Ectoparasite loads decrease the fitness of alpine marmots (Marmota marmota) but are not a cost of sociality

Parasites arc often considered an unavoidable cost of living in groups. We examined this hypothesis by investigating the ectoparasite fauna of a free-living alpine marmot population, a highly social sciurid species. Only a single important ectoparasite was found, the mite Echinonyssus blanchardi. Heavy infestation with this mite correlated with reproductive success of marmots in two ways. Infant winter mortality increased with ectoparasite load, probably because ectoparasites reduced the ability of parents and alloparents to warm infants during hibernation. Litters produced by females exposed to a high number of mites during the preceding winter were weaned later. Late weaners had a lower chance of surviving hibernation. However, we found no relationship between ectoparasite load and group size or marmot density. Instead, the number of E. blanchardi per animal was lower in areas where marmot groups were clumped, i.e., in favorable habitats. Although ectoparasites most likely decrease the fitess of alpine marmots to some extent, they should not be considered a cost of social life in this species.