Do Parasitism and Nutritional Status Interact to Affect Production in Snowshoe Hares?

We evaluated the hypothesis that the effects of parasitism on animal body condition are accentuated when host nutritional status is compromised. In a field experiment we manipulated natural burdens of five nematode species via anthelmintic treatment in one-half the free-ranging snowshoe hares (Lepus americanus) on each of six study areas and supplemented overwinter food level on three areas, to assess whether treatments interacted synergistically. We predicted that the effects of parasitism would be aggravated in malnourished hares (i.e., hares not administered supplemental food), and that animals receiving both treatments would have (1) lower parasite burdens, and (2) higher body mass, fat storage, and reproduction, than those receiving only one or no treatments. Mean numbers of Trichuris leporis were lower in hares subjected to food supplementation, suggesting that nutritional status affected immunity to that species. Obeliscoides cuniculi, Nematodirus triangularis, Dirofilaria scapiceps, and Protostrongylus boughtoni numbers were not directly affected by supplementation. Body mass of parasite-normal controls (i.e., not administered the anthelmintic treatment) was negatively correlated to nematode abundance during May–June, implying that parasitism had a deleterious effect on hare mass at that time. However, the anthelmintic treatment failed to directly affect hare body mass, fat storage, or reproduction. Food supplementation had a positive effect on body mass, fat storage, and reproduction, although effects of food were neither strong nor consistent among study areas and body condition parameters being measured. Only 2 of 22 analyses of hare production yielded marginally significant (i.e., 0.05 < P < 0.10) interactions between food and parasitism; therefore, we concluded that the two factors were not strongly linked. However, because our snowshoe hare population was at low densities and likely exposed to neither severe food shortage nor high rates of nematode parasitism, failure to detect an interaction between treatments may not comprise a general refutation of the hypothesis. Rather, hare populations subject to more serious food limitation (i.e., during a cyclic peak in numbers) may be apt to display stronger interactions between nutrition and parasitism.