Seasonal biochemical variations suggest that nutrient and energy dynamics in guanacos differ from typical ruminants.

Abstract In seasonal environments, herbivorous mammals employ physiological, morphological, and behavioral strategies to meet their nutritional demands despite fluctuating environmental conditions. Biochemical parameters that represent nuanced indicators of nutritional status and physiological function can provide a framework to understand animal–habitat interactions at a population level. Here, we assessed variation in Guanaco, Lama guanicoe, nutritional physiology in a temperate, semiarid region (Monte León National Park, in southern Patagonia, Argentina), testing the hypothesis that their nutritional state responds to the seasonal variation in resources and energy demands of their annual cycle. We captured 36 free-ranging, adult guanacos between austral early spring and early autumn, and measured biochemical variables in blood serum that are proxies of nutritional intake and mobilization of stored energy (glucose, urea, creatinine, urea:creatinine, albumin, total protein, alkaline phosphatase, nonesterified fatty acids). Urea was highest in early spring, following winter food deprivation, contrary to the expectation of enhanced urea retention and recycling. High urea levels may reflect the constraint of the energy demand of such recycling (e.g., cost of urea transfer or subsequent protein synthesis with recycled nitrogen) when food intake is still low. Guanacos maintained high and steady levels of glucose from early spring to early autumn, probably reflecting continuous hepatic gluconeogenesis from amino acid precursors. Low levels of nonesterified fatty acids in early spring suggest an apparent lack of seasonal mobilization of lipid stores, which is consistent with previous reports of very low body fat content for related species. Urea and creatinine were higher during pregnancy for females, potentially reflecting seasonal variations in food resources rather than the use of body proteins as an energy source, although future research is needed to address this issue. Our findings suggest that guanacos have a distinctive phenotype of macronutrient utilization compared to typical ruminants, indicating that seasonality in their nutritional state may differ from the general pattern in other large-bodied herbivores.