Hot bumble bees at good food: thoracic temperature of feeding Bombus wilmattae foragers is tuned to sugar concentration.

The ability of bees to generate metabolic heat plays an important role in their ability to forage and pollinate because they must achieve a minimum temperature to activate their flight muscles. In honey bees and stingless bees, the thoracic temperature of feeding foragers is correlated with the caloric value of sucrose solution provided at feeders outside the nest. We provide the first detailed data showing that this phenomenon also occurs in the closely related bumble bee and thus may be homologous in all social bees of the Apidae. Using infrared thermography, we measured T(th) for Bombus wilmattae foragers (mass 0.17+/-0.11 g, length 15.0+/-1.5 mm) from six wild colonies, foraging on a range of sucrose concentrations (0.5-2.5 mol l(-1), 16-65% by mass) in foraging arenas. For all colonies, we measured significant increases in DeltaT(th) (P<0.0001) with increasing sucrose concentration, with significant differences (P<0.0001) between colonies due to different linear regression slopes (0.28-2.4) and y-intercepts (2.7-5.5). We suggest that this modulation of pitching T(th) to sucrose concentration is a general phenomenon in all social bees and may be a widespread adaptation facilitating rapid food collection in flying Hymenoptera.