Sociometry and sociogenesis of colonies of the harvester ant, Pogonomyrmexbadius: worker characteristics in relation to colony size and season

As colonies of all monogyne ants grow from a single, colony-founding queen to a mature colony with many workers, they develop the species-typical char acteristics of the mature colony. This ontogeny, and these species-typical characteristics and their seasonal changes were studied in the Florida harv ester ant by excavating 31 colonies of the full range of sizes, on 4 dates representing major phases of the annual cycle. Worker characteristics varie d strongly with colony size, location in the nest, and season. All but inci pient colonies contained both major and minor workers. The proportion of th e colony which was major workers was unaffected by colony size, averaging a bout 7%,but showed a small increase in mid-summer. Minor workers increased in size as colonies grew larger, but major workers did not. Most of the cha nges in minor worker weight were caused by changes in fat stores, which var ied by 85% or more. The patterns of weight variation supported the followin g life and seasonal history of minor workers. The young, lighter-colored wo rkers (callows), occurred near the bottom of the nest between June and Octo ber confirming a strong age-stratification in the nest. For a given headwid th, callow miners were fatter than their older, darker sisters. In the cour se of the season, young workers gained lean weight, but lost fat as they mo ved to nest chambers ever closer to the surface. Finally, when their fat co ntent was less than 10%, they became foragers on the surface. Along with th ese age-related changes, minor workers were lightest and least fat in July after the colony had produced its annual crop of sexual alates, and gained an average of 24% by winter. Workers were heavier and fatter if they came f rom a larger colony, and heavier and younger deeper in the nest. Seasonal v ariation in fat weight and dry weight was greater in smaller colonies than large. Although less variable, lean weight was lower in July and in the bot tom of the nest, and higher in a larger colony These patterns were similar, though less precise, for major workers. Altogether in the life history of harvester ant colonies, the large pulse o f early-spring sexuals probably cannot be produced solely from current fora ging intake. The metabolic and labor resources needed for sexual production are stored in the bodies of the young workers, whose fat content reaches t he annual minimum after producing sexuals. After these sexuals have flown, the colony once again switches to producing workers and storing excess fora ging intake as worker fat for over-wintering, and for producing the next ye ar's sexuals. As colonies get larger, this proportional excess increases, g iving rise to fatter and larger workers.