The Evolution of Masturbation in Birds

Most theories on sexual behaviours are based on adaptive explanations. However, masturbation, a common but scarcely discussed sexual behaviour, appears a Darwinian puzzle. Why would individuals waste valuable resources such as time, energy and, for males, sperm? We combined targeted surveys with information from published accounts to test hypotheses about why masturbation occurs, using a phylogenetically broad dataset on the presence or absence of masturbation across 120 bird species. We find masturbation is widespread in birds, but strongly phylogenetically conserved, typically being fixed (as present or absent) or nearly so within broad clades. Masturbation is more common in males, though the widespread evidence for masturbation in females suggests that maintaining fresh sperm in testes cannot be the single explanation. We find no difference in masturbation occurrence between juveniles and adults, further suggesting that it does not solely represent practice copulations before maturity. Species with indiscriminate matings are more likely to masturbate than socially monogamous species or species with long-term pair bonds. Importantly, as masturbation is more commonly reported in wild than captive birds, we find it is therefore not a negative or maladaptive response to captivity. Instead, it is part of a wider repertoire of sexual behaviours exhibited in birds. Overall, our data are consistent with masturbation serving as an outlet for increased sexual arousal, a means of increasing reproductive success through postcopulatory selection, or both.