Sex-biased parasitism in monogamous arctic foxes is driven by diet

Male-biased parasite loads are common in vertebrates, particularly in species with intense sexual selection, yet few studies have examined sex-biased parasite loads in monogamous species with low sexual selection and no differential investment in ornaments or other sexually selected traits. The circumpolar arctic fox (Vulpes lagopus) is a socially monogamous predator with low sexual selection; both parents invest heavily in their young. To determine if parasite loads (abundance and intensity) in arctic foxes vary with sex, age, or diet, we identified and enumerated parasites in arctic fox carcasses collected in winter from local trappers at Churchill, Manitoba, Canada. We measured stable isotope ratios in muscle samples as a proxy for diet. Males had more cestodes, which are only transmitted through prey, but numbers of nematodes, which are acquired by direct transmission, did not differ between sexes. delta N-15 values were lower in males, suggesting greater reliance on small mammals. Age did not affect diet or the number of cestodes, but the species of cestodes differed by age; for example, Echinococcus multilocularis was present only in subadults (<1 year old). Nematodes were more abundant in subadults, likely because pups spend more time at dens in high densities, increasing exposure. Intraspecific differences in arctic fox parasites are best explained by variation in diet and foraging patterns, rather than hormone-mediated reduction in immunity. These results suggest that in monogamous species, males may not suffer compromised immunity as seen in species with strong sexual selection.