Behavioral discretization and the number of castes in an ant species

Summary

• 1. Two extreme possibilities in the evolution of temporal castes can be envisaged. First, workers can undergo changes in responsiveness to various kinds of stimuli in a strongly discordant manner as they grow older, so that each task is addressed by a distinctly different frequency distribution of workers belonging to different age groups. Because these age-frequency distributions change almost gradually from one task to another in covering many such tasks, the resulting temporal caste system is referred to as continuous. At the opposite extreme, the aging worker can undergo changes in responsiveness to different stimuli in a highly concordant manner, so that all of the tasks are attended by one or relatively few frequency distributions of workers belonging to different age groups. The resulting temporal caste system is referred to as discrete, and the evolutionary process leading to it is called behavioral discretization (Fig. 1).

• 2. The temporal system of the minor worker caste of Pheidole dentata proves to be much closer to the discrete state, although it is not extreme in form (Figs. 3, 4). On the basis of ethograms constructed of stressed and unstressed colonies in which the approximate ages of the minor workers were known, it is possible to recognize five discrete female castes: the queen, a single temporal subcaste of the major worker, and three temporal subcastes of the minor worker. These are the elements which can now be employed in ergonomic analyses of the species' caste system.