Spatial variation in food availability predicts extrapair paternity in the arctic fox

Extrapair paternity (EPP) is described in many socially monogamous species, but within-population variability in its frequency is poorly documented. Availability and distribution of food may influence polyandrous behaviors, either directly by affecting the need for paternal care or indirectly via their effect on population density. We quantified the frequency of EPP in a population of arctic foxes (Vulpes lagopus), a predominantly socially monogamous species with high paternal investment. We then tested whether spatial (presence of a goose colony) and temporal (cycles of lemmings) variations in food availability were linked to variations in mating systems within the population. From 2003 to 2008, we studied the mating systems of arctic foxes on Bylot Island (Nunavut, Canada) by combining molecular analyses with behavioral observations during cub rearing. Although the dominant social mating system was monogamy, at least 31% of cubs with known social father were born from extrapair matings (in 10 of 38 litters sampled). Likelihood of EPP was associated with food availability. It was greatest (86%) at the center of the goose colony and decreased sharply with increasing distance from it. EPP can thus be frequent in the socially monogamous arctic fox, and intraspecific variability in extrapair matings is strongly correlated with food availability during cub rearing.