Spontaneous nocturnal erections and masturbation in captive male chimpanzees (Pan troglodytes)

Abstract Nocturnal penile tumescence (NPT) has been reported for males of only a few mammalian species, including humans, albeit this scarcity might be an artefact of the difficulty of documenting it. We investigated NPT in 12 adult male chimpanzees living in an all-male group at Kumamoto Sanctuary, Japan. Recorded non-invasively with infrared video cameras across 72 nights (6 per individual), we observed NPT in two thirds of the individuals (8/12), with events in total. More than three quarters occurred during a transition from sleep to wakefulness, suggesting NPT may be associated with rapid eye movement sleep, similar to humans. Masturbation occurred in conjunction with NPT, including the likely consumption of ejaculate, in 4–6 individuals. While detailing nocturnal erections in chimpanzees may help us to better understand this robust physiological phenomenon in humans, further comparative research is necessary to reconstruct its phylogenetic history. At present, our findings support hypothetical functional explanations such as the facilitation of nocturnal emissions in order to increase ejaculate quality or an increase in tissue oxygenation to prevent erectile dysfunction.