Sociometry and sociogenesis of colonies of the harvester ant, Pogonomyrmexbadius: distribution of workers, brood and seeds within the nest in relation to colony size and season

1. The vertical distribution in the nest of chambers, workers, callow worke rs, brood and seeds was studied in the harvester ant, Pogonomyrmex badius, in northern florida. On each of four sample dates (May, July, October, Janu ary), six to seven colonies, chosen to represent the full range of sizes, w ere excavated. All chamber contents were collected and counted. Chambers we re mapped and measured. In a preliminary study, two nests were excavated af ter preventing vertical migration by driving barriers into the wall of a pi t next to the nest, severing the vertical tunnels. The vertical distributio n of these barrier-nests differed little from unrestrained nests, indicatin g that unrestrained excavation produced a reasonable picture of vertical di stributions. 2. Nest depth, chamber number and total area increased with colony size. Ch amber area declined sharply with depth, as did chamber number, such that mo re than half of the total area was found in the upper quarter of the nest. 3. The proportion of dark-coloured (older) workers also declined strongly w ith depth, but this decline was weaker in the spring, and depended to a mod est degree on colony size. Conversely, in the distribution of callow (young ) workers, the proportion increased towards the bottom of the nest. Mean wo rker age was inversely related to the depth at which workers were found. Th e proportion of the brood also increased towards the bottom of the nest, wi th worker brood, sexual brood, pupae and larvae all being distributed simil arly. 4. By contrast, seeds were stored at a preferred absolute depth between 40 and 100 cm. Colonies shallower than 100 cm stored seeds in their deepest ch ambers. Larger colonies stored most seeds in the upper third of the nest, b ut patterns were somewhat erratic because chambers were either filled compl etely with seeds or were empty. 5. Because chamber area decreased sharply with depth, the densities (indivi duals cm(-2)) Of all colony members, including dark workers, were lowest ne ar the surface cm and highest in the deepest parts of the nest. Here, worke r densities ranged from 2 to 8 cm(-2), and brood from 2 to 25 cm(-2). 6. The regularity of the patterns of distribution suggests that harvester a nt colonies have considerable spatial and temporal structure, which serves or is the outcome of important colony processes. A simple mechanism that co uld generate several of these patterns is discussed. New workers produced d eep in the nest move upwards as they age. As they leave the brood zone they change from brood care to general nest duties, including increased nest ex cavation, leading to the top-heavy pattern of nest area. As they appear at the surface, they change to guarding and foraging. Thus, age polyethism may be partly the result of this upward migration of workers.