Mating Success and Survival Rate in a Population of Damselflies: Results at Variance with Theory?

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Abstract
Sexual-selection theory makes two predictions. (1) Males usually vary more in mating success than do females (the Bateman principle), and this difference should be more pronounced in sexually dimorphic, polygamous species than in non-dimorphic, monogamous ones. (2) Brightly colored males trade an increased risk of predation for greater mating success and experience higher mortality than cryptically colored females. We tested these predictions by comparing lifetime mating success and mortality rates between males and females of Ischnura gemina, a sexually dimorphic, polygamous damselfly. Contrary to prediction, we found no significant differences between the sexes in variance in mating success, evenness of mating success, mean number of matings per lifetime, or mean life span. Comparison of the potential for sexual selection in I. gemina with that in other species for which lifetime mating success has been measured shows values for I. gemina most similar to those of the monogamous, non-dimorphic kittiwake, suggesting that sexual dimorphism in I. gemina is not the result of sexual selection. It is possible, however, that sexual selection was important in past evolutionary change in I. gemina but is not currently important, perhaps because of a lack of genetic variance in genes controlling color pattern and mating behavior. A number of characteristics of its mating system could be responsible for the apparently low potential for sexual selection in the I. gemina population we studied. These include habitat structure, non-territoriality of males, and contact guarding of females by males. The unprofitable-prey hypothesis of Baker and Parker (1979) provides an alternative explanation for the evolution of sexual dimorphism and predicts a higher mortality for cryptically colored females than for brightly colored males. Current data do not support this hypothesis as an explanation for the evolution of sexual dimorphism in I. gemina.