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Male and female primates from all extant radiations engage in autosexual behaviors, implying that masturbation is a facet of the sexual repertoire of our hominid ancestors. At a proximate level, masturbation results in relaxation and reduced aggressiveness. At an ultimate level, male masturbation might be beneficial because old and low-quality sperm are discarded, while fresh sperm production is stimulated. Convincing functional explanations for female masturbation are still lacking.

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Masturbation is one of the most common sexual behaviors in humans. It is also a phylogenetically widespread trait of various other mammalian and some non-mammalian species. Several hypotheses have been proposed aiming to explain the function of masturbation in primates and other species. These were mainly based on observations of nonhuman primates such as rhesus macaques or bonobos and rodents such as African ground squirrels. Based on these observations various scholars suggested that masturbation improves ejaculate quality, decreases the risk of contracting sexually transmitted infections or is merely a by-product of sexual arousal and thus an alternate outlet to copulation. While these theories may explain some facets of masturbation in some species, they do not explain why masturbation is so widespread and has developed in various species as well as our hominid ancestors. Moreover, the research on which these theories are based is scarce and heavily focused on male masturbation, while female masturbation remains largely unexplored. This sex difference may be responsible for the one-sided theorizing that attributes a specific biological benefit to masturbation. We propose that the widespread prevalence of masturbation in the animal kingdom may be better explained by viewing masturbation as a primarily self-reinforcing behavior that promotes pleasure both in human and in nonhuman species.
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This book provides an authoritative and comprehensive synthesis of current research on the evolution and physiological control of sexual behaviour in the primates - prosimians, monkeys, apes, and human beings. This new edition has been fully updated and greatly expanded throughout to incorporate a decade of new research findings. It maintains the depth and scientific rigour of the first edition, and includes a new chapter on human sexuality, written from a comparative perspective.
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Assessments of primate male fertility via semen analyses are so far restricted to captivity. This study describes a non-invasive method to collect and analyse semen in wild primates, based on fieldwork with Yakushima macaques (Macaca fuscata yakui). Over nine mating seasons between 1993 and 2010, 128 masturbatory ejaculations were recorded in 21 males of 5 study troops, and in 11 non-troop males. In 55 %, ejaculate volume was directly estimated, and in 37 %, pH-value, sperm vitality, numbers, morphology and swimming velocity could also be determined. This approach of assessing semen production rates and individual male fertility can be applied to other primate taxa, in particular to largely terrestrial populations where males masturbate frequently, such as macaques and baboons. Furthermore, since explanations of male reproductive skew in non-human primate populations have until now ignored the potential role of semen quality, the method presented here will also help to answer this question.
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Much sexual selection theory is based on the idea that ejaculate is cheap. Since further details are unknown our aim was to determine the energy that primate males require for ejaculate production. We addressed this problem by measuring the energy content (in kJ) of ejaculates from Japanese macaques (Macaca fuscata) using standard bomb calorimetry. Then, we estimated the relative amount of energy that individuals require for ejaculate production by relating the net energy content of ejaculates to males' daily basal metabolic rate (BMR). Fresh macaque ejaculate contains 3.0 kJ ml(-1). Assuming a mean volume of 2.7 ml an average macaque ejaculate contains 8.1 kJ. Depending on the individuals' body mass (6-13 kg) and the number and volume of the ejaculates, macaque males are assumed to use between at least 0.8% and at most 6.0% of their BMR for ejaculate production per day during the breeding season. Even when regarding only the minimal energy investment of 0.8% of daily BMR for ejaculate production, clearly ejaculates come with some cost for primate males.
Masturbation in Male Primates: Taxonomic Distribution, Proximate Causes and Potential Evolutionary Functions
  • C. Frearson