Homosexual behaviour in birds: frequency of expression is related to parental care disparity between the sexes

Homosexual behaviour occurs in over 130 species of birds, yet explaining its maintenance in evolutionary terms appears problematic at face value, as such sexual behaviours do not seem in immediate pursuit of reproductive goals. Parental care sexual conflict theory predicts that release from parental care translates to an increased propensity towards polygamous sexual behaviour. We hypothesized that homosexual behaviour(s) may be expected to increase in frequency for the sex that invests less in parental care and potentially enjoys increased mating opportunities. Consistent with our predictions, lower relative contribution to parental care for a particular sex is related to increased frequency of occurrence of homosexual behaviour. For males, highly polygynous species with minimal male parental investment exhibit higher frequencies of male homosexual behaviour, including male–male mounting and especially courtship. In socially monogamous species, male parental investment is greater, and the expression of male homosexual behaviour is lower. Similarly, among pair-bonding species, frequencies of male–male pair bonding increase with decreases in male contribution to care relative to females. When females of socially monogamous species provide less care than males, they exhibit higher frequencies of homosexual behaviour, namely pair bonding and courtship activities. Conversely, when females of polygynous species provide the bulk of parental care, female–female sexual behaviour is infrequently expressed. Homosexual behaviour in birds is more likely to occur under scenarios of enhanced mating opportunity without necessarily influencing reproductive success and thus may exist neutrally, or alternatively provide a behavioural template co-opted for adaptive design.