Despite its ubiquity in the animal world, masturbation presents an evolutionary conundrum. Why would a creature expend resources and time for self-gratification instead of engaging in procreative activities?
Insights from individual species have provided some rationale. For instance, subordinate Japanese macaques (Macaca fuscata) utilize masturbation to ensure their sperm remains viable for the infrequent opportunities to procreate. Yet, the origins and the evolutionary reason for masturbation remain uncertain.
A ground-breaking research paper released on June 7 in the Proceedings of the Royal Society B proposes that the evolution of this behavior in primates can be traced back around 40 million years. It postulates that this act serves to keep male primates primed for breeding opportunities, and may also serve as a mechanism to prevent diseases.
According to Lateefah Roth, a biologist at the Institute of Forensic Psychiatry and Sex Research at the University of Duisburg-Essen, Germany, the authors are pioneers in employing a comparative species methodology to understand the role of masturbation.
To ascertain the inception of this behavior in primates, Matilda Brindle, an evolutionary biologist from University College London, and her team, examined published scientific works to gather information on primates observed masturbating, both in natural and controlled environments.
Given the inherent difficulty in observing masturbation, and in some instances, the absence of research focus on this behavior, Brindle supplemented her data by conducting surveys among researchers. The questionnaires solicited information on unreported instances of primate masturbation, with particular emphasis on female primates, as signs of female arousal are less evident. Using computational models, the team endeavored to determine the probable point of origin of this behavior in primate evolution.
While there were gaps in the data that made it impossible to assert if the earliest primates masturbated, Brindle was able to deduce that it appears from around 40 million years ago, the forebears of all monkeys and apes were likely practicing this act. This timeframe aligns with the period when simians diverged from tarsiers, small, large-eyed primates native to Southeast Asia.
Having identified when, Brindle next sought to understand why. She evaluated if masturbation was more common among primates with multiple mating partners, since such mating systems could exert evolutionary pressures to control the breeding process, either by speeding up copulation, preferential arousal for certain partners or enhancing sperm quality.
Additionally, Brindle investigated if there was a correlation between masturbation and a higher incidence of infection by disease-causing pathogens, including sexually transmitted infections. For instance, male Cape ground squirrels (Geosciurus inauris) are known to masturbate post copulation to purge their bodies of potential infectious agents.
Her findings suggest that male primates who have multiple mates and are exposed to more pathogens are more likely to masturbate, but this is not the case for females. Masturbation seems to assist males to be prepared to mate swiftly with fresh sperm while clearing their reproductive tract of pathogens. In females, however, these two hypotheses do not align. The vagina's acidic environment, which typically protects against pathogens, becomes less acidic during arousal to ensure the survival of sperm, inadvertently providing a safer environment for pathogens.
The study did not find a connection between female primate masturbation and having multiple partners, but Brindle suspects further data may reveal a link. She attributes the scarcity of data on females to the historical perception of female animals as merely the recipients of male actions.
Brindle observes a decline in the stigma surrounding research on masturbation and sexual behavior, which gives her hope for future breakthroughs. But Roth cautions that to truly understand the evolutionary implications of this behavior, we must widen our scope beyond primates, given that other mammals, birds, and reptiles exhibit similar behavior.