Ant societies buffer individual-level effects of parasite infections.

Parasites decrease host fitness and can induce changes in host behavior, morphology, and physiology. When parasites exploit social insects, they influence not only infected individuals but also the society as a whole. Workers of the ant Temnothorax nylanderi are an intermediate host for the cestode Anomotaenia brevis. We studied a heavily parasitized population and found that, although parasite infection had strong and diverse consequences for individual workers, colony fitness remained unchanged. At the individual level, we uncovered differences among the three worker types, infected and healthy workers from parasitized colonies and healthy workers from nonparasitized colonies. Infected workers were smaller than healthy ones and had smaller heads as parasite load increased. Behavioral changes extended to all workers from parasitized colonies; such workers were less active than workers in nonparasitized colonies but engaged in more grooming. Healthy workers from parasitized colonies showed behavioral patterns intermediate to those of infected workers and healthy workers from nonparasitized colonies. Despite the lower activity level found in ants from parasitized colonies, an important fitness parameter, per-worker productivity, remained unaltered in parasitized colonies. However, the investment strategies of parasitized colonies changed as their sex ratio became male biased and as male body size increased. In short, ant colonies can buffer the drain of resources by the parasite despite strong effects on individual workers.