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 two small mammal species: the Californian red-backed vole (Clethrionomys californicus) and the northern flying squirrel (Glaucomys sabrinus). Although the nitrogen concentration of sporocarps was high, much of it was in non-protein form or associated with cell walls, suggesting that it may be of low nutritional value or protected from mammalian digestive enzymes. Sporocarps also had high concentrations of cell wall constituents, indicating low availability of digestible energy. When fed a diet of this fungus alone in a controlled 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 voles digested the various components of sporocarps at least as well as the flying squirrels, even though they were almost six-fold smaller in body mass. This observation supports the notion that red-backed voles, like other microtine rodents, have morphological and physiological adaptations of the digestive system that are postulated to permit greater digestion of fibrous diets than predicted on the basis of body size. Despite this, our results re-affirm previous conclusions that hypogeous fungi are only of moderate nutritional value for most small, hindgut-fermenting mammals. Future studies should focus on the importance of mixed-species of fungi in the diet of small mammalian mycophagists.

Production of milk and nutrition of the dependent young of free-ranging koalas (Phascolarctos cinereus)

The production of milk by lactating females, and energy expenditure and foliage intake of their dependent young, were investigated in free-ranging koalas. Koalas had the lowest mass-specific daily milk-energy production at peak lactation so far recorded in a mammal, but the duration of reproduction was 58% longer than the combined marsupial and eutherian average. As a consequence, the total energy input to reproduction in koalas was similar to that in other mammals. We propose that the prolonged lactation and low daily rate of energy transfer to the young by female koalas is an adaptation to the low energy availability from their diet of Eucalyptus foliage. Energy requirements (field metabolic rates) of young koalas were lower than those expected for typical marsupials (only 60% at permanent pouch exit), which may be a necessary preadaptation that allows the low rate of maternal energy transfer. However, the energy requirements of the adult females were no lower than expected for marsupials. This pattern of energy requirements and age resulted in a linear relationship between field metabolic rate and mass for the koalas in this population. Differences in milk production between the years of the study coincided with fluctuations in the availability of preferred young foliage, which suggests that lactational output by koalas may be flexible and affected by diet quality. Despite the interannual differences in milk production, growth of the young was similar in the two years.

Glucose absorption by a nectarivorous bird: the passive pathway is paramount

We tested the hypothesis that most sugar absorption across the small intestine's brush border is normally by a mediated pathway, i.e., the Na(+)-glucose cotransporter. In nectar-eating rainbow lorikeets (Trichoglossus haematodus, 120 g), we measured mediated D-glucose uptake in vitro using the everted-sleeve technique. The apparent Michaelis constant (7.7 mM) was similar to that observed in hummingbirds and other birds in general. Maximal mediated D-glucose uptake summed along the entire length of intestine (48 cm) was not notably high (5.34 mumol/min) when compared with other avian species and was an order of magnitude too low to explain observed rates of glucose absorption in vivo (54.5 mumol/min). This implied that nonmediated glucose absorption predominated, and independent verification of that was sought. We applied a pharmacokinetic technique to measure in vivo absorption of L-glucose, the stereoisomer that does not interact with the Na(+)-glucose cotransporter. Eighty percent of L-glucose that was ingested was absorbed, confirming that nonmediated absorption can be substantial. We discuss how equating L-glucose absorption with passive D-glucose absorption depends on certain assumptions regarding the relative importance of diffusive and convective passive flux. In either case, the conclusion about the relative importance of passive absorption should still hold.

Nutritional-Value of Hypogeal Fungal Sporocarps for the Long-Nosed Potoroo (Potorous-Tridactylus), a Forest-Dwelling Mycophagous Marsupial

Although mycophagy (fungus-feeding) is widespread among small ground-dwelling mammals, there has been little evaluation of the nutritional benefits of this feeding habit. In Australia, some members of the Potoroidae (or rat-kangaroo family) consume large amounts of hypogeal fungi throughout the year. Hypogeal fungi appear to be of marginal nutritional quality for small mammals with simple stomachs but potoroos have an enlarged forestomach in which microbial fermentation takes place, and this may allow more effective utilisation of protected nitrogenous components and structural carbohydrates of fungi. In a feeding experiment, we evaluated the nutritional value of sporocarps of the hypogeal fungi Mesophellia glauca (Mg) and Rhizopogon luteolus (R1), for the long-nosed potoroo (Potorous tridacytlus). Although the concentration of nitrogen was high in both fungi, much of that nitrogen was in non-protein form or associated with cell walls and may be either of low nutritional value or protected from digestive enzymes. The concentration of cell-wall constituents (fibre) was high in both fungi, suggesting low availability of digestible energy. Despite these features, the digestibilities of dry matter (Mg 86%, Rl 80%), ingested nitrogen (Mg 72%, Rl 72%) and energy (kJ kg-1) (Mg 93%, Rl 76%) of both fungi were high. Consequently, P. tridactylus maintained positive nitrogen balance and high intakes of digestible and metabolisable energy. We conclude that the sporocarps of hypogeal fungi represent a nutritionally valuable food for rat-kangaroos and suggest that lack of a foregut-fermentation strategy in other similar-sized ground-dwelling mammals in the forests of south-eastern Australia explains why they use the hypogeal fungal resource to a lesser extent than do rat-kangaroos.

Foliage of Eucalyptus-Punctata and the Maintenance Nitrogen Requirements of Koalas, Phascolarctos-Cinereus

Nitrogen metabolism and balance were investigated in captive koalas fed mature foliage of a staple food tree, Eucalyptus punctata, in six feeding trials at various times of year. Winter foliage contained less nitrogen than summer foliage but koalas compensated by eating more in winter. Thus, nitrogen balance did not differ significantly between seasons. Urinary nitrogen excretion similarly showed no seasonal trend but losses of non-dietary (i.e. endogenous and microbial) faecal nitrogen were greater in winter than in summer, presumably because of the higher food intake. Estimates of the requirements of dietary and truly digestible nitrogen for maintenance of nitrogen equilibrium (283 mg . kg-0.75 . d-� and 271 mg . kg-0.75 . d-� respectively) were comparable with several estimates for other marsupials. Although urinary nitrogen.excretion by koalas was low compared with that by other marsupials, consistent with the koala's standard metabolic rate, losses of non-dietary faecal nitrogen were relatively high. It was concluded that relatively high losses of endogenous andlor microbial nitrogen in faeces are a consequence of feeding on Eucalyptus foliage and that the koala's low standard metabolic rate is an important adaptation permitting it to utilize this diet by partly compensating for faecal nitrogen losses.