Sociometry and Sociogenesis of Colony-Level Attributes of the Florida Harvester Ant (Hymenoptera: Formicidae)

Abstract

This study addressed how collective, supra-organismal colony characteristics change during growth (sociogenesis) from the founding queen to maturity and through the seasons. A total of 31 harvester ant colonies representing a full range of sizes was excavated on 4 sample dates during the year. The data were analyzed for growth-related and seasonal patterns. The subterranean nests consisted of horizontal chambers connected by vertical tunnels. The total area of these nests increased more slowly than did the colony population, leading to an increase in mean worker density, which also varied seasonally. Nest area grew somewhat more rapidly through nest deepening and addition of new chambers than it did through the enlargement of chambers. The mean weight and headwidth of workers increased with colony size, but this size increase was limited to the minor workers. The major workers marle up 7% of the worker population, a proportion that did not vary with colony size. The total weight and number of seeds stored by colonies increased isometrically with colony size, so that the milligram of seeds per milligram of ant biomass did not change. However, the weight of seeds increased more rapidly than did the population of dark (older) workers who collected them, so that the foragers of larger colonies each collected more seeds than their counterparts in smaller colonies. Large colonies contained up to 300,000 seeds. Colonies of fewer than 60 workers collected smaller seeds, but larger colonies showed no relationship between mean worker size and seed size. Brood production was limited to May-September, Sexuals were produced only in the early spring, before any workers, by colonies with >2,500 workers ( 1/3 of maximal colony size). In the summer, colonies of all sizes produced only workers. The seasonal patterns of colony fat reserves suggest that colonies draw on these reserves to overwinter and produce sexuals, then they rebuild the reserves during the summer. The size of the fat reserves increased much more rapidly than did colony size, suggesting that changes in colony composition and function lead to better nutrition. All indicators of colony growth rate increased more slowly than did colony size. These included the ratio of broond to workers, callow workers to dark workers, and the number of developing follicles in the queen's ovaries. These decreases were also reflected in raw birth rates (individuals per day) that increased half as rapidly as did colony size, and colony growth rates (percent per day) that decreased by half with each 10-fold increase in colony size. These patterns are discussed in relation to colony function, ontogeny and life history.