Predicting the direction of sexual selection

Our current understanding of the operation of sexual selection is predicate d on a sex difference in parental investment, which favours one sex becomin g limiting and choosy over mates, the other competitive and nonchoosy. This difference is reflected in the operational sex ratio (OSR), the ratio of s exually receptive males to females, considered to be of fundamental importa nce in predicting the direction of sexual selection. Difficulties in measur ing OSR directly have led to the use of the potential reproductive rates (P RR) as a measure of the level of investment in offspring of males and femal es. Several recent studies have emphasized that other factors, such as vari ation in mate quality and sex differences in mortality patterns, also influ ence the direction of sexual selection. However, as yet there has been no a ttempt to form a comprehensive theory of sex roles. Here ne show that neith er OSR nor PRR is the most fundamentally important determinant of sex roles , and that they are not interchangeable. Instead, the cost of a single bree ding attempt has a strong direct effect on competition and choosiness as we ll as consistent relationships to both OSR and PRR. Our life history based approach to mate choice also yields simple, testable predictions on lack of choice in either sex and on mutual mate choice.