Division of labor is one of the most basic and widely studied aspects of colony behavior in social insects. Studies of division of labor are concerned with the integration of individual worker behavior into colony level task organization and with the question of how regulation of division of labor may contribute to colony efficiency. Here we describe and critique the current models concerned with the proximate causes of division of labor in social insects. The models have identified various proximate mechanisms to explain division of labor, based on both internal and external factors. On the basis of these factors, we suggest a classification of the models. We first describe the different types of models and then review the empirical evidence supporting them. The models to date may be considered preliminary and exploratory; they have advanced our understanding by suggesting possible mechanisms for division of labor and by revealing how individual and colony-level behavior may be related. They suggest specific hypotheses that can be tested by experiment and so may lead to the development of more powerful and integrative explanatory models.

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We examined the distribution and ancestral relationships of genetic caste determination (GCD) in 46 populations of the seed-harvester ants Pogonomyrmex barbatus and P. rugosus across the east-to-west range of their distributions. Using a mtDNA sequence and two nuclear markers diagnostic for GCD, we distinguished three classes of population phenotypes: those with GCD, no evidence of GCD, and mixed (both GCD and non-GCD colonies present). The GCD phenotype was geographically widespread across the range of both morphospecies, occurring in 20 of 46 sampled populations. Molecular data suggest three reproductively isolated and cryptic lineages within each morphospecies, and no present hybridization. Mapping the GCD phenotype onto a mtDNA phylogeny indicates that GCD in P. rugosus was acquired from P. barbatus, suggesting that interspecific hybridization may not be the causal agent of GCD, but may simply provide an avenue for GCD to spread from one species (or subspecies) to another. We hypothesize that the origin of GCD involved a genetic mutation with a major effect on caste determination. This mutation generates genetic conflict and results in the partitioning and maintenance of distinct allele (or gene set) combinations that confer differences in developmental caste fate. The outcome is two dependent lineages within each population; inter-lineage matings produce workers, while intra-lineage matings produce reproductives. Both lineages are needed to produce a caste-functional colony, resulting in two reproductively isolated yet interdependent lineages. Pogonomyrmex populations composed of dependent lineages provide a unique opportunity to investigate genetic variation underlying phenotypic plasticity and its impact on the evolution of social structure.

To place social insect foraging behavior within an evolutionary context, it is necessary to establish relationships between individual foraging decisions and parameters influencing colony fitness. To address this problem, we examined interactions between individual foraging behavior and pollen storage levels in the honey bee, Apis mellifera L. Colonies responded to low pollen storage conditions by increasing pollen intake rates 54% relative to high pollen storage conditions, demonstrating a direct relationship between pollen storage levels and foraging effort. Approximately 80% of the difference in pollen intake rates was accounted for by variation in individual foraging effort, via changes in foraging activity and individual pollen load size. An additional 20% resulted from changes in the proportion of the foraging population collecting pollen. Under both high and low pollen storage treatments, colonies returned pollen storage levels to pre-experimental levels within 16 days, suggesting that honey bees regulate pollen storage levels around a homeostatic set point. We also found a direct relationship between pollen storage levels and colony brood production, demonstrating the potential for cumulative changes in individual foraging decisions to affect colony fitness.