The switch point theorem posits that both sexes should exhibit behavioural flexibility, switching between discriminant and indiscriminate mating, as their perceived demographic conditions change. The theorem also predicts that an individual’s mate choice should be more dependent on its own intrinsic characteristics and ecological conditions than on the exaggerated sexually selected traits possessed by potential mates. To test these predictions, we manipulated the operational sex ratio (OSR), a demographic factor, of adult Jamaican field crickets (Gryllus assimilis) under controlled laboratory conditions and evaluated whether the mate choices of females and males were influenced by their social environment, their own body size and the difference in the body size of available potential mates. Body size is under sexual selection in this species, with females preferring larger males and larger males exhibiting repeatable mate choice. We predicted that changes in the OSR should influence the mating behaviour of both sexes, as the OSR determines the probability that an individual will encounter a potential mate. We observed that the OSR affected the flexibility of male, but not female, mate choice. Additionally, the OSR influenced male behaviour, with larger males from the female-biased OSR taking longer to make a choice than larger males in the male-biased and even ratio OSR treatments. Mate choice in females was influenced by their intrinsic body size (only large females were choosy) and by the exaggerated traits expressed by potential mates (large females were only choosy when potential mates differed considerably in body size). Collectively, these findings provide partial support for the predictions of the switch point theorem as both sexes exhibited flexible mating decisions, switching from discriminant to indiscriminate mating under varying demographic conditions.
Department of Biology

Villarreal, A.E. (Amy E.), Godin, J.-G.J, & Bertram, S.M. (2018). Influence of the operational sex ratio on mutual mate choice in the Jamaican field cricket (Gryllus assimilis): Testing the predictions of the switch point theorem. Ethology, 124(11), 816–828. doi:10.1111/eth.12816