High thermal tolerance in high elevation species and laboratory-reared
colonies of tropical bumble bees
1. Bumble bees are key pollinators with some species reared in captivity
at a commercial scale, but with evidence of population declines and with
alarming predictions under climate change scenarios. While studies on
the thermal biology of temperate species are still limited, they are
entirely absent from the tropics where the effects of climate change are
expected to be greater. 2. Herein we test if tropical bumble bees’ lower
(CTMin) and upper (CTMax) critical thermal limits decrease with
elevation and if the stable optimal conditions used in laboratory-reared
colonies reduces their thermal tolerance. 3. We assessed changes in
CTMin and CTMax of four species at two elevations (2600 and 3600 m) in
the Colombian Andes and of laboratory-reared individuals of B.
pauloensis. In addition, we examined the effect of body size and
compiled information on bumble bees’ thermal limits from the literature
to assess potential predictors for broad-scale patterns of variation. 4.
CTMin decreased with elevation while CTMax did not. CTMax was slightly
higher (0.84 °C) in laboratory-reared than in wild-caught bees while
CTMin was similar. CTMin decreased with increasing body size while CTMax
did not. Latitude is a good predictor for variations in CTMin while
annual mean temperature and extreme monthly temperatures are good
predictors for both CTMin and CTMax. 5. The stronger response in CTMin
with increasing elevation supports Brett’s heat-invariant hypothesis.
Tropical bumble bees appear to be about as heat tolerant as those from
temperate areas, suggesting that other aspects of climate besides
temperature (e.g., water balance) might be more determinant
environmental factors for these species under global warming.
Laboratory-reared colonies are adequate surrogates for addressing
questions on thermal tolerance and global warming impacts.