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.

The effects of elevated CO2 atmospheres on the nutritional quality of Eucalyptus foliage and its interaction with soil nutrient and light availability.

Seedlings of Eucalyptus tereticornis (Smith) were grown under two levels of availability each of CO2 (352 and 793 µmol mol-1), soil nutrients (1/24 and 1/4 Hoagland's solution) and light (full and 30% sunlight). Low soil nutrient availability or high light increased the C:N ratio of leaves, leading to lower leaf nitrogen concentrations, higher leaf specific weights and higher levels of both total phenolics and condensed tannins. These results were consistent with other studies of the effect of environmental resource availability on foliage composition. Similar results were observed when the C:N ratio of leaves was increased under elevated CO2. The changes in leaf chemistry induced by the treatments affected the performance of 4th-instar larvae of Chrysophtharta flaveola (Chapuis) fed on the leaves. Increased C:N ratios of leaves reduced digestive efficiencies and pupal body sizes and increased mortality. Below a threshold nitrogen concentration of approximately 1% dry mass, severe reductions in the performance of larvae were recorded. Such changes may have significant consequences for herbivores of Eucalyptus, particularly in view of projected increases in atmospheric CO2.

Consequences of biotransformation of plant secondary metabolites on acid-base metabolism in mammals-A final common pathway?

Regulation of acid-base homeostasis is essential for mammals and birds. Biotransformation and metabolism of absorbed plant secondary metabolites (PSMs) results in the production of organic acids that threaten acid-base homeostasis. Consequently these acids must be buffered and excreted from the body. The production of an acid load from detoxified PSMs should occur in herbivorous mammals and birds and with most PSMs and so may provide a unifying theme to explain many effects of PSMs on animal metabolism. Since the organic acids will be largely ionized at physiological pH, disposal of the hydrogen ion and the organic anion may proceed independently. Most hydrogen ions (H(+)) from organic acids are eliminated by one or more of three ways: (1) when they react with bicarbonate in the extracellular fluid to form carbon dioxide and the carbon dioxide is exhaled, (2) when they bind to dibasic phosphate and are excreted by the kidney as monobasic phosphate, and (3) when they are buffered and retained in the skeletal system. The secretion of phosphate ions and ammonium excretion are two ways in which the kidney replaces bicarbonate ions that have been eliminated as carbon dioxide. Secretion in the kidney tubule is an important means of excreting excessive organic anions rapidly. This process is saturable and may be subject to competition from a variety of different metabolites. Lagomorphs have limited capacity to form new bicarbonate from ammonium excretion and may therefore be obliged to excrete other cations such as sodium to balance the excretion of organic anions from PSMs. Acidemia has wide-ranging impacts on animals but browsing mammals and birds may have to break down muscle tissues to provide for urinary ammonium in order to generate bicarbonate for buffering. Acidemia also can affect the extent of urea recycling. Animals consuming browse diets may have to regulate feeding so that the rate of formation of hydrogen ions does not exceed the rate of disposal. The mechanisms by which this could occur are unknown.

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.

Use of fibrous diets by small herbivores: How far can the rules be 'bent'?

Allometric considerations suggest that small mammals should be unable to eat highly fibrous diets. A combination of the selective and more rapid passage of fibrous material through the gut, together with changes in gut capacity when energy requirements increase, may allow small mammals to escape these allometric constraints. Recent evidence that birds can absorb essential amino acids from the caecum (which has hitherto been considered insignificant in mammals) suggests that birds and mammals have evolved very different ways to be a herbivore.

Methods and pitfalls of extracting condensed tannins and other phenolics from plants: Insights from investigations onEucalyptus leaves.

Optimal conditions for extraction of tannins and other phenolics from tree foliage and their subsequent storage rarely have been investigated. We investigated methods of drying leaves, optimal solvents, and the effects of light and temperature on the extractability and stability of condensed tannins (proanthocyanidins) and total phenolics from leaves ofEucalyptus trees. Aqueous acetone was a better solvent than aqueous methanol for condensed tannins and total phenolics, but condensed tannins were less stable in aqueous acetone than aqueous methanol. Stability of condensed tannins also was decreased substantially by room temperature versus 4°C and by exposure to indirect sunlight, although the assay for total phenolics was unaffected. For quantitative estimation of condensed tannins, extraction with 50% acetone was better than methods of direct analysis of leaf tissue. The highest estimates of total condensed tannins were obtained by exhaustive extraction with 50% acetone followed by direct analysis of the residue. Lyophilization of fresh leaf increased yield of condensed tannin (although usually by less than 10%). Lyophilization and subsequent storage of extracts had little effect on assays for condensed tannins or total phenolics.