SEXUAL SELECTION AND SEXUAL DIFFERENCES IN MORMON CRICKETS (ORTHOPTERA: TETTIGONIIDAE, ANABRUS SIMPLEX).

Abstract

Darwin (1871) suggested that differences in the intensity of sexual selection on the sexes was a cause of secondary sexual differences. He proposed that behavioral and morphological traits possessed by males had evolved in the context of sexual competition, because fertilizable females are typically in short supply. Emlen and Oring (1977) pointed to several factors that are expected to change the ratio of fertilizable females to sexually active males (the operational sex ratio) and thus, the intensity of sexual selection on the sexes; theoretically both the investment of the sexes in offspring (parental investment; Trivers, 1972) and certain features of the social and physical environment are important. Parental investment theory (Trivers, 1972) predicts that females in species where males invest most in individual offspring should exhibit typically masculine traits by competing for access to sexually active males and males should exhibit typically feminine traits by being selective of mates. In vertebrate species where males provide the majority of parental care there is good evidence for a role-reversal in courtship behavior (Wittenberger, 1979; Petrie, 1983). In insects, paternal care of eggs and offspring is rare (Smith, 1980) but males of several groups supply their mates with nutritious prey items or glandular products (Thornhill, 1976a). These male efforts represent mating effort, as they apparently function in acquiring copulations (Alexander and Borgia, 1979). They are, however, nonpromiscuous mating efforts, potentially able to limit female reproduction (Thormhill, 1976b; Gwynne, 1984a) and thus are expected to influence the evolution of sexual differences in a similar way to male parental investment (Gwynne, 1984b). Males of many katydids (Orthoptera: Tettigoniidae) feed their mates with a large spermatophore which enhances female reproduction (Gwynne, 1984a). Mormon cricket males (Anabrus simplex Haldeman) produce a large spermatophore (Gillette, 1904), investing some fifth of their body weight in a single mating. I reported a reversal in the typical courtship roles for this species; females competed for access to singing males and males discriminated among females by selecting heavier individuals as mates (Gwynne, 1981). Here I show that this courtship rolereversal is found at certain sites but not at others. I examine sexual selection and its consequences at different sites by (1) testing the prediction that sexual selection on females, as estimated by variance in mating success, should be greater at sites where sexual competition among females is observed, (2) detailing the differences in courtship behavior between sites that have apparently resulted from differences in sexual selection, (3) testing the prediction that female body size should be greater at role-reversed sites as a consequence of intersexual selection for larger females, and (4) describing possible environmental differences between sites that have caused interpopulation variation in sexual differences.