Morphometric variation in North American Pogonomyrmex and Solenopsis ants: caste evolution through ecological release or dietary change?

Abstract

Polymorphism in the ant Pogonomyrmex badius was studied using morphometric analysis. Head shape in P. badius was compared to 14 closely related monomorphic Pogonomyrmex believed to differ in worker morphology due to character displacement. Head shape in P. badius and two polymorphic Solenopsis species, one known to have a seed-milling caste (S. geminata) and the other continuously polymorphic (S. invicta), was also compared. Monomorphic species of Pogonomyrmex differed from P. badius in the rules of transformation that accompany changes in worker head size. Although workers of the Pogonomyrmex species studied changed in size primarily by increases in the area of the occipital region and the width of the head, polymorphic P. badius workers showed a significantly greater allometric increase in the occipital region. The comparative morphometrics of P. badius, monomorphic Pogonomyrmex and two polymorphic Solenopsis species did not support the hypothesis that size-related changes in head shape are due to competitive displacement, because head shapes in P. badius were not similar to head shapes in the monomorphic Pogonomyrmex. In contrast, results suggest that a dietary change may have led to the evolution of polymorphism in P. badius through selection for a seed-milling caste. This conclusion is supported by additional analyses that indicate the existence of two distinct modal head shapes in the caste distribution of P. badius and S. geminata, suggesting that bimodal shape variation might be a common evolutionary response to seed processing.