Potential Reproductive Rates and the Operation of Sexual Selection

Abstract

In most animals, members of one sex compete more intensely for mates than members of the other sex, and show a greater development of secondary sexual traits. The relative intensity of mating competition in the two sexes depends on the operational sex ratio (OSR) (the ratio of males that are ready to mate to females that are ready to mate) at the site and times when mating occurs. The extent and direction of biases in the OSR is closely related to the potential rates of reproduction that individual males and females can achieve, although the distribution of the two sexes in space and time, sex differences in development time or life expectancy, and biases in the sex ratio at birth or hatching can also be important. The potential rates of reproduction in the two sexes are, in turn, affected by the proportion of time and energy expended by male and female parents on their progeny, though other factors may constrain reproductive rate in one or both sexes. We outline a simple model of the factors affecting the OSR, where relative parental expenditure by the two sexes and the adult sex ratio are fixed, and a more complex model where the adult sex ratio varies in relation to the reproductive activity of the two sexes. This framework for relating sex differences in mating competition to the OSR, potential reproductive rates, and parental expenditure differs from Triver's concept of the relation between parental investment and mating competition in three ways: first, it identifies the OSR as the immediate factor determining which sex completes most intensely for mates; second, it recognizes that factors other than the future fitness costs of rearing offspring can affect the potential reproductive rate of the two sexes; third, it suggests an empirical measure (potential reproductive rate) that can be estimated in natural populations and used to predict the distribution of mating competition.