Sexual selection and the potential reproductive rates of males and females

PRONOUNCED sex differences in mating competition are a prominent feature of many animal breeding systems. These differences are widely attributed to sex differences in parental investment1,2 which bias the ratio of sexually receptive females to males3 (the operational sex ratio), generating more intense competition between members of one sex, usually males3-5. Unfortunately, relative parental investment1 is usually impossible to measure in species where both sexes invest in their offspring6,7 and there is currently no empirical basis for predicting the pattern of mating competition in these species. In contrast, the potential rate of reproduction by males and females (measured as the maximum number of independent offspring that parents can produce per unit time) is both more directly related to the operational sex ratio and more easily estimated in natural populations7. Here we show that among species where males care for the young, the sex with the higher potential reproductive rate competes more intensely for mates than the sex with the lower potential rate of reproduction. © 1991 Nature Publishing Group.