I investigate a social parasitism by examining the lives the parasite involved. I use a variety of techniques to understand the population genetics, host acquisition behaviors, and space-use of an obligate parasitic ant (Polyergus mexicanus) that conducts raids to steal the brood of its host species (several Formica spp.) and makes them “slave” workers in the parasite’s nest. I consider the interacting roles of intraspecific competition, kinship, and host specialization on each of these topics.

In Chapter 1, I use microsatellite loci to characterize the genetic structure of the parasite population at three scales: subpopulations, neighboring nests, and among nestmates. I find that there are three very distinct subpopulations that often predict host-use patterns, but not always. Neighboring nests are sometimes closely related, enabling potential kin selection. Lastly, parasite nestmates are overwhelmingly full siblings, yet there were numerous nestmates that must have come from different parents, suggesting that raiding parasites may also steal conspecific brood for enslavement.

In Chapter 2, I characterize raids from an optimal foraging perspective. I found that parasitic colonies that send more raiders further distances obtain more brood on their raids. Nests that are capable of larger raids in general had a higher success rate, and more variability in raid sizes, but not distances. These findings suggest that raids are tailored to the expected quality of the host nest target, and that larger nests have more flexibility in their raiding tactics.

In Chapter 3, I investigate intraspecific competition, aggression, and territoriality among parasites by looking at the spatial arrangement of P. mexicanus nests and raids relative to their conspecific neighbors and their host species. I found that nests were often overdispersed, a classic sign of intraspecific competition. However, contrary to my predictions, raids from neighboring nests that shared host species overlapped more than those that used different host species. Overall, these spatial results suggest that competition among parasites may play out at different life stages, and that the specific identity of host species might determine the nature of conspecific parasitic interactions.

This work documents the complexity and far-reaching implications of parasite-parasite interactions.