Ecological determinants of sex roles and female sexual selection

Abstract

Darwinian sex roles characterized by competitive males and choosy females were long thought to prevail in animals, driven by differences in energetic investment between male and female sex cells. Here we review research on the flower-feeding bushcricket Kawanaphila nartee which has been instrumental in our understanding of the evolution of sex roles. It is now clear that sex roles are determined by many factors, including ecological variables. The sex roles of K. nartee were shown to reverse dynamically according to environmental food availability: when food is scarce, females compete for nuptial gifts and males become choosy; when food is abundant, sex roles are conventional. Interestingly, female K. nartee possess more developed hearing than do males. It has been suggested that female hearing sensitivity is beneficial in intrasexual competition during sex role reversal, when females scramble for access to acoustically signaling males, suggesting that even temporary periods of sexual selection can result in the evolution and maintenance of sex-specific characters in females. Research on K. nartee has underscored the importance of ecology to reproduction by showing that food availability can affect the operational sex ratio and thus sex roles, indicating that sex roles are not necessarily fixed but rather respond plastically to environmental factors.