Mycophagy by small mammals in the coniferous forests of North America: nutritional value of sporocarps of Rhizopogon vinicolor, a common hypogeous fungus

We evaluated the nutritional value of sporocarps of Rhizopogon vinicolor, a common hypogeous fungus in the coniferous forests of North America, for tw o small mammal species: the Californian red-backed vole (Clethrionomys cali fornicus) and the northern flying squirrel (Glaucomys sabrinus). Although t he nitrogen concentration of sporocarps was high, much of it was in non-pro tein form or associated with cell walls, suggesting that it may be of low n utritional value or protected from mammalian digestive enzymes. Sporocarps also had high concentrations of cell wall constituents, indicating low avai lability of digestible energy. When fed a diet of this fungus alone in a co ntrolled feeding experiment both mammal species lost a small amount of body mass. Digestibilities of dry matter, nitrogen, cell wall constituents and energy from sporocarps by both species were lower than the digestibilities of other food types by other similarly sized small mammals. Red-backed vole s digested the various components of sporocarps at least as well as the fly ing squirrels, even though they were almost six-fold smaller in body mass. This observation supports the notion that red-backed voles, like other micr otine rodents, have morphological and physiological adaptations of the dige stive system that are postulated to permit greater digestion of fibrous die ts than predicted on the basis of body size. Despite this, our results re-a ffirm previous conclusions that hypogeous fungi are only of moderate nutrit ional value for most small, hindgut-fermenting mammals. Future studies shou ld focus on the importance of mixed-species of fungi in the diet of small m ammalian mycophagists.