Cyclic changes in snowshoe hare (Lepus americanus) fecundity have been attributed primarily to changes in winter forage availability and predation pressure. Disentangling how nutrition and predation pressure (direct predation and indirect predator avoidance behavior) affect snowshoe hare physiology is complex and understudied. As an herbivore of the northern boreal forests, snowshoe hares cope with extreme seasonal changes in diet, ambient temperature and energy demands.

Quantification of seasonal hare physiology may elucidate how changes in food, fear and mortality interact and play a role in the development of cyclic patterns of population abundance. We examined seasonal variation in blood biomarkers indicative of nutritional status, fecal cortisol metabolite concentrations, and a recognized snowshoe hare body condition index (BCI), between five ecologically distinct times of year and in relation to adult survival rates. Snowshoe hares sampled from a high-density population in northern Alaska during 2018 appeared to show decreases in survival rates and mean values of nutritional markers total protein (TP), blood urea nitrogen (BUN), hematocrit (Hct), Chloride (Cl) and glucose during spring and autumn. Increases in adult survival as well as increases in blood biomarker mean concentrations of Cl, TP, BUN, Hct, Na, and glucose were observed during summer. A reduction in mass and survival from autumn to winter suggests forage became limited. Mean increases in TP, BUN, Hct, and glucose concentrations during winter may be suggestive of high metabolic turnover. Cortisol metabolite concentrations did not appear associated with seasons with low nutritional condition. A two-fold increase in mean fecal cortisol metabolite concentrations was observed during summer and was possibly associated with breeding and/or increased densities of predators. Results from this study provide physiological evidence reinforcing previous findings of associations between low survival rates and energetically costly times of year. Furthermore, this work provides the first snowshoe hare reference range of blood biomarkers (N = 165) indicative of nutritional status, and introduces the idea of using a collective biomarker approach to advance our understanding of seasonal variation in the physiology and nutritional condition of snowshoe hares.