Fundamental dietary specialisation explains differential use of resources within a koala population

The diets of individual animals within populations can differ, but few studies determine whether this is due to fundamental differences in preferences or capacities to eat specific foods, or to external influences such as dominance hierarchies or spatial variation in food availability. The distinction is important because different drivers of dietary specialisation are likely to have different impacts on the way in which animal populations respond to, for example, habitat modification. We used a captive feeding study to investigate the mechanisms driving individual dietary specialisation in a population of wild koalas (Phascolarctos cinereus) in which individuals predominantly ate either Eucalyptus viminalis or Eucalyptus obliqua foliage. All six koalas that primarily ate E. viminalis in the wild avoided eating E. obliqua for more than 1 month in captivity. In contrast, all seven koalas that primarily ate E. obliqua could be maintained exclusively on this species in captivity, although they ate less from individual trees with higher foliar concentrations of unsubstituted B-ring flavanones (UBFs). Our results show that fundamental differences between individual animals allow some to exploit food resources that are less suitable for others. This could reduce competition for food, increase habitat carrying capacity, and is also likely to buffer the population against extinction in the face of habitat modification. The occurrence of fundamental individual specialisation within animal populations could also affect the perceived conservation value of different habitats, translocation or reintroduction success, and population dynamics. It should therefore be further investigated in other mammalian herbivore species.

Four species of arboreal folivore show differential tolerance to a secondary metabolite

The marsupials that eat Eucalyptus in south-eastern Australia provide an example of animals with similar niche requirements occurring sympatrically. They certainly differ in size, ranging from about 1 kg in the greater glider (Petauroides volans) and the closely related common ringtail possum (Pseudocheirus peregrinus), to 4 kg (common brushtail possum, Trichosurus vulpecula) and up to 15 kg in the koala (Phascolarctos cinereus). All species, however, may eat considerable amounts of eucalypt foliage, often favouring the same species, and thus appear to compete for food. In order to better understand the degree of competition for food, we measured feeding by the greater glider in response to increasing concentrations of a specific group of eucalypt plant secondary metabolites (PSM), the sideroxylonals, and then compared it to results published for the other species. The greater glider was more resilient than the other species to increasing concentrations of sideroxylonals. We suggest this allows gliders to feed on leaves from the eucalypt subgenus, Symphyomyrtus, while its small size and gliding ability allow it to feed where koalas cannot, on the young leaves on top of the canopy. In contrast, the common ringtail possum is well adapted to feeding from species of the subgenus Eucalyptus, which do not produce sideroxylonals but contain less available nitrogen (AvailN) than do the symphyomyrtles. These 'nutritional niches' segregate the forest and along with other factors, such as generalist and specialist feeding strategies and differences in body size and requirements for shelter, presumably minimise competition between the marsupial species.

Age-dependent changes in gross and histological morphology of the thyroid gland in South Australian koalas (Phascolarctos cinereus)

Studies characterising the thyroid gland structure of koalas are limited and have not previously been undertaken in South Australian populations. Hence, this study aimed to describe the thyroid gland morphology of koalas from the Mount Lofty region, South Australia. Results showed that thyroid gland morphology was highly variable between individual koalas (n = 36), ranging from that considered typical for healthy mammals, in which small colloid-filled follicles were lined by cuboidal epithelium, to that consistent with colloidal goitre, in which macrofollicles distended with colloid were lined by flattened epithelium. Juvenile koalas more frequently showed typical thyroid gland morphology than adults, with significantly higher thyroid follicle density (P < 0.05) and a higher proportion of follicles lined by cuboidal epithelium compared with flattened epithelium (P < 0.05). Thyroid glands of most adult koalas were characterised by colloidal macrofollicles (P < 0.01), and classified as colloidal goitre. There were no significant differences in thyroid gland morphology based on health status or sex of koalas. These findings suggest that an age-dependent colloidal goitre occurs in adult koalas, which is unlikely to have ill effects and may be associated with the low metabolic rate of this species, or exposure to a dietary goitrogen.

