Sperm Competition and the Function of Masturbation in Japanese Macaques (Macaca fuscata)

摘要: Male masturbation or sexual-auto stimulation is well documented in humans. This study dealt with the occurrence of non-human primates general, and in Japanese macaques (Macaca fuscata) particular, from perspective behavioural ecology. In an interspecific comparison 52 primate species, male was found to be a common behaviour that correlates more strongly to species exhibit multi-male multi-female breeding system than living monogamous, one-male units, solitary or dispersed groups. This result direct contrast established theories sperm competition, which predict higher risk sperm competition (i.e. those with multi-male multi-female systems) should invest large ejaculate volumes so as out-compete conspecifics. They not, therefore, produce seemingly waste ejaculate by masturbating. In wild (on Yakushima Island) captive Japanese macaques, seasonal high incidence female male promiscuity, two hypotheses concerning how may be beneficial males in regard were tested. First, I tested whether not flush out low quality their genital tracts when they masturbate. Second, for or not subsequent "fresher". The mechanism parameters change response storage time the male tract determined performing experiments singly-caged Japanese macaque males. longer stored, larger its volume total sperm number became. However, lower sperm swimming velocity percentage vital became. Based on this result, roughly types ejaculates could distinguished and correlated commonly known mating strategies. Guarders tend older, high ranking who have opportunity perform many consecutive matings same female. bring into games Type A ejaculates, been stored for less lengthy periods are characterised comprised slow swimming dead sperm. In contrast, sneakers, tend younger middle low rank, usually able mate only opportunistically (almost never consecutively almost always covertly) their ejaculates inevitably face guarders’ (or other sneakers’) ejaculates. Sneakers masturbate before thus only small volume but one fast moving sperm, all live (Type B). In contrast, guarders exclusively long stored days there are no estrus females available troop. seen before mating. Masturbation, then, ranking attempt make the best limited opportunities characterized levels of sperm investing quality as opposed quantity. is supported DNA-paternity-exclusion analysis study troop Island, in which six nine babies sired sneakers. Masturbation is, thus, physiologically adaptive primates. Both, sneakers use to improve quality. Therefore, can regarded evolutionarily stabilised strategy (ESS).