1. Many field studies have shown that small herbivorous mammals include fungus (usually hypogeous sporocarps of ectomycorrhizal fungi) in their diets. However, the dietary importance of fungus relative to other foods is generally unclear because of limitations on the power of conventional techniques of diet analysis. Stable isotope analysis in conjunction with faecal analysis was used in an attempt to overcome these limitations.
2. Two foregut-fermenting marsupials (the Northern Bettong Bettongia tropica and Rufous Bettong Aepyprymnus rufescens) and a hindgut fermenter (the Northern Brown Bandicoot Isoodon macrourus) were studied. The Northern Bettong and Northern Brown Bandicoot are of similar body size (around 1 kg); the Rufous Bettong is significantly larger at 3 kg. Faecal analysis showed that the two bettongs ate a variety of grasses, lilies and fungi; the bandicoot ate these foods and also invertebrates.
3. Ratios of 15N/14N and 13C/12C differed in major food types collected in the field (fungus, grass, lily and invertebrates). Grass was clearly separated from the other food types by its low 13C/12C ratio, while fungus was separated from the other types by its high 15N/14N ratio. Invertebrates and lilies differed slightly in 13C/12C ratios.
4. Isotope ratios in body tissue (sampled in hair) of the three mammals were also discrete, showing that the species differed in the predominant sources of their C and N. Estimates of the proportion of C assimilated in body tissue that was derived from grass were 80% for the Rufous Bettong, 40% for the Northern Bettong and 45% for the Northern Brown Bandicoot. Analysis of 15N/14N ratios suggested that the Northern Bettong derived almost all its N from fungus, the Northern Brown Bandicoot derived practically no N from fungus, and the Rufous Bettong was intermediate.
5. The results confirm that for the Northern Bettong, fungus is a predominant source of N and C assimilated into body tissue. Differences between the use of fungi by the Northern Bettong and the Northern Brown Bandicoot strengthen conclusions from other studies that foregut fermentation confers on small mammals a greater ability to utilize fungus than does hindgut fermentation. It is hypothesized that the limited use of fungus by the Rufous Bettong is due to the patchy distribution of hypogeous sporocarps, which would result in a high energy cost of foraging for this larger-bodied species with a higher absolute food requirement.