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 operat ional sex ratio-and thereby determines which sex engages in mating com petition. Because both parental investment and the operational sex rat io can be difficult to measure, the maximum number of independent offs pring that can be produced per unit time-the potential reproductive ra te-has been advocated as a general measure for predicting the directio n of mating competition. Katydids show reversals in the pattern of mat ing competition that are associated with reversals in relative parenta l expenditure. Here I show that relative parental expenditure and rela tive mating rates, equivalent to the operational sex ratio, are negati vely correlated, as predicted by classical parental investment theory. However, I show that the control of sexual selection is independent o f the maximum number of offspring produced per unit time. Sperm compet ition is shown to reduce realized male reproductive rate relative to f emales and could have an indirect effect on the operational sex ratio if males adjust their investment in accord with their confidence of pa ternity.