Task allocation and reproductive skew in social mass provisioning carpenter bees in relation to age and size

Abstract

The mass provisioning carpenter bees comprise two tribes, the Xylocopini and the Ceratinini. Although social nesting occurs in both tribes, no morphological castes have evolved and females are totipotent, which makes the tribe as a whole highly suitable to test predictions of reproductive skew models. We review current information for the two tribes with respect to reproductive competition and reproductive skew and then investigate whether the observed skew fits with predictions from optimal skew theory. Social nests of Xylocopa species include a non-foraging guard and a foraging egg layer who completely dominates reproduction. Reproductive dominance is settled by aggression, and the probability of winning this fight is influenced by both age and size. In Ceratina species, task allocation is also very clear: one female guards the nest, while the other female(s) forage(s). Although the guard is usually the first to produce an egg, her eggs are frequently replaced by those of the forager, and skew is incomplete. Using comparisons between species and genera the impact of ecological constraints on solitary nesting, relative group productivity and relatedness on reproductive partitioning between dominants and subordinates are investigated in a qualitative way. In support of the optimal skew model, strong constraints on solitary nesting coincided with strong skew. However, the predicted effects of relatedness and group productivity on skew were not found. Furthermore, no support was found for the predictions of the optimal skew model that high skew coincides with frequent aggressive testing and risky task performance by subordinates.