Male field cricket song reflects age, allowing females to prefer young males

Sexual selection often involves female preference for males of a certain age, and a body of theory predicts preference for old males. We measured a comprehensive set of traits from the acoustic sexual display of male field crickets, Gryllus bimaculatus, and found that nearly all song traits changed predictably as males aged, involving a general slowing down of the wing movements during song production. Our female preference experiments indicated a strong and repeatable preference for the songs of young males, contradicting the existing literature, which argues that female crickets prefer older males on the basis of changes in song carrier frequency. Rather, female preference for young male song was determined by its high energetic quality. We develop the 'old flight muscle' hypothesis, arguing that age-related degradation of stridulatory muscle performance is likely to result in the observed changes with age. Secondary sexual characters may be subject to oxidative somatic degradation suggesting that, when males provide only sperm, females should prefer the sexual displays of young males. Our results support new modelling approaches and a growing body of empirical evidence suggesting that old males are not always preferred by females.