Extreme Worker Polymorphism in the Big-headed Pheidole Ants

Abstract

Polymorphism is the existence of multiple phenotypes within a given species in a population. In social insects, worker polymorphism plays a key role in maximizing the effectiveness of the division of labor. Some ants have an extreme form of worker polymorphism where workers have a broad size range and multiple size modes. Pheidole ants, for example, consist of both highly polymorphic species as well as those with relatively low polymorphism. Here, we examined why different Pheidole species have different degrees of polymorphism and how polymorphism is produced. We thoroughly characterized the worker caste systems of P. spadonia, P. rhea, P. obtusospinosa, and P. tepicana. There were significant differences among the four species in size range, number of size modes, caste ratios, allometry, and caste biomass allocation. An examination of worker caste traits of P. spadonia, P. rhea, and P. obtusospinosa revealed that as head size increased for all three species: (1) mandibles became broader and less serrated, (2) head muscle volume increased, and (3) bite force increased. These traits of large supersoldiers are likely adapted for crushing while those of small minors are likely for cutting. Foraging experiments showed that P. spadonia, P. rhea, and P. obtusospinosa used their workforce in different ways for food processing outside the nest. For P. rhea, the frequency of supersoldiers involved in food processing increased as the processing level required increased. However, P. obtusospinosa supersoldiers were rarely found processing food outside the nest and P. spadonia soldiers assisted in processing dead prey but did not help at all in processing live prey. P. obtusospinosa and P. spadonia workers may be more involved with other colony tasks. This hypothesis was confirmed when field observations of P. obtusospinosa showed supersoldiers participating in head-blocking at their entrance to fend off invading army ants; no other castes exhibited this behavior. Lastly, we tested genetic influences on worker polymorphism. We found that as colony genetic diversity increased (via polyandry), the degree of polymorphism increased. We also showed evidence of paternal genes influences on the development of worker castes in the highly polymorphic P. rhea.