THE ADAPTIVENESS OF WORKER DEMOGRAPHY IN THE ATTINE ANT TRACHYMYRMEX-SEPTENTRIONALIS

Caste theory states that worker size distributions in ant colonies hav e evolved for efficient division of labor and predicts that they shoul d vary with habitat and have an effect on fitness. We tested these pre dictions in a comparative study of worker size variation and colony fi tness in Florida and Long Island populations of Trachymyrmex septentri onalis. Colonies from the two populations were excavated shortly befor e the time of the mating flights, so that workers and reproductive for ms could be censused. We measured worker head widths in each colony an d analyzed the resulting size frequency distributions using the mean, standard deviation, skewness, and kurtosis, and also principal compone nts derived from these parameters. Size frequency distributions were u nimodal and close to normal, with a tendency to negative skewness. Col onies in the Long Island population had both larger workers and greate r size variation than those in Florida. Florida colonies followed a gr owth trajectory in which the mean and standard deviation of worker hea d width both increased with colony size, whereas in Long Island there was an inverse relationship between the mean and standard deviation th at was independent of colony size. Analysis based on the principal com ponents showed that worker size variation significantly affected fitne ss in both populations, and this was not a secondary effect of colony size. Population differences in colony life history strategies may acc ount for the observed patterns: the Long Island population appears to be adapted for surviving the winter diapause, while the Florida popula tion has experienced different selective pressures, possibly for rapid colony growth. Size variation that has evolved in the context of life history strategies may serve as a preadaptation for division of labor .