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 if bees’ thermal tolerance decreases with elevation and if the stable optimal conditions used in laboratory-reared colonies reduces their thermal tolerance. We assessed changes in the lower (CTMin) and upper (CTMax) 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 B. pauloensis. We also compiled information on bumble bees’ thermal limits and assessed potential predictors for broad-scale patterns. We found that CTMin decreased with increasing elevation while CTMax was similar between elevations. CTMax was slightly higher (0.84 °C) in laboratory-reared than in wild-caught bees while CTMin was similar, and CTMin decreased with increasing body size while CTMax did not. Latitude is a good predictor for CTMin only while annual mean temperature, maximum and minimum temperatures of the warmest and coldest months are good predictors for both CTMin and CTMax. The stronger response in CTMin with increasing elevation, and similar CTMax, 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. 

We conducted thermal limits assays with four of the nine species of bumble bees that occur in Colombia: Bombus (Cullumanobombus) hortulanus Friese, B. (Cullumanobombus) funebris Smith, B. (Thoracobombus) pauloensis Friese, and B. (Cullumanobombus) rubicundus Skorikov. Between February and May 2021, we collected bumble bees from two locations in the Department of Cundinamarca, Colombia, chosen for their accessibility, abundance of bumble bees, and range of elevations: Tenjo (4˚51.410’N, 74˚06.468’W, 2589 m), an agricultural area on the Bogota’s high plain, and Matarredonda (4˚33.121’N, 73˚59.927’W, 3400–3600 m), an area with preserved Paramo vegetation about 40 km southeast of Tenjo (Fig 1S). The composition and abundance of bumble bees varied between sites, with B. pauloensis occurring in Tenjo while B. hortulanus, B. funebris, and B. rubicundus in Matarredonda. At each location, we collected bees with a net and transferred them individually to plastic containers, each made with a vial covered with a net mesh. To assess for differences in the thermal limits between wild caught and laboratory-reared bees, we tested individuals from colonies of B. pauloensis that were initiated from gynes captured in Sopó, Cundinamarca (4˚55’N, 73˚56’W, 2600 m).

We measured heat and cold tolerances using a dynamic (ramping temperature) protocol with the Elara 2.0 (IoTherm, Laramie, WY), a portable fully programmable heating/cooling anodized aluminum stage. We placed bees individually inside glass vials (50×15 mm, 3.70 cm3) and plugged them with a moistened cotton ball (~ 0.2 mL of distilled water per cotton ball) to ensure enough humidity during the assays. We used an initial temperature of 22 °C and held bees for 10 minutes at this temperature before increasing it or decreasing it at a rate of 0.5°Cmin−1. To estimate the temperature inside the vials, we placed a K-type thermocouple inside two empty glass vials plugged with a cotton ball. We individually tracked these vial temperatures using a TC-08 thermocouple data logger (Pico Technology, Tyler, TX, USA). As an approximation of bees’ thermal limits, we used the temperature at which bees show signs of curling (CTMin, Oyen and Dillon, 2018) or lost muscular control, spontaneously flipping over onto their dorsa and spasming (CTMax, Lutterschmidt and Hutchison, 1997; García-Robledo et al., 2016; 2018). Then, for each specimen, we recorded its minimum intertegular distance (ITD) as a proxy of body size (Cane, 1987). As in Oyen & Dillon (2018), we tested the same individual for CTMax and CTMin, starting by measuring CTMin with a period of acclimation at room temperature (20 min at 20–22 °C) before measuring CTMax.