Mycophagy as a nutritional strategy for small mammals in the Rocky Mountains

Several small mammals, including red-backed voles and northern flying squirrels, practice mycophagy, or the consumption of fungi and lichens, but the nutritional benefits of mycophagy have not been determined. Red-backed voles occur in old-growth forests, and a poor renal water conservation mechanism and dependence on hypogeous fungi have been proposed as reasons for habitat restriction. To test the renal water conservation hypothesis, I deprived voles of water for 12 hours and measured the urine osmolarity. Voles could concentrate urine quite well, and therefore habitat restriction is likely due to some other factors, potentially the presence of hypogeous fungi in old-growth forests. Because fungi may influence distribution, I analyzed hypogeous fungi and arboreal lichens for nutrient content. Fungi contained higher protein, lipid, neutral detergent fiber, acid detergent fiber, ash, potassium, and phosphorous concentrations than arboreal lichens, but lichens were higher in calcium. Two fungi contained very high concentrations of vitamin D. Because fungi are eaten in spring through fall and lichens are eaten frequently in winter, the nutritional composition of the diet differs with season. To determine diet preferences of voles and squirrels, I conducted feeding trials with fungi and lichens eaten in the wild. Fungi were preferred over lichens, and most preferences among fungi were partial. Animals appeared to prefer fungi high in protein, potassium, and phosphorous. Lastly, nitrogen and dry matter digestion experiments were conducted to determine if squirrels and voles are able to survive on diets of only arboreal lichen (Bryoria spp.) or hypogeous fungi (Rhizopogon spp. and Hysterangium sp.). Animals lost weight in all experiments, and voles and squirrels were not able to maintain positive nitrogen balance when fed Bryoria spp. lichens. As a result, small mammals must supplement a winter lichen diet with a protein source. We conclude that red-backed voles and northern flying squirrels are able to survive on diets of fungi and lichens by diet mixing. Lichens are eaten because they are high in calcium and are easily digested, but fungi provide nitrogen (protein) and many other vitamins and minerals including vitamin D.