Nutrient Balancing by a Wild Browsing Herbivore: Nutritional Geometry of Snowshoe Hares (Lepus americanus)

Browsing herbivores must consider food digestibility while balancing the intake of multiple nutrients (i.e., protein and energy) simultaneously. Nutritional Geometry (NG) is a framework that is used to assess how nutrients interact to impact animal feeding behavior and body condition. Here, we use NG combined with detailed digestibility trials to evaluate how snowshoe hares (Lepus americanus), a common boreal browser that experiences 10-year population cycles, balance energy and protein. We conducted 65 no-choice and 15 multi-choice feeding trials on 17 hares in Kluane, Yukon (Canada) during the winters of 2022 and 2023. We tested four diets ranging from the low protein (5.6%) and high fiber content of hare winter food (twigs) to the high protein (16.7%) and low fiber content of rabbit chow. We measured daily intake per kg^0.75 per day in multi-choice trials and daily intake, weight change, and digestibility in no-choice trials. We analyzed the effect of diet treatment on each response and the effect of protein and energy intake, in both crude and digestible terms, on feeding rates and weight change. In multi-choice trials, hares chose a diet balanced in energy and protein, but with a protein content above that in twigs. On single diets, hares were fed to meet a minimum daily digestible energy intake of approximately 1000 kJ/kg^0.75/day, regardless of protein content, after which digestible protein influenced weight change (p = 0.02). We found that hares could maintain their weight after they acquired 6 g of digestible protein per kg^0.75/day. Our results suggest that snowshoe hares choose to consume food items on the basis of the interaction between energy and protein, and these choices influence weight change. Our work supports previous hypotheses that declines in twig quality at peak hare densities could contribute to the subsequent increase in over-winter weight loss that occurs during the population crash.