Subcaste-specific evolution of head size in the ant genus Pheidole

An organism's morphology is constrained by its evolutionary history and the need to meet a variety of potentially competing functions. The ant genus Pheidole is the most species-rich ant genus and almost every species has a dimorphic worker caste (a few are trimorphic). This separation of workers into two developmentally distinct subcastes (smaller minors and larger majors with distinctively large heads) may partially release individuals from functional constraints on morphology, making Pheidole an ideal genus for addressing questions on the evolution of morphology in relation to ecological specialization. Major workers can perform a variety of tasks, although they are usually specialized for defence, as well as food retrieval and processing. Pheidole species vary in their diet, although many species gather seeds. The major workers mill the seeds using large jaws powered by mandible closer muscles that occupy a large proportion of the head cavity. In the present study, we examined the relationship between seed-harvesting and morphology in Pheidole, hypothesizing that majors of seed-harvesting species would have larger heads relative to non-seed-harvesters to accommodate the powerful mandibular muscles needed to mill seeds. By taking a phylogenetically controlled comparative approach, we found that majors of seed-associated Pheidole did not have larger heads (width and length) than majors of non-seed-harvesting species. However, the head length of minors (and to a lesser extent head width) was smaller in seed-harvesters. Additionally, we found the difference in head size between majors and minors was greater in seed-harvesting species. These morphological differences in diet, however, were not related to changes in the rate of evolution in either seed-harvesting or non-seed-harvesting lineages. These findings suggest that the morphologies of worker subcastes can evolve independently of each other, allowing colonies with polymorphic workers to specialize on new resources or tasks in ways not possible in monomorphic species.