What good are soldiers during peacetime? Large worker ants enhance transport capacity during foraging

Polymorphic worker castes have evolved repeatedly in ants, yet the selective advantages of larger workers are not always clear. Because larger workers require greater energetic investment to produce, they must provide disproportionate benefits to colony performance. While larger workers are typically assumed to function in colony defense as soldiers, defensive encounters are sporadic, raising a central question: what maintains these costly individuals during periods of relative peace? Here, we tested whether larger workers enhance foraging performance through increased running speed or improved load transport in the red imported fire ant, Solenopsis invicta. We quantified locomotor performance of workers spanning a 23-fold range in body mass under unloaded and loaded conditions. Larger workers ran faster in absolute terms when unladen, but velocity scaled sublinearly with body size and declined when standardized to body length. When carrying brood, running speed was independent of worker size, and free-running assays showed that larger workers moved faster only during outbound foraging, suggesting that larger workers do not substantially increase transport rates when carrying loads. In contrast, momentum increased steeply and disproportionately with body size across all conditions, reflecting a substantial increase in transport capacity rather than speed alone. Morphological analyses revealed positive allometry of the head, thorax, and abdomen, alongside relatively slender legs, suggesting a reorganization of body form to enhance force production rather than locomotor speed. These results suggest that larger workers function as high-capacity transport specialists, providing one potential explanation for the evolution and maintenance of worker polymorphism in ants beyond colony defense.