RELATIVE PARENTAL EXPENDITURE, POTENTIAL REPRODUCTIVE RATES, AND THE CONTROL OF SEXUAL SELECTION IN KATYDIDS

Traditional theory predicts that relative parental investment controls the ratio of fertilizable females to sexually active males-the operational sex ratio-and thereby determines which sex engages in mating competition. Because both parental investment and the operational sex ratio can be difficult to measure, the maximum number of independent offspring that can be produced per unit time-the potential reproductive rate-has been advocated as a general measure for predicting the direction of mating competition. Katydids show reversals in the pattern of mating competition that are associated with reversals in relative parental expenditure. Here I show that relative parental expenditure and relative mating rates, equivalent to the operational sex ratio, are negatively correlated, as predicted by classical parental investment theory. However, I show that the control of sexual selection is independent of the maximum number of offspring produced per unit time. Sperm competition is shown to reduce realized male reproductive rate relative to females and could have an indirect effect on the operational sex ratio if males adjust their investment in accord with their confidence of paternity.