MECHANISMS OF ORGANIZATION IN SOCIAL INSECTS: THE INFLUENCES OF SPATIAL ALLOCATION, DOMINANCE INTERACTIONS, AND WORKER VARIATION IN BUMBLE BEES

Abstract

Social insect workers can vary in terms of body size, space use, division of labor, and reproductive potential. Here, I begin to 'dissect' a social insect colony, using the bumble bee Bombus impatiens, to determine how this variation affects colony organization. I found that workers are spatially organized inside the nest and they remain at a specific distance from the colony center. Bees that feed larvae tend to remain in the center, whereas foragers are more often found on the periphery when not foraging. Smaller workers are more likely to feed larvae and incubate brood, and larger workers are more likely to fan or guard the nest. Still, workers perform multiple tasks throughout their life. The size of this task repertoire does not depend on body size or age. Furthermore, workers that remain further from the queen while inside the nest and avoid energy-expensive tasks during the ergonomic phase are more likely to reproduce by the end of the colony cycle. Inactive bees are not, however, defensive reserves. Although inactive bees increase their speed inside the nest when the nest was disturbed, they were not more likely to leave the nest (presumably to attack the simulated attacker) or switch to guarding behavior. This suggests that inactive bumble bees that remain farther from the queen may be storing fat reserves to later develop reproductive organs. Finally, I examined how within-group variation affects colony performance. I reduced variation in body size or temperature response thresholds by removing individuals from the colony with extreme phenotypes, and compared colony performance to colonies where random bees were removed. Colonies took longer to cool down the nest after bees were removed, but this effect was most striking when variation in temperature thresholds was reduced. Further, although larger bees are better at carrying items and are more likely to fan, the ability of colonies to perform undertaking behavior or thermoregulation was not affected when size variation was reduced. These studies provide evidence that (1) within-group variation affects colony organization and (2) variation among workers in their inherent tendencies to respond to stimuli positively affects colony performance.