The evolution of head size and shape relative to seed harvesting in the ant genus Pheidole

Pheidole is presently the most species rich of the 288 ant genera described. Workers of Pheidole are almost always dimorphic, comprising of a small minor caste and a major or soldier caste with a particularly large head. Major workers can perform a variety of tasks, but are usually associated with defense, and the retrieval and processing of food. Pheidole species vary in their diet but many species gather seeds and the majors are known to mill the seeds using large jaws powered by mandible closer muscles filling the head cavity.  We examined the relationship between seed harvesting and head size, hypothesizing that seed harvesters will have majors with larger heads compared to non-seed harvesting species to accommodate more powerful mandibular muscles.  Using the comparative method we found that seed associated Pheidole do not have majors with larger heads (width and length) than non-seed harvesting species, but there is a positive relationship between seed harvesting and minor head length.  Additionally there is a significant relationship of head width and length to seed harvesting when both minors and majors are considered together.  Seed harvesting Pheidole have smaller minors and larger majors than their non-seed harvesting congeners.  This suggests that head size of major and minor castes can evolve for diet, but they do so relative to the morphology of other worker castes.