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.

abstract: A key factor in the success of social animals is their organization of work. Mathematical models have been instrumental in unraveling how simple, individual-based rules can generate collective patterns via self-organization. However, existing models offer limited insights into how these patterns are shaped by behavioral differences within groups, in part because they focus on analyzing specific rules rather than general mechanisms that can explain behavior at the individual-level. My work argues for a more principled approach that focuses on the question of how individuals make decisions in costly environments. In Chapters 2 and 3, I demonstrate how this approach provides novel insights into factors that shape the flexibility and robustness of task organization in harvester ant colonies (Pogonomyrmex barbatus). My results show that the degree to which colonies can respond to work in fluctuating environments depends on how individuals weigh the costs of activity and update their behavior in response to social information. In Chapter 4, I introduce a mathematical framework to study the emergence of collective organization in heterogenous groups. My approach, which is based on the theory of multi-agent systems, focuses on myopic agents whose behavior emerges out of an independent valuation of alternative choices in a given work environment. The product of this dynamic is an equilibrium organization in which agents perform different tasks (or abstain from work) with an analytically defined set of threshold probabilities. The framework is minimally developed, but can be extended to include other factors known to affect task decisions including individual experience and social facilitation. This research contributes a novel approach to developing (and analyzing) models of task organization that can be applied in a broader range of contexts where animals cooperate. / Dissertation/Thesis / Doctoral Dissertation Applied Mathematics for the Life and Social Sciences 2016