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 livingin groups. We examined this hypothesis by investigating theectoparasite fauna of a free-living alpine marmot population,a highly social sciurid species. Only a single important ectoparasitewas found, the mite Echinonyssus blanchardi. Heavy infestationwith this mite correlated with reproductive success of marmotsin two ways. Infant winter mortality increased with ectoparasiteload, probably because ectoparasites reduced the ability ofparents and alloparents to warm infants during hibernation.Litters produced by females exposed to a high number of mitesduring the preceding winter were weaned later. Late weanershad a lower chance of surviving hibernation. However, we foundno relationship between ectoparasite load and group size ormarmot density. Instead, the number of E. blanchardi per animalwas lower in areas where marmot groups were clumped, i.e., infavorable habitats. Although ectoparasites most likely decreasethe fitess of alpine marmots to some extent, they should notbe considered a cost of social life in this species.