Brood-provisioning strategies in Dawson's burrowing bee, Amegilla dawsoni (Hymenoptera : Anthophorini)

Males of Dawson's burrowing bee are dimorphic in size. Although large (majo r) males defeat smaller ones in competition for emerging females and theref ore are more likely to mate, majors are greatly outnumbered by half-sized ( minor) males. Nesting females might produce many minor males, despite their low reproductive value, because female behaviour is governed by a mixed ev olutionarily stable strategy (ESS), in which case the ratio of majors to mi ners should not be affected by changes in female condition. In contrast, a conditional-strategy hypothesis predicts that older, wing-worn or stressed females unable to forage efficiently should be especially likely to produce minor offspring, which require less brood provisions. To test these altern ative hypotheses, we manipulated the condition of nesting female bees by th e addition of weights and the removal of their wing margins. These manipula tions, done early in the flight season, failed to increase the production o f minor males, a result consistent with the mixed-ESS hypothesis. However, unmanipulated females were far more likely to produce minor males if they w ere small or if they were nesting late in the season, when foraging conditi ons had deteriorated, findings that are consistent with a conditional provi sioning strategy. Thus it appears that the abundance of minor males is the result of a conditional provisioning strategy of nesting females, which may be superimposed on a fixed tendency to produce large offspring early in th e season and small ones later.