Influence of pile length and body size on rates of heat loss in the bumble bee, Bombus vosnesenskii, The

Variation in bumble bee body size and pile length across environmental gradients has been addressed by a number of studies due to it’s presumed impact on cooling rate and, therefore, foraging success. However, very little work has been done to actually quantify the effects of that measured variation on thermal biology. We measured morphological variation across the range of a widespread bumble bee (Bombus vosnesenskii) for both field captured and common garden reared workers as well as the effects of that morphological variation on the passive cooling rates of the bees. We find that increasing body size and pile length both significantly decrease cooling rates, though the effect of pile is rather weak. Also, unexpectedly, bees from warm climates tend to be larger, hairier, and cool slower than their cold climate counterparts. Based off the weak effect of pile length on convective cooling and the counterintuitive trend of long haired bees living in warm climates, we suggest that in B. vosnesenskii pile does not act primarily as a means of insulation from convective cooling, but instead as a shield from radiative heat gain as their mostly black hair absorbs the radiative input and looses the heat to convection before it conducts down to the animal’s body.