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Nutritive Value Of Fungi

Introduction

Hypogeous fungi clearly play an important part in the nutrition of at least some Australian mammals. A great diversity of fungi is consumed and the fungi are eaten for most of the year by at least some animals. Thus the fungi must fulfil the dietary requirements of these animals.

However, fungi consist largely of indigestible complex polymers. Unless animals have some mechanism to break apart the fungal structures and break down these polymers, their dietary needs will not be met. Small mammals require specific mechanisms to utilise their complex polymers because the materials in hypogeous fungi otherwise pass too rapidly throuh the gastrointestinal tract.

Nutritive Components of Fungi

Few studies have examined the nutritive qualities of fungi. Cork and Kenagy (1989) found that fruit bodies accumulated higher concentrations of phosphorus, nitrogen and potassium than seed and leaves. Along with other minerals, the few fungi they examined also contained a wide variety of organic compounds, including vitamins, amino acids, steroids and various polymers. Lipids are common storage compounds in the cytoplasm, and the walls consist of complex polymers including proteins and chitin. LINK Thus fungal sporocarps have the potential to be adequate foods for animals with an appropriate digestive system. The potential may be met if the animal can also select different foods that contain the various requirements not provided by fungi.

In a feeding trial, squirrels fed sporocarps of Elaphomyces lost weight. The diet contained adequate total nitrogen and energy. However, the nitrogen and energy were in a form that was inaccessible to the squirrel. Further, a considerable proportion of the spores appear to pass through the GIT unaffected. The study cannot be applied that easily to all animals in the field. One fungus was used and animals would not be expected to subsist on one food source. Squirrels have an omnivorous diet and normally feed on a range of foods. However, the study indicates some of the difficulties faced by mycophagous animals.

Little is known of the dietary qualities of Australian fungi. While lipids are likely to be abundant in spores, lipid only becomes available if the spores are broken apart. Protein may be adequate but the composition of amino acids is probably unbalanced and some may be unavailable. The fungi Mesophellia and Rhizopogon appear to have high total nitrogen, though much of it in a protected complex form. Cell walls are also fibrous indicating that they are similar to Elaphomyces, which incidentally is also found in Australia. As animals will consume a wide variety of fungi during the year, is can be assumed that they will be exposed to a range of quality and quantity of each dietary need. However, these issues indicate that specific digestive processes are needed to maximise the benefits of consuming fungi.

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Mycophagous Animals


Complex stomach of a Bettong indicating foregut fermentation.

Diagram of the gastro-intestinal system of a Bandicoot.

In Australia, mycophagy is reported most commonly among members of the rat kangaroos (Potoroidae) and rodents (Muridae). Most animals are quite small, mostly weighing between 100 and 3000 g. Of these, only the potoroos appear to have the digestive system needed to utilise hypogeous fungi. Indeed, some members of the Potoroidae are known to rely on fungi for most of their nutrition. LINK These animals have foregut fermentation. Foregut fermentation enables a more complete fragmentation of food, with microbial digestion, followed by further digestion and extraction of nutrients by the host. Indeed, the variety of microbes in the GIT might be expected to modify the balance of amino acids and produce vitamins needed by the host. LINK These nutrients would then be absorbed on passage through the gut.

Studies indicate that rat kangaroos can maintain weight on a limited array of fungi, when the fungi are the only source of food. Further, field studies indicate that some rat kangaroos appear to gain weight when fruiting bodies are readily available. This suggests that foregut fermentation is necessary to maximise the nutritive benefits from the highly protected fungal structures.

In contrast, Rats and Bandicoots lack foregut fermentation. They consume a smaller proportion of fungi in their diet, presumably using the fungi for its specific qualities. Hindgut fermentation is apparently a less efficient form for extracting nutrients from complex polymers. The immediate passage to excretion reduces the opportunity for removal of essential amino acids and vitamins formed by the microbes. Thus rodents need other sources of nutrition to balance their diet.

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Conclusion

A small number of Australian animals rely extensively on fungi for their nutrition. They all appear to have foregut fermentation. Thus the nutritive value of fungi is supplemented by microbial modification and digestion.

Mycophagous animals that lack foregut fermentation use fungi, but do not rely on fungi for all their dietary needs. The precise benefit of fungi in the diet to these animals is unclear. However, we might assume that specific requirements are being met from fungi with the other dietary components supplementing the fungi.

Overall, hypogeous fruiting bodies are potentially valuable sources of food. However, the fungal structures are protected in a way that prevents all but a few animals with foregut fermentation from using the full potential of the fruiting bodies.

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References

Claridge AW (2001) Ecological role of hypogeous ectomycorrhizal fungi in Australian forests and woodlands. Plant and Soil.

Claridge AW, Castellano MA & Trappe JM. (1996) Fungi as a food resource for mammals in Australia. In: Fungi of Australia Vol 1B Fungi in the Environment. ABRS, Canberra.

Cork SJ & Kenagy GJ (1989) Nutritional value of a hypogeous fungus for a forest-dwelling ground squirrel. Ecology 70, 577 – 586.

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