Behavioral differences between Pogonomyrmex rugosus and dependent lineage (H1/H2) harvester ants

The discovery of genetic caste determination (GCD) in populations of Pogonomyrmex harvester ants raises many questions about the evolution and persistence of such populations. The genetic caste determination arises from the existence of two distinct, but mutually dependent, genetic lineages within a population. Workers always develop from a combination of the two lineages, but their sister queens develop from within-lineage matings. Maintaining genetic caste determination appears to be costly because many queen-destined eggs are wasted when a colony is not in the reproductive stage, yet these populations appear to be widespread. We investigated whether inter-lineage workers have novel traits that give GCD colonies a selective advantage in certain environments. In particular, we compared ecologically relevant behavioral characteristics of inter-lineage workers in H-lineage colonies with co-occurring normal colonies of P. rugosus. First, we measured colony defensive response toward a simulated vertebrate predator. Second, we set up direct competitive foraging and recruitment experiments between dependent lineage and P. rugosus colonies. Last, we measured individual aggressive response to foreign inter-lineage and P. rugosus workers. We found that H1/H2 inter-lineage workers explored objects on the nest more thoroughly and responded much more aggressively to simulated predator disturbance than the P. rugosus colonies. In individual encounters, H1/H2 inter-lineage and P. rugosus workers were equally aggressive toward foreign ants, but both worker types could discriminate P. rugosus from inter-lineage intruders and were more aggressive toward ants of the alternate type to themselves. When competing directly for resources, however, P. rugosus colonies consistently dominated seed piles. In summary, H1/H2 GCD colonies show distinct behavioral differences, but there is no clear ecological advantage from the traits we examined.