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b'Male masturbation or sexual-auto stimulation is well documented in humans. This\\nstudy dealt with the occurrence of masturbation in non-human primates in general, and in\\nJapanese macaques (Macaca fuscata) in particular, from the perspective of behavioural\\necology.\\nIn an interspecific comparison of 52 primate species, male masturbation was found to\\nbe a common behaviour that correlates more strongly to species that exhibit a multi-male\\nmulti-female breeding system than to species living in monogamous, one-male units, solitary\\nor dispersed breeding groups. This result is in direct contrast to established theories of sperm\\ncompetition, which predict that species with a higher risk of sperm competition (i.e. those\\nwith multi-male multi-female breeding systems) should invest in large ejaculate volumes so\\nas to out-compete conspecifics. They should not, therefore, produce and seemingly waste\\nejaculate by masturbating.\\nIn wild living (on Yakushima Island) and captive Japanese macaques, a multi-male\\nmulti-female seasonal breeding species with a high incidence of female and male\\npromiscuity, two hypotheses concerning how masturbation may be beneficial to males in\\nregard to sperm competition were tested. First, I tested whether or not males flush out low\\nquality sperm from their genital tracts when they masturbate. Second, I tested for whether or\\nnot the subsequent ejaculate was "fresher".\\nThe mechanism of how ejaculate parameters change in response to storage time in the\\nmale genital tract was determined by performing experiments with singly-caged Japanese\\nmacaque males. I found that the longer an ejaculate was stored, the larger its volume and total\\nsperm number became. However, the longer an ejaculate was stored, the lower the sperm\\nswimming velocity and percentage of vital sperm became.\\nBased on this result, roughly two types of ejaculates could be distinguished and\\ncorrelated with commonly known male mating strategies. Guarders tend to be older, high\\nranking males who have the opportunity to perform many consecutive matings with the same\\nfemale. They bring into sperm competition games Type A ejaculates, which have been stored\\nfor more or less lengthy periods and are characterised by a large volume and a large total\\nsperm number comprised of slow swimming and many dead sperm. In contrast, sneakers,\\ntend to be younger males of middle or low rank, who are usually able to mate only\\nopportunistically (almost never consecutively and almost always covertly) and their\\nejaculates inevitably face sperm competition from guarders\\u2019 (or from other sneakers\\u2019) ejaculates. Sneakers masturbate before mating and thus bring into the female only a small\\nvolume of ejaculate but one with fast moving sperm, all of which are a live (Type B). In\\ncontrast, guarders exclusively masturbate out long stored ejaculate only on days when there\\nare no estrus females available in the troop. Guarders never were seen to masturbate before\\nmating.\\nMasturbation, then, is one mechanism by which lower ranking males attempt to make\\nthe best of their limited mating opportunities in a species characterized by high levels of\\nsperm competition by investing in ejaculate quality as opposed to quantity. This result is\\nsupported by a DNA-paternity-exclusion analysis in the study troop on Yakushima Island, in\\nwhich six of nine babies were sired by sneakers. Masturbation is, thus, physiologically\\nadaptive to sperm competition in primates. Both, guarders and sneakers use masturbation to\\nimprove ejaculate quality. Therefore, masturbation can be regarded as an evolutionarily\\nstabilised strategy (ESS).'