Nutritional Value of Hypogeous Fungus for a Forest‐Dwelling Ground Squirrel

Hypogeous fungi often contain high concentrations of nitrogenous compounds, vitamins, and minerals and are major dietary items for many rodents and other small mammals in North American forests. For these reasons ecologists have presumed hypogeous fungi to be of high nutritional value for small mammals. In a series of feeding experiments we investigated the nutritional quality of sporocarps (fruiting bodies) of the hypogeous fungus Elaphomeyces granulatus in the diet of the golden—mantled ground squirrel Spermophilus saturates, which is highly mycophagogus throughout its season of activity. We compared the digestive utilization of the fungus by this small mammal with digestibility of a range of plant foods available in nature. The sporocarps of E. granulatus had a high concentration of nitrogen, but 80% of this nitrogen was in the totally indigestible spores and the relatively indigestible cell walls of the peridium, and only half of the remaining 20% was protein nitrogen. Even though the squirrels ate the peridium in preference to the spore—laden core, the digestibility of the ingested nitrogen was only °50%, which is much lower than the digestibility of most plants. The digestible energy content of the fungus was low compared with that of conifer seeds and similar to that of grasses and legumes that are naturally eaten by the ground squirrels in small amounts. The overall (dry—matter) digestibility of E. granulatus sporocarps (60%) appears to be near the minimum digestibililty on which golden—mantled ground squirrels are able to maintain themselves. Therefore, sporocarps of E. granulatus cannot be regarded as a high—quality dietary items in terms of the availability of nutrients. We suggest that this is true for hypogeous fungi in general and that their value as a food resource derives mainly from their great abundance during the active season of small mammals such as ground squirrels and from their emission of strong odors, which makes mature sporocarps highly detectable. These features should maximize the yield of energy and nutrients to small mammalian mycophagists in relation to foraging effort.