The ecology and evolution of colony demography and division of labor in the fungus gardening ant Trachymyrmex septentrionalis

The demographic characteristics of ant colonies involve variation in worker age, and also size variation, or polymorphism. Polymorphism is thought to be an adaptation for efficient division of labor, because it could allow a colony to divide labor among workers according to which size of worker can most efficiently perform each task. If this hypothesis is correct, then ant colonies should exhibit adaptive patterns of demography that are correlated with their division of labor, with the ecological features of their habitats, and with colony fitness.; I studied colony demography and ecological differences between two populations of the weakly polymorphic attine ant Trachymyrmex septentrionalis, from Florida and Long Island, NY. Long Island colonies had larger workers and greater size variation than Florida colonies. Colony fitness, measured by alate production, was correlated with measurements of the worker size frequency distribution. This entire distribution appears to be the adaptive demographic trait; the presence or absence of any size class, or the proportions of size classes present, was not related to fitness.; Division of labor was studied by observing laboratory colonies in which workers were marked either as individuals or by age and size. The repertoire is among the largest reported for any social insect. It consists of forty-five acts that can be grouped into five roles: fungus tending, brood care, substrate processing, nest excavation and maintenance, and foraging. Division of labor was related to worker age and size, such that the younger and smaller workers remain on the fungus comb and perform fungus tending and brood care, and older and larger workers perform tasks in the nest periphery and outside the nest. Foraging was done almost exclusively by the oldest workers, but age and size classes were not otherwise specialized since all classes performed all roles. Individual worker repertoires were generalized, with most workers performing three or more of the five roles. The Florida and Long Island populations had the same repertoire and exhibited the same patterns of division of labor by age and size.; The worker size variation found in the two populations could be explained by hypothesizing that growing colonies are selected to carefully balance the size and numbers of workers they produce, and that the larger workers found in the Long Island population increase colony survival during overwintering. This size variation, imposed on an existing pattern of division of labor by age, could serve as a preadaptation for the evolution of highly polymorphic workers and an elaborate size-based division of labor.