Background/Question/Methods Understanding the dynamics of contact transmitted parasites and pathogens is an emerging concern in evolutionary ecology and conservation. Parasitism is a frequent burden of group-living species as a result of increased conspecific interaction. Several studies have linked high parasitism to decreased fitness in social animals. However, a model that predicts parasite prevalence (percentage of host individuals infected), intensity (parasite abundance per host individual), and richness (number of parasite species per host individual), in social animals has not been formalized. We present a modified island model that explains how the size of social groups, and the intergroup contact rate may predict parasite loads in social animals. This model predicts that large social groups with high intergroup contact will have greater parasite prevalence, intensity, and richness than smaller, isolated social groups. Our model assumes that parasites are transmitted by contact, have an ideal free distribution dependent on host resources, and that host allogrooming is ineffective. We test this model using field data from the social, diurnal rodent, Octodon degus, in central Chile. During 2006 and 2007, we determined group size and extragroup interactions using a combination of telemetry and burrow trapping.

Results/Conclusions Ectoparasitism appears to be a cost of group-living in Octodon degus. Our 2006 data showed a positive relationship between group size and ectoparasite intensity for 13 social groups. Furthermore, we found a strong negative relationship between per capita fitness and parasite intensity of females during lactation. Ectoparasite prevalence for adults was greater than 98% in 2006 and 2007. Adult males had greater variability in ectoparasite intensity than adult females in both years, and adult males had a tendency for higher ectoparasite intensity than females. The intergroup movements of males may be an important component in determining parasite migration between groups. These data provide preliminary support for our island model predictions.