Seasonal indices of nutrition and stress in a northern population of snowshoe hares.

Cyclic changes in snowshoe hare (Lepus americanus) fecundity have been attributed to changes in winter forage availability and predation pressure. Disentangling how nutrition and predation pressure affect snowshoe hare physiology is complex. As an herbivore of the northern boreal forests, snowshoe hares cope with extreme seasonal changes in diet, ambient temperature, and energy demands. We examined seasonal variation in the body condition index, blood biomarkers indicative of nutritional status, and fecal cortisol metabolite concentrations, in snowshoe hares across five ecologically distinct times of year in relation to adult survival rates. Snowshoe hares sampled from a high-density population in northern Alaska during 2018 showed decreases in survival and in plasma concentrations of total protein (TP), blood urea nitrogen (BUN), hematocrit (Hct), Chloride (Cl) and glucose during March and October. Increased survival and concentrations of Cl, TP, BUN, Hct, sodium (Na) and glucose were observed during August. Decreases in mass and survival from August to October suggest limited forage. Increases in TP, BUN, Hct and glucose in December suggest higher metabolic turnover. Fecal cortisol concentrations were not significantly associated with seasonal nutritional condition. A two-fold increase in mean cortisol was observed during August, potentially associated with energetically costly processes such as increased movement and reproduction. This work provides seasonal observations of snowshoe hare plasma biochemical values (N = 164) indicative of nutritional status, and supports the idea of using a collective biomarker approach to advance our understanding of how seasonality may play a role in snowshoe hare physiology.