Patterns of parasite prevalence and individual infection in yellow‐bellied marmots

Patterns of infection and prevalence result from complex interactions between hosts and parasites, the effects of which are likely to vary by species. We investigated the effects of age, sex and season on the likelihood of individual infection, and the effects of host population size, sex ratio and age structure on parasite prevalence. We capitalized on data from a long‐term study of yellow‐bellied marmots M armota flaviventris potentially infected with fecal–orally transmitted intestinal parasites ( A scaris sp., E imeria spp. and E ntamoeba sp.), ectoparasitic fleas T hrassis stanfordi , and a flea‐ and louse‐transmitted blood parasite T rypanosoma lewisi . Patterns of individual‐ and group‐level infection varied widely by parasite. Yearlings were more likely to be infected with Tr . lewisi and A scaris . Yearlings were also slightly more likely than adults to have E imeria , but female yearlings had higher infection levels than female adults, while male yearlings had lower infection levels than male adults. E ntamoeba infection decreased as the season progressed. Adults and males were more likely to be infected with Th . stanfordi . A scaris prevalence increased with colony size. There were no significant relationships between colony size and prevalence of E ntamoeba , Tr . lewisi , E imeria or T hrassis . There was a small, but significant positive correlation between male‐biased sex ratio and prevalence of fleas. The host population's age structure affected the prevalence of infection of A scaris and E imeria . Overall intestinal parasite diversity increased with colony size. Taken together, our results show a great deal of variation in the likelihood of individual infection and patterns of parasite prevalence in marmots.