Testing Parental Investment and the Control of Sexual Selection in Katydids: The Operational Sex Ratio

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Abstract
Theory predicts that relative parental investment by the sexes controls the ratio of the numbers of fertilizable females and sexually active males (the operational sex ratio, OSR) and thus sexual selection and sexual differences. This paper tests the hypothesis that variation in parental investment controls the numbers of males and females available for mating. In katydids, males and females invest parentally by providing material investment to eggs, with the male donation derived from spermatophore materials eaten by the female. Previous work with katydids in nature has shown intraspecific variation in sexual selection acting on females; only certain populations show a courtship role reversal in which females complete for access to males. These observations led to the hypothesis that the availability of food in nature determines the frequency of spermatophore production and thus the number of males available for mating. In the present study, male and female katydids were maintained on diets that differed in food quality. Both sexes were given daily access to sexually receptive members of the opposite sex. As predicted, males on a low-quality diet mated less frequently than males on a high-quality diet. The opposite trend was observed for females: females on a low-quality diet increased mating frequency, apparently to obtain additional spermatophore nutrients. These data suggest that male parental investment in the spermatophore increases in its relative importance when diet is low in quality. The effects of diet on mating frequency result in different estimates of the OSR, which accord with observations in nature: for the high-quality diet, there are fewer fertilizable females than sexually active males; similar conditions in nature would be predicted to result in a male-to-male competitive (polygynous) mating system. For the low-quality diet, there were slightly fewer fertilizable females, indicating that such a diet may result in competition among females.