Functional morphology and the role of bite loading in Pheidole (Hymenoptera: Formicidae) worker evolution

Cristian L. Klunk, Marco A. Argenta

& Marcio R. Pie

Worker polymorphism is a remarkable feature of many ant lineages. Besides body size differences, morphological polymorphism in the worker caste can be manifested in mandible and head shapes, which drive task specialization and improve the colony division of labor. The genus Pheidole shows at least two discretely recognized worker subcastes, the majors and minors. Minors are smaller with slender mandibles, and run most of the colony non-reproductive tasks. Majors are bigger, with a disproportional larger head and more robust mandibles, being commonly responsible for specialized tasks such as defense and food processing, which demands stronger bites. Although the improvement of the colony division of labor is commonly associated to the evolution of worker dimorphism in Pheidole, little is known about the influence of head and mandible shape to task specialization in ants. To relate morphological disparity to functional performance, an explicit biomechanical approach is essential. We aim to apply finite element analysis to simulate the mechanics of bite in head and mandibles of workers, to investigate how the shape of those structures influences the patterns of stress dissipation induced by distinct bite loading demands. Specifically, we aim to simulate different bite and muscular contraction scenarios in Pheidole worker mandibles and heads, respectively, to investigate if the shape of those structures can improve biomechanical performance, and if these morphological specializations are consistent with Pheidole workers task specialization. We hypothesize that majors are best suited to deal with stronger bites, having mandibles and head shapes that improve stress dissipation patterns and allows the generation of higher bite forces than minors, which agrees with the specialized roles of majors in the colony division of labor. Alternatively, a lack of shape effect in biomechanical performance would suggest that other morphological features, such as size and cuticle thickness, drive task specialization of Pheidole workers.