Small worker bumble bees (Bombus impatiens) are hardier against starvation than their larger sisters.

In bumble bees (Bombus spp.), where workers within the same colony exhibit up to a tenfold difference in mass, labor is divided by body size. Current adaptive explanations for this important life history feature are unsatisfactory. Within the colony, what is the function of the smaller workers? Here, we report on the differential robustness to starvation of small and large worker bumble bees (Bombus impatiens); when nectar is scarce, small workers remain alive significantly longer than larger workers. The presence of small workers, and size variation in general, might act as insurance against times of nectar shortage. These data may provide a novel, adaptive explanation, independent of division of labor, for size polymorphism within the worker caste.

Comparative study in stingless bees (Meliponini) demonstrates that nest entrance size predicts traffic and defensivity.

Stingless bees (Meliponini) construct their own species-specific nest entrance. The size of this entrance is under conflicting selective pressures. Smaller entrances are easier to defend; however, a larger entrance accommodates heavier forager traffic. Using a comparative approach with 26 species of stingless bees, we show that species with greater foraging traffic have significantly larger entrances. Such a strong correlation between relative entrance area and traffic across the different species strongly suggests a trade-off between traffic and security. Additionally, we report on a significant trend for higher forager traffic to be associated with more guards and for those guards to be more aggressive. Finally, we discuss the nest entrance of Partamona, known in Brazil as boca de sapo, or toad mouth, which has a wide outer entrance but a narrow inner entrance. This extraordinary design allows these bees to finesse the defensivity/traffic trade-off.