Abstract: Physiological adaptations to temperature shape the spatial and temporal distribution of species. Thermal tolerance can be modified by individual morphology and behavior, and often links tightly with other, more easily measured intrinsic traits, such as body size and life-history stage. Linking patterns of life-history traits with thermal sensitivity may be useful for predicting species vulnerability to future climate scenarios, especially for species key to ecosystem functioning, such as wild bees. Among bees, bumble bees (Bombus sp.) appear to be especially sensitive to climatic warming. In addition, bumble bees are eusocial, with distinct castes within the colony. Each caste (queens, workers, males) is active at different times of the year and represents a different life stage of the colony, thus there may be caste-specific vulnerabilities to abiotic conditions. Effectively predicting bumble bees’ vulnerability to future climate scenarios therefore requires an integrated understanding of traits, life stage, and physiology.
Here, we ask how bumble bee thermal tolerance correlates to body size and caste. Our ongoing laboratory experiments with Bombus impatiens suggest that there are caste-specific relationships with critical thermal maximums (CTmax). Specifically, CTmax declined 0.08 ºC per 10 mg of body weight in workers and 0.1 ºC/10 mg in males, while we find no relationship between size and CTmax in queens. Males exhibited higher upper thermal limits than workers, suggesting that thermal tolerance is influenced by the temperatures an individual habitually experiences. Workers return to the nest between foraging bouts and may benefit from colony-level thermoregulation of the nest regulation, while males leave their natal nest and must contend with temperature extremes as individuals. Thus, there may be caste-specific vulnerabilities to extreme temperatures with differential consequences for colony growth and reproduction.
