Trait scaling and foraging in bumble bees

The relationships between organismal form and function are key components of understanding diversity, especially among morphologically similar groups. As organisms scale in size and shape, trait covariance can influence processes such as resource use, both through direct foraging traits and cognitive correlates, such as brain size. We investigated the patterns of trait scaling and covariance, and their relationships to foraging specialization, in an assemblage of wild bumble bees (genus Bombus). Specifically, we examined how foraging-relevant traits scaled with body size and covaried across the whole organism, and how these relationships correlated to social caste and diet breadth. Bumble bees largely exhibited hypoallometric scaling relationships: larger species and individuals had proportionally smaller heads and pollen load capacity. Larger head to body ratios, i.e., larger brain volumes, were positively associated with more generalized diets in queens. Among workers, higher trait covariation (i.e., morphological integration) was negatively associated with body size and positively associated with diet breadth – foragers of larger species show lower trait covariation and more specialized foraging, suggesting adaptive morphological constraints related to the flexibility of resource use. Our findings indicate that morphological variation is important to bumble bee foraging ecology, but trait integration across the whole organism exhibits idiosyncratic patterns among castes.