Social task regulation in the dimorphic ant, Pheidole pallidula: the influence of caste ratio.

We investigated whether the physical castes of the dimorphic ant Pheidole pallidula (Nylander) (Hymenoptera: Formicidae), are involved in determining within-nest activities and how their social investment in everyday tasks is influenced by large changes in the colony's caste ratio. Although the large-headed majors are morphologically distinct from minors, they are similar in size, exhibit similar behavioral repertoires and carry out nearly the same tasks as minors. Changes, even large ones, in the colony's caste ratio have no significant effect on the repertoire size of either caste. Majors do not compensate for the depletion of minors by expanding their repertoire or increasing their activity level. Instead of being an idle stand-by caste as suggested for other Pheidole specie s, P. pallidula majors are nearly as totipotent as minors. Moreover, their performance rate of social behaviors is remarkably high and constant regardless of the colony caste ratio. Such high investment of the major caste helps the colony to keep social behaviors at a baseline even in colonies undergoing large demographic changes. Alternative schemes of social regulation in polymorphic ant species are discussed. A possible methodological bias accounting for between-species differences in the level of majors' specialization is described.