DOI: 10.5061/dryad.pzgmsbcqb
title: High thermal tolerance in high elevation species and laboratory-reared colonies of tropical bumble bees

description: 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. 
description: 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).<br><br>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.