The effect of competitors on calling effort and life span in male field crickets

Temporal variation in the strength of selection on male sexual traits is often attributable to changes in the social environment that alter the number of competitors. Selection could favor phenotypic plasticity in male investment into sexual traits if there are cues indicative of current and, possibly, future levels of mating competition. In many taxa, males court more intensely when rivals are present, but the extent to which phenotypically plastic responses differ predictably among males is less well studied. For example, will larger males show a greater or smaller change in courtship in response to the presence of rivals? In addition, the effects of any changes in courtship on key life-history traits have been understudied. In this study, we experimentally tested male crickets (Teleogryllus commodus) from 3 populations to determine how the presence or absence of a rival affects: 1) calling effort, 2) life span, and 3) whether male body size (correlated with dominance) influences any changes in life span or calling effort. Calling effort increased significantly with body size, mainly due to daily calling effort increasing with age and larger males living longer. Considering all males, there was no effect of rival presence on lifetime calling effort. However, within the "rival present" treatment, after correcting for age, the longer lived of 2 "paired" males called significantly more before his rival died than afterwards. This implies that there is a plastic shift in courtship effort. Finally, larger males lived significantly longer and, crucially, males housed with a rival had, on average, a significantly shorter life span.