Patricia Gowaty: UCLA Department of Ecology and Evolutionary Behavior

Do genes for choosy females and indiscriminate mates determine typical sex roles; or, do ecological and social constraints determine sex roles? Or is sex role determination due to more complex interactions of sex-associated genes and ecological conditions? Answering this question has been difficult, largely because until now there has been no null theory of ecological constraints on sex-associated “sex role” behavior. Here we describe the switchpoint theorem (SPT) that analytically solves the problem of what fraction of potential mates in a population a focal individual of either sex should find acceptable to mate in order to maximize relative lifetime fitness; the SPT is a null model of ecological constraints on reproductive decision-making. The SPT assumes that demographic stochasticity affects time available for mating and that there is variation in fitness that would be conferred by mating with alternative potential mates, to prove that environmental induction of flexible, reproductive decision-making of individuals of both sexes is adaptive. The SPT provides a reasonable scenario for the assumption that all individuals assess likely fitness outcomes before accepting or rejecting a potential mate. Rather than assuming sex differences in genes for choosy females and indiscriminate males, the SPT is a sex-neutral hypothesis that begins with individual differences in ecological constraints to predict induced and adaptive sex role variation. The predictions of the SPT depend on five parameters: individual survival and encounter probabilities, population size, the distribution of fitness that would be conferred by mating with alternative potential mates, and individual latencies (time outs after copulation). The SPT predicts that all else equal, higher probabilities of individual survival and encounters, higher population sizes, and longer latencies decrease the fraction of acceptable mates for focal individuals, so that focal individuals reject more potential mates. Similarly, it predicts that when instantaneous survival and encounter probabilities decline, adaptively flexible individuals accept more potential mates. The SPT provides a novel, quantitative, unified framework for studying how sexual selection and sexual conflict may operate when individuals manipulate the time available for mating of potential mates, their mates, and rivals.