Body size, metabolism, and longevity in worker bumble bees (Bombus impatiens)

Longevity is an important life history trait. Body size and metabolism generally affect longevity such that larger organisms have lower mass-specific metabolic rates and live longer. However, results from intraspecific studies that analyze the relationships between body size, metabolism, and longevity are inconsistent. In social insects, defining these relationships can be useful in understanding social organization and the evolution of sociality. Bumble bee workers (Bombus impatiens), which show significant variation in body size, were used in this study which investigated the relationships between body size, metabolic rate, and lifespan. In the first experiment, the relationship between body size (measured as thorax width) and lifespan were analyzed. In the second experiment, respirometry was used to track worker metabolic rate from eclosion to death. We found that 1) larger bees lived longer; 2) workers with lower mass-specific metabolic rates at eclosion had significantly longer lifespans; and 3) in workers, mass-specific metabolic rate decreased with age. Our study shows that in B. impatiens, body size affects lifespan, and workers that expend greater energy per unit mass die sooner. Moreover, mass-specific metabolic rate decreases with worker age. Our study provides evidence for energetic senescence in bumble bees and shows a unique trade-off between energy use and lifespan. We plan to study the relationship between energy use and work effort. Combined with lifespan data, this future work will provide information on the dynamics of energy allocation within colonies and the organization of work effort among colony members.