Sperm Competition and the Function of Masturbation in Japanese Macaques (Macaca fuscata)
type: CreativeWork
description: 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 hav…
abstract: 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 hav…
author: Thomsen, Ruth
date published: 2001-02-27
date modified: 2026-07-24T02:51:21Z
provider: Ludwig Maxmilians Universität München
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: Ludwig-Maximilians-Universität