Bumblebee response thresholds and body size: does worker diversity increase colony performance?

Although models of colony organization in social insects often rely on the assumption that within-group variation increases group performance, empirical support for this is mostly confined to studies of genetic variation. However, workers in ant or bee colonies often vary in behaviour and morphology even when genetic variation is low. Bumblebees provide a unique opportunity to explore the consequences of such variation: colonies have a wide range of worker body sizes compared to other social bee species, and workers also vary in response thresholds (i.e. stimulus levels at which workers respond by performing a task), in spite of queens being singly mated (and thus, low genetic variation). Here we test how body size and response threshold diversity affect colony performance in two unrelated in-nest tasks (thermoregulation and undertaking). We manipulated worker diversity using worker removals to restrict threshold or body size variation within the colony. We also quantified the degree of intracolony variation across colonies and related this to colony performance. In general, colonies took longer to cool the nest after bees were removed, but there was no significant effect of treatment on fanning or undertaking success. Furthermore, when intracolony variation was analysed as a continuous variable, we found no effect on colony-level thermoregulation or undertaking performance. Instead, average threshold was a more useful predictor of thermoregulation success, and colonies with a narrower range of size variation had more success at undertaking. These results emphasize the importance of understanding how different types of variation (e.g. behavioural, morphological, etc.) contribute to colony performance.