Diet and the demands of defense: Testing resource tradeoffs with deer mice and Rocky Mountain wood ticks

Immunological and behavioral defenses of vertebrates can limit blood loss to ectoparasites and affect pathogen transmission. Previously we determined deer mice (Peromyscus maniculatus) acquire resistance to blood feeding by larval Rocky Mountain wood ticks (RMWTs, Dermacentor andersoni), but observed variation in resistance among mice. It is thought that resource tradeoffs may produce variation in defense and shape host-parasite interactions. We tested if dietary factors affect the development of tick resistance relative to the maintenance of body weight. To test diet quality we had 2 diet groups with a mix of insects and seeds totaling 18kcal/day for each mouse. One diet was skewed to include more insects than seeds and the other contained more seeds than insects. The total amount provided was sufficient for mice to maintain body weight under non-parasitized conditions. To test diet quantity we made 2 diet groups, adequate and restricted, using rodent chow. In both experiments mice were fed the diets before and during 2 tick infestations. Engorged ticks were recovered from both infestations to determine the relative resistance to ticks for each diet group. Mice in the insect-rich diet group acquired resistance to RMWTs after one exposure, while mice in the seed-rich diet group did not. Mice fed a diet skewed toward insects lost more weight when parasitized than mice fed a diet richer in seeds. These data suggest dietary differences affect body mass during parasitism, but defense remains intact. When food quantity changed only mice fed the adequate diet acquired resistance to ticks, whereas mice fed the restricted diet did not. The restricted group lost weight when parasitized, unlike the adequate diet group. This suggests diet quantity affects the level of resistance. Though host condition may be influenced by the interaction of food resources and infection, defenses may remain intact even when dietary resources change over space and time.