Sociometry and Sociogenesis of Colonies of the Fire Ant Solenopsis Invicta During One Annual Cycle

Abstract

In social insects, colonies as well as individuals have evolving life histories. Identification of the life history tactics of a social insect requires data on colony attributes and their development. To this end a full range of fire ant (Solenopsis invicta) colony sizes was sampled and censused on seven dates throughout 1 yr. Data included: mound volume; the number, dry masses, and fat contents of sexual and worker adults and immatures; stratified nest temperatures; worker distribution within the nest throughout the year; duration of the pupal stages; and respiration rates. Analysis showed: 1. Colonies reached their annual maximum population size in midwinter and their maximum biomass in spring. During the spring sexual production period they declined to a midsummer minimum. Calculations showed that the magnitude of this decline increased with colony size. During January to July, worker mortality exceeded natality, causing colony decline, while from July to December, natality predominated, causing growth. 2. Mound volume was closely related to the total mass of ants in the colony, and varied with season paralleling the mass of ants. 3. The mean size and variability of workers, and the percent major workers, increased with colony size and changed over the year. 4. The fat content (percent fat) of workers increased with worker size and colony size. Worker percent fat was lowest in summer after sexual production, climbed immediately to the annual maximum and then declined gradually through winter and spring. 5. Although sexual male and female pupae were close in mean dry mass (2.55 mg and 3.10 mg, respectively), males gained only 6% during adult maturation while females gained 290%. Females gained fat more rapidly than lean tissue causing their percent fat to increase from 31% to 49%. Mean mass of male and female sexual adults did not change with colony size. 6. The cost of worker maintenance declined from nearly 100% of total colony cost in winter to 46% in late spring when brood production peaked. 7. Production rates peaked in spring, with colonies investing 50% of their daily production in sexuals. This peak production was not sustained through the summer, and was probably fueled by stored worker fat. Worker production dominated in the latter part of the summer. All measures of production rate as well as total annual production increased with colony size, but most did so less rapidly than colony size, resulting in a declining efficiency of production and a declining natality rate. 8. The percent of annual production invested in sexuals increased sharply in colonies of between 20 000 and 50 000 workers, then remained at ≈33% for the remainder of colony growth, showing that the transition from the ergonomic to the reproductive stages is sharp, and that colonies must grow in order to produce more sexuals. 9. Many quantitative colony attributes were related to one another by differential growth, and can thus be seen as isometric or allometric measures. Rules of relative growth may thus constrain the possible combinations of attributes and their evolution. The methods of morphometric size and shape analysis are discussed as tools for understanding suites of colony attributes, and comparing them among species. 10. The sociometric/sociogenic method is discussed as a way to compile, analyze and compare data on social insect colony attributes and their growth and development.