Alternative mating strategies in a desert grasshopper: evidence of density-dependence

Field observations revealed that, on a given day, male mating behaviour in a population of the grasshopper Ligurotettix coquilletti ranged from little or no stridulation (inactive) to relatively persistent singing (active signalling). Inactive males were usually located in the territories of active signallers. Actively signalling male achieved more matings, and also more frequently approached and mounted females in incidents terminating in the male departing the female without copulating. Individuals switched between inactive and actively-signalling behaviour during their adult lives, and males that were usually active signallers achieved greater lifetime mating success. Thus, we obtained no evidence to support the hypothesis that variation in mating behaviour was maintained by negative frequency-dependent selection. By comparing the behaviour of males in two field plots maintained at different population densities, however, we found that high density was directly related to a higher incidence of inactive behaviour. Males perched on creosote (Larrea tridentata) bushes (their host plant), and certain bushes. Mating behaviour was independent of body size and, while young (<7 days old) males tended to be inactive, neither age nor the time of adult maturation during the season could fully account for the incidence of inactive behaviour among males. Instead, we suggest that the adoption of inactive behaviour resulted partly from aggressive encounters between males.