Seasonal shifts in thermoregulatory behaviour of bumble bee queens.

Many organisms cope with highly variable environmental temperatures by differentiating body temperature from that of the environment through thermoregulation. Heterotherms can both endogenously maintain body temperature with metabolic heat and behaviourally thermoregulate by selecting suitable microclimates. Expending excess energy to maintain stable high body temperatures may be prioritized during certain times of the year, or for certain activities (e.g. reproduction, escape from predation). Alternatively, behavioural thermoregulation may take precedence when energy savings are critical. The degree to which heterotherms rely on these different strategies has rarely been studied. To address this gap, we measured body temperature and selected temperatures on a thermal gradient for heterothermic bumble bee queens (Bombus huntii) at two life stages: during spring, when ovary development prior to colony establishment is critical, and in autumn, when they build energy stores for overwintering. Not only did spring queens have a narrower range of body temperatures than autumn queens, but they maintained higher body temperatures at cooler gradient temperatures. These results suggest that thermoregulatory strategy varies seasonally to sustain key activities but is context dependent: when ambient temperatures are cool, metabolic heat production is relied upon if reproductive pressure is high and reduced if accumulating energy reserves is critical.