Intraspecific variation in diet and behavior affects parasite loads in arctic fox
Differences in host behavior, age, sex, and condition, as well as variation in parasite specificity, drive variation in parasite infection, and ultimately determine host exposure. Intrinsic biological differences between males and females may cause one sex to be more prone to infection by parasites. Many species have evolved sex-based differences in behaviour, morphology, and physiology, due to differences in reproductive strategies. Differences in reproductive investment by males could increase their vulnerability to parasites. The ‘immunocompetence handicap hypothesis’ (ICHH) suggests a trade-off between immune function and testosterone. Testosterone suppresses the immune system, making males with higher testosterone levels more vulnerable to infection by parasites. The age of a host often influences its parasite load; differences in exposure overtime, plus changes in acquired immunity and diet over time can cause changes in the parasite load due to age. The circumpolar arctic fox (Vulpes lagopus) is a socially monogamous predator with no difference in morphology or home range size between sexes. Sexual selection is low in this species and both parents invest heavily in their young. Offspring leave their maternal den a few months after birth. To compare differences in parasite infections due to host age, sex, or diet we identified and enumerated parasites in arctic fox carcasses collected from local trappers in Churchill, Manitoba, Canada in the winter of 2010-2011 and 2011-2012. Tissues were sampled to measure stable isotope ratios as a proxy for diet.
Males had more cestodes than females. Stable isotope ratios of nitrogen were lower in males, suggesting greater reliance on small mammals. No differences were found between males and females in nematodes, suggesting that immunity did not differ between sexes and lending poor support to the ICHH hypothesis in this socially monogamous species. The trophically transmitted nematode Spirocerca lupi was more prevalent in adults, but overall, nematodes were more abundant in juveniles, likely because pups spend more time at dens in high densities, increasing exposure. Diet did not differ based on age. Likewise, the overall load of cestodes, which are primarily transmitted through prey, did not differ based on age. However, the species of cestodes found in adults and juveniles differed; e.g., Echinococcus multilocularis was present only in juveniles. These results suggest that intraspecific differences in arctic fox parasites are best explained by variation in diet and foraging pattern, rather than hormone-mediated reduction in immunity.