Sexual Selection and Sexual Differences in Mormon Crickets (Orthoptera: Tettigoniidae, Anabrus simplex)

Gwynne, D

The reproductive behavior of Mormon crickets (Anabrus simplex) at sites of high population densities differs from a site of low population density; at low densities males competed for females as mates and there was evidence that females rejected males. In contrast, at high densities the courtship roles were reversed-females competed and males were the discriminating sex. Sexual selection on high density females is thus expected to be higher; supporting this prediction, variance in the mating success of high density females was higher than that of low density females. Disproportionate mating by large females occurred at high densities but not at low densities; this was partly a result of male discrimination against smaller females at high densities. Sexual selection for larger females at high densities has apparently resulted in the sexual dimorphism in body size observed at these sites but not observed at the low density site. The very large male investment in a nutritious spermatophore, relative to female parental investment, is apparently the cause of the observed reversal in the typical differences between the sexes of Mormon crickets. However, this difference in investment does not explain the lack of role-reversal at the low density site where nutritional investment by the male does not appear to be any less than investment by high density males. It is suggested that shortage of males able to produce spermatophores at high population densities results in males becoming limiting for female reproduction. Mormon crickets at high densities competed for nutritious food sources and showed evidence of food-limitation. Shortage of food in the environment is expected to decrease the number of males able to produce spermatophores at any one time. The number of males with large reproductive accessory glands (which produce the spermatophore) available at each site supports this hypothesis.

Sexual Selection and Sexual Differences in Mormon Crickets (Orthoptera: Tettigoniidae, Anabrus simplex)

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