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 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 while parasite infection had strong and diverse consequences for individual workers, colony fitness remained unchanged. On the individual level, we uncovered differences among the three worker types: infected and healthy workers from parasitized colonies and healthy workers from non-parasitized colonies. Infected workers were smaller than healthy ones and had, as parasite load increased, smaller heads. Behavioral changes extended to all workers from parasitized colonies, which were less active but groomed more. Healthy workers from parasitized colonies showed behavioral patterns intermediate to those of infected workers and healthy workers from non-parasitized colonies. Despite the lower activity level, 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 male body size increased. In short, ant colonies can buffer the drain of resources by the parasite despite strong effects on individual workers.

Scharf_et_al_Data_FileThe file contains the main data used for the analyses presented in the manuscript. The first sheet contains information on the behavioral and position observations of the ants, the second one on their morphology and the third sheet contains data on life history and investment patterns differences between parasitized and healthy ant colonies.