Masturbation

Whether or not masturbation confers a direct fitness advantage or emerged as an evolutionary byproduct is subject to debate. Myriad theories exist as to the adaptive function of masturbation, but three main hypotheses have been explored most extensively. The ejaculate-quality-improvement (EQI) hypothesis contends that masturbation eliminates degraded gametes and avoids polyzoospermia (excessive sperm concentration in ejaculate; decreases fertility due to compromised sperm motility and migration), increasing the quality of ejaculate and thus improving the probability of impregnation upon mating. The STI-reduction hypothesis suggests that masturbation cleanses the male reproductive tract, decreasing the likelihood of contracting sexually transmitted infections. Finally, the sexual-outlet hypothesis argues that masturbation does not provide a direct fitness benefit at all, but rather arose as a non-adaptive byproduct of neuroendicrine specializations for increased sexual arousal and performance, necessitating an alternative outlet to copulation when mating partners are not available.

Studies of male rhesus macaques in the wild correlated frequency of masturbation with low mating opportunity; regardless of rank, males were most likely to masturbate on days in which they did not mate, and lower-ranking males masturbated more frequently than higher-ranking males. Patterns of masturbation in Japanese macaques seemed to depend on preferred mating strategy: Guarders, which tend to be higher-ranking males who mate repeatedly, often consecutively, with the same females, exclusively masturbate only on days when no estrus females are available for mating. Conversely, sneakers, which tend to be lower ranking males who mate opportunistically and often covertly, regularly masturbate prior to mating. The latter practice may serve to eliminate lower quality ejaculate prior to mating, as theorized by the EQI hypothesis. Examination of ejaculate quality between these groups indicates larger volume and total sperm number, but also a larger presence of degraded or slow-swimming sperm, in guarders, in contrast to smaller but higher-quality ejaculate yields from sneakers. These results suggest that masturbation may be a mechanism by which lower ranking, opportunistically mating males more effectively capitalize on limited mating opportunities by increasing their quality of ejaculate and consequent probability of impregnation. Indeed, DNA-paternity-exclusion analysis of the population observed for this study revealed six of nine babies within the troop to be sired by sneakers.

Despite similar masturbatory practices, studies in rhesus macaques determined that only 15% of masturbation events ended with ejaculation, casting doubt on the EQI hypothesis as the sole explanation for the widespread emergence of autoerotic behavior. Furthermore, red colobus monkeys, another species of Old World monkey, masturbate very rarely, and autoerotic behavior is performed almost exclusively by alpha males during intergroup encounters. Male Cape ground squirrels, a highly promiscuous squirrel species, have been shown to masturbate primarily after copulation, and far more frequently on days of female estrus. This pattern aligns more logically with the STI-reduction hypothesis than the EQI hypothesis. In short, current hypotheses on the adaptive function of masturbation appear insufficient to universally address this question, and patterns of masturbation most relevant to human autoerotic behavior have not yet been resolved. Masturbation may be differentially adaptive depending on species, or may have emerged independently of any direct fitness benefit. Further research is necessary to better resolve the functional role of masturbation in humans and other animals. More comprehensive research into female autoerotic behavior in non-human animals may also better inform the emergence of this behavior.