Nutritional value and diet preference of arboreal lichens and hypogeous fungi for small mammals in the Rocky Mountains

Abstract

Many small mammals consume lichen and fungi, but southern red-backed voles ( Clethrionomys gapperi (Vigors, 1830)) and northern flying squirrels ( Glaucomys sabrinus (Shaw, 1801)) exhibit strong mycophagy compared with other North American taxa. We analyzed nutrient content of lichen and fungi and observed feeding preferences of voles and flying squirrels to understand the foraging behavior of these mammals and their strategy for surviving on relatively low-quality diets dominated by lichen and fungi. We analyzed nutrient characteristics of 10 hypogeous (fruiting belowground) fungi and four arboreal lichens eaten by red-backed voles and northern flying squirrels in the Rocky Mountains. Hypogeous fungi contained higher nitrogen, lipid, neutral detergent fiber, acid detergent fiber, ash, potassium, and phosphorous concentrations than arboreal lichens, but lichens were higher in calcium. To assess diet preferences, 10 pairwise feeding trials using four hypogeous fungi and two arboreal lichens were conducted with voles and seven trials using three hypogeous fungi and two lichens were conducted with squirrels. In general, squirrels and voles preferred hypogeous fungi over arboreal lichens. We then calculated dry matter and nitrogen digestibilities for flying squirrels and red-backed voles fed diets of arboreal lichen and hypogeous fungi. Overall mean dry matter digestibilities were ≥70% for all diets. For hypogeous fungi, nitrogen digestibility was 12.3% and 24.9% for squirrels and voles, respectively, suggesting that most nitrogen was indigestible. Animals maintained positive nitrogen balance when fed fungi but were unable to maintain positive nitrogen balance when fed lichens. Maintenance nitrogen requirements for flying squirrels were lower than predicted. Low requirements may allow for increased consumption of lichen in winter, but lichen diets must be supplemented with a source of nitrogen because animals were unable to maintain nitrogen balance when fed lichen alone. Consumption of numerous fungi and lichen taxa is necessary to sustain these animals year-round.