Female mate choice plasticity is affected by the interaction between male density and female age in a field cricket

Abstract

Female mate choices influence the strength and direction of evolution by sexual selection. Such choices are often plastic, changing in response to changes in environmental (e.g. predator abundance or conspecific density) and internal (e.g. female age) conditions. Although these factors can interact naturally, the effects of such interactions are only beginning to be studied. We used female variable field crickets, Gryllus lineaticeps, to examine how female age and perceived male density interact to affect the plasticity of an aspect of female mate choice: female choosiness. Female choosiness is a measure of female responses to male traits that deviate from the female's preferred male trait; the more choosy a female is, the weaker her response will be to male traits that deviate from the preferred value. We exposed females to either a high or a low male-calling density environment, then tested them at one of three ages. We assessed female choosiness by measuring the female's response to low-quality male song: females spending less time near the low-quality song were deemed more choosy. We found that the interaction between female age and perceived male density significantly affected female responses. Young females were the most plastic, showing significantly more choosiness in the high male density environment compared to the low male density environment. Intermediate and old females showed less plasticity and intermediate choosiness. Our results suggest that in populations in which most females are young, sexual selection may be strongest at high male densities.