has not yet been comprehensively studied, although various hypotheses have been proposed (Chapter 01). In this thesis, I conduct the first systematic, comparative study into this common sexual behaviour. I began by compiling and categorising the scattered published information on primate masturbation, for both females and males, supplementing this with questionnaire responses (Chapter 02). The resulting database consisted of nearly 400 sources, covering 105/281 species (37.4 %), 54/68 genera (79.4 %) and 18/19 (sub)families (94.7 %). I then investigated the natural history of masturbation across the primate order, highlighting the distribution and diverse forms the behaviour takes (Chapter 03). I found masturbation was present in females of 19/45 genera (42.2 %) and in males of 36/51 genera (70.6 %). The most targeted body parts were the genitals, most frequently stimulated by the hands and fingers, and masturbation occurred in wild-living as well as captive individuals, indicating this behaviour is not a pathological outcome of captivity. Next, I examined the evolutionary history of masturbation for the first time, using a Bayesian phylogenetic approach to reconstruct its ancestral state at eight key nodes (Chapter 04). Although the occurrence of this behaviour in early primate evolution was equivocal, masturbation was consistently present in the ancestor of all platyrrhines and catarrhines, and appears to have been retained at all subsequent nodes with varying probabilities. Finally, I employed Bayesian phylogenetic methods to test adaptive hypotheses about masturbation (Chapter 05). I found that masturbation in males tended to occur in species exhibiting a multi-male mating system, larger testes, and a higher pathogen load, while masturbation in females tended to occur only in species with large testes. These findings support both the Postcopulatory Selection Hypothesis, in females and males, and the Pathogen Avoidance Hypothesis, in males. Together, the results of this research will help to increase our understanding of a very common, but little understood, behaviour (Chapter 06).

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