Can minor males of Dawson's burrowing bee, Amegilla dawsoni (Hymenoptera: Anthophorini) compensate for reduced access to virgin females through sperm competition ?

Dawson's burrowing bees (Amegilla dawsoni) exhibit a conditional mating strategy with two alternative tactics. Large (major) males exclusively patrol emergence sites in search of about-to-emerge females, whereas small (minor) males usually search the periphery of emergence sites for females that escape patrollers. About 80% of the male population are miners, despite the fact that patrolling emergence sites is apparently the more profitable mating tactic. We tested the hypothesis that minor males gain fitness by mating with nonvirgin females and engaging in sperm competition with rival ejaculates. If the sperm competition hypothesis applied, it would help explain why nesting females produce so many minor sons. Contrary to this hypothesis, however, we found that minor males do not exhibit traits frequently associated with sperm competition. Minor and major males did not differ in testis mass after controlling for body size. Neither did they differ significantly in the duration or pattern of copulation nor in the volume of ejaculate transferred. In addition, and also contrary to the sperm competition hypothesis, females apparently mated only once. Loss of female sexual receptivity occurred quickly after the onset of copulation, and nesting females appeared completely unreceptive. Thus, all aspects of the bee's mating system strongly indicate that sperm competition does not occur in Dawson's burrowing bee, so that miners cannot compensate even partially via sperm competition for their mating disadvantage with virgin females.