Is diversity in worker body size important for the performance of bumble bee colonies?

Abstract

Specialization and plasticity are important for many forms of collective behavior, but the interplay between these factors is little understood. In insect societies, workers are often predisposed to specialize in different tasks, sometimes with morphological or physiological adaptations, facilitating a division of labor. Workers may also plastically switch between tasks or vary their effort. The degree to which predisposed specialization limits plasticity is not clear and has not been systematically tested in ecologically relevant contexts. We addressed this question in 20 freely-foraging bumble bee (Bombus terrestris) colonies by continually manipulating colonies to contain either a typically diverse or reduced ("homogeneous") worker body size distribution, over two trials. Pooling both trials, diverse colonies did better in several indices of colony performance. The importance of body size was further demonstrated by the finding that foragers were larger than nurses even in homogeneous colonies with a very narrow body size range. However, the overall effect of size diversity stemmed mostly from one trial. In the other trial, homogeneous and diverse colonies showed comparable performance. By comparing behavioral profiles based on several thousand observations, we found evidence that workers in homogeneous colonies in this trial rescued colony performance by plastically increasing behavioral specialization and/or individual effort, compared to same-sized individuals in diverse colonies. Our results are consistent with a benefit to colonies of predisposed (size-diverse) specialists under certain conditions, but also suggest that plasticity or effort, can compensate for reduced (size-related) specialization. Thus, we suggest that an intricate interplay between specialization and plasticity is functionally adaptive in bumble bee colonies.