Metabolic fate of dietary terpenes fromEucalyptus radiata in common ringtail possum (Pseudocheirus peregrinus)

Arboreal marsupials consume terpenes in quantities that are toxic to other mammals, indicating that they possess special detoxification mechanisms. The metabolic fate of dietary terpenes was studied in the common ringtail possum (Pseudocheirus peregrinus). Three animals were fedEucalyptus radiata leaf for 10 days. Leaf consumption increased over three days to an average steady state of about 10-15 mmol total terpenes per day. GCMS analysis identified six urinary terpene metabolites, which were dicarboxylic acids, hydroxyacids, or lactones. Another nine metabolites could only be shown to be terpene-derived but of unknown structure. The amounts excreted were estimated by GC-FID, using response factors based on carbon content. Total 24-hr excretion of terpene-derived metabolites increased to 6.2-7.6 mmol on days 5-10, while glucuronic acid excretion remained constant at about 1.5 mmol. No other conjugates of terpene metabolites were found. The strategy used by the possum to detoxify dietary terpenes seems to be to polyoxygenate the molecules forming highly polar, acidic metabolites that can be readily excreted. Conjugation is minimal, perhaps to conserve carbohydrate and amino acids.

The physiology of the honey possum, Tarsipes rostratus, a small marsupial with a suite of highly specialised characters: a review

Field and laboratory studies of the iconic nectarivorous and 'pollenivorous' honey possum, Tarsipes rostratus, are reviewed with the aim of identifying aspects of its physiology that are as yet poorly understood and needed to implement management strategies for its long-term conservation. Dietary specialisations include the loss of teeth, a modified gut with a high rate of passage, exceptionally low minimum nitrogen requirements, an apparently high basal metabolic rate and a permanently polyuric kidney. In contrast, its reproductive physiology is plesiomorphic, combining aspects such as a post-partum oestrus, embryonic diapause, photoperiodicity and extended maternal care that are usually separate characteristics of other marsupial groups. In common with a number of other marsupials, the honey possum has the potential for trichromatic colour vision and has been the subject of several studies attempting to correlate visual quality with ecological realities. Field physiological studies have established its high rates of nectar and pollen intake needed to maintain energy balance and highlight the need for a constant intake from floral sources. Early allometric studies suggesting that the honey possum's relatively low reproductive rate may be linked to a diet limited in protein have not been supported and nitrogen intakes in the field exceed by a factor of 10 the animal's basic requirements for balance. Measurements of rates of protein turnover in field-caught lactating females suggest that they divert nitrogen from the protein pool to milk production by reducing rates of degradation, rather than by increasing rates of synthesis of protein. Although not yet an endangered species, the honey possum's habitat has been drastically reduced since European occupation of Australia and future-targeted research on the animal's unique physiology and habitat linkage is needed that can be translated into effective management practices. Only then will its long-term survival be assured.

Winter as a nutritional bottleneck for North American porcupines (Erethizon dorsatum)

North American porcupines are distributed across a wide variety of habitats where they consume many different species of plants. Winter is a nutritional bottleneck for northern populations, because porcupines remain active when environmental demands are high and food quality is low. We used captive porcupines to examine physiological responses to low-quality diets at high energy demands during winter at ambient temperatures as low as -39 degrees C. We did not observe an endogenous pattern of body mass gain or loss when porcupines were fed a low nitrogen diet (1.1% dry matter) ad libitum through winter. Dry matter intake declined from 43.6 to 14.6 g kg(-0.75) d(-1) even though ambient temperatures declined from -3 to -30 degrees C, which indicates a seasonal decrease in metabolic rate. Porcupines consuming white spruce needles maintained digestive efficiency for energy (61%) and neutral detergent fiber (NDF) (50%). However, low requirements for energy (398 kJ kg(-0.75) d(-1)) and nitrogen (209 mg kg(-0.75) d(-1)) minimized the loss of body mass when intakes were low and plant toxins increased urinary losses of energy and nitrogen. Porcupines were also able to tolerate low intakes of sodium, even when dietary potassium loads were high. Porcupines use a flexible strategy to survive winter: low requirements are combined with a high tolerance for dietary imbalances that minimize the use of body stores when demands exceed supply. However, body stores are rapidly restored when conditions allow. Porcupines posses many physiological abilities similar to specialist herbivores, but retain the ability of a generalist to survive extreme conditions by using a variety of foods.

Woodrat (Neotoma) herbivores maintain nitrogen balance on a low-nitrogen, high-phenolic forage, Juniperus monosperma

The acquisition of adequate quantities of nitrogen is a challenge for herbivorous vertebrates because many plants are in low nitrogen and contain secondary metabolites that reduce nitrogen digestibility. To investigate whether herbivores maintain nitrogen balance on plant diets low in nitrogen and high in secondary compounds, we studied the effect of juniper (Juniperus monosperma) ingestion on the nitrogen balance of two species of herbivorous woodrats (Neotoma stephensi and N. albigula). These woodrat species feed on the foliage of juniper: N. stephensi is a juniper specialist, whereas N. albigula is a generalist that incorporates some juniper in its diet. Based on the nitrogen contents of the natural diets of these woodrats, we predicted that the generalist would be in negative nitrogen balance on a juniper diet whereas the specialist would not be affected. We found that both species of woodrat had low-nitrogen requirements (334.2 mg N/kg0.75/day) and that a diet of 50% juniper did not result in negative nitrogen balance for either species. However, excretion patterns of nitrogen were altered; on the 50% juniper diet, fecal nitrogen losses increased approximately 38% and urinary nitrogen losses were half that of the control diet. The results suggest that absorption and detoxification of juniper secondary compounds may be more important for restricting juniper intake by the generalist than nitrogen imbalance.

Nitrogen requirement of the omnivorous greater bilby, Macrotis lagotis (Marsupialia: Peramelidae)

Mainly due to their utilisation of relatively low-fiber diets compared to herbivorous mammals, omnivores are expected to have correspondingly low maintenance nitrogen requirements (MNRs). The limited studies examining nitrogen requirements of omnivorous mammals to date have shown this to be the case. In this article, we determine the dietary MNR of greater bilbies (Macrotis lagotis), arid-zone omnivorous marsupials, by feeding them varying proportions of mixed seeds and dried currants (sun-dried grapes). We also examine the possibility that bilbies conserve nitrogen by recycling endogenous urea to their gastrointestinal tract. The dietary MNR of 127 mg N kg(-0.75) d(-1) calculated for the bilby falls within the range calculated for other marsupial omnivores and is lower than that of any of the herbivores. This low requirement for nitrogen was correlated with significant recycling of endogenous urea to the gut; bilbies recycled between 44% and 80% of urea synthesised in the liver, but the proportion recycled was independent of dietary nitrogen intake. The relatively low MNR of the bilby is consistent with its low rates of basal metabolism and the recycling of endogenous urea to the gut, both of which reduce urinary nitrogen losses, and a low-fiber diet that minimises metabolic fecal nitrogen loss. Because the bilby inhabits an environment where its food supply is unpredictable and of seasonally low nitrogen content, this species is likely to benefit from its low requirement for nitrogen.

A comparison of the nitrogen requirements of the eastern pygmy possum, Cercartetus nanus, on a pollen and on a mealworm diet

The eastern pygmy possum, Cercartetus nanus, is known to feed both on flower products and on invertebrates. This study compares its ability to meet its nitrogen requirements on pollen and on insect larvae. Captive C. nanus were fed diets in which nitrogen was provided either by Eucalyptus pollen or by the mealworm Tenebrio molitor. The apparent digestibility of