The ecology and evolution of colony-size variation

Animals often breed in colonies that can vary in size by several orders of magnitude. Colony-size variation is perplexing because individuals in some colony sizes have lower fitness than those in other colony sizes, yet extensive size variation persists in most populations. Natural variation in colony size has allowed us to better quantify the costs and benefits of coloniality, but what causes and maintains size variation is in general unknown. Ecological correlates of colony-size variation potentially include local availability of resources, such as food or nesting sites, and may also reflect individuals' sorting among colonies (based on life-history traits, morphology, or behavioral propensities) to find the social environment to which they are best suited. Preferences for particular colony sizes are genetically based in some species. The fitness differences observed among colony sizes may reflect unmeasured tradeoffs among life-history components and also could vary temporally or spatially. Colony-size variation might be maintained by fluctuating directional or stabilizing selection that alternately favors individuals in different group sizes and leads to stasis in the colony-size distribution over the long term. Recent focus on the cues individuals use to select breeding habitat (e.g., conspecific attraction, reproductive success of others) does not satisfactorily explain variation in colony size. Costs of dispersal, reliance on imperfect information, and collective nonrandom movement can also lead to colony-size variation in the absence of fitness-based site selection. Our understanding of factors generating and maintaining variation in colony size remains in its infancy and offers many opportunities for future research with broad implications for behavioral ecology.