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

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