Abstract
Bumble bees are key pollinators with some species reared in captivity at a commercial scale, but with significant evidence of population declines and with alarming predictions of substantial impacts under climate change scenarios. While studies on the thermal biology of temperate bumble bees are still limited, they are entirely absent from the tropics where the effects of climate change are expected to be greater. Herein, we test whether bees' thermal tolerance decreases with elevation and whether the stable optimal conditions used in laboratory-reared colonies reduces their thermal tolerance. We assessed changes in the lower (CT(Min)) and upper (CT(Max)) critical thermal limits of four species at two elevations (2600 and 3600 m) in the Colombian Andes, examined the effect of body size, and evaluated the thermal tolerance of wild-caught and laboratory-reared individuals of Bombus pauloensis. We also compiled information on bumble bees' thermal limits and assessed potential predictors for broadscale patterns of variation. We found that CT(Min) decreased with increasing elevation, while CT(Max) was similar between elevations. CT(Max) was slightly higher (0.84°C) in laboratory-reared than in wild-caught bees while CT(Min) was similar, and CT(Min) decreased with increasing body size while CT(Max) did not. Latitude is a good predictor for CT(Min) while annual mean temperature, maximum and minimum temperatures of the warmest and coldest months are good predictors for both CT(Min) and CT(Max). The stronger response in CT(Min) with increasing elevation, and similar CT(Max), supports Brett's heat-invariant hypothesis, which has been documented in other taxa. Andean bumble bees appear to be about as heat tolerant as those from temperate areas, suggesting that other aspects besides temperature (e.g., water balance) might be more determinant environmental factors for these species. Laboratory-reared colonies are adequate surrogates for addressing questions on thermal tolerance and global warming impacts.