Ecology and the evolution of worker morphological diversity: a comparative analysis with Eciton army ants

1. Certain ants have considerable morphological diversity within the workforce, with individuals physically specialized for particular jobs. The ecological conditions that select for this phenomenon are still poorly understood. 2. We use a comparative analysis to address the hypothesis that prey type has selected for greater morphological diversity in the small, monophyletic army ant genus Eciton. These ants are nomadic group-predators and transport loads in the limited space below the body, so bulkier prey types should select for more diverse worker morphology. 3. We show that five Eciton species differ in the presence or absence of a distinct ‘submajor caste’, the morphological exaggeration of submajors when present, and the allometric scaling of standard workers over a broad size range. Crucially, the presence and morphological exaggeration of submajors is associated with the variety of awkwardly shaped prey types that the species takes, and this caste specializes in prey transport. 4. Our findings support the importance of dietary composition in the evolution of worker morphological diversity in Eciton and demonstrate the utility of our general approach. Future critical tests of this hypothesis are discussed, as well as how our approach can contribute to a broader understanding of the relationship between ecology and the evolution of worker morphological diversity.