Seasonal variation in selection on male calling song in the field cricket, Gryllus rubens

Abstract

Conspicuous male traits, such as calling to attract females, evolve through the joint action of sexual and natural selection. When the costs of natural selection increase relative to the benefits of sexual selection, males should become less conspicuous. Male calling song in field crickets attracts both phonotactic females for mating and acoustically orienting parasitic flies that deposit larvae on calling males, killing their infected hosts. Flies are present mainly during the autumn, which means that crickets that become adults at this time have a different selective regime from those that emerge in the spring. On this basis we hypothesized that seasonal changes in parasitism would favour seasonal changes in male calling behaviour. We evaluated this hypothesis by recording the calls of individual males and by conducting nightly surveys of male calling in the field. Because seasonal changes in sexual selection may favour seasonal changes in male calling behaviour, we used pitfall traps to determine whether the number of females attracted to male calls changed seasonally. As predicted, fewer males called during the autumn than during the spring, and autumn males were less likely to call at dusk, when flies were most active. Contrary to prediction, however, males that called spent more time calling per night in the autumn than in the spring. Pitfall trap data revealed that more calling per night resulted in more female arrivals in the autumn but not in the spring. This result suggests that males that call for long periods attract more females in autumn but not in spring. This means that seasonal variation in both natural and sexual selection has resulted in seasonal differences in male calling behaviour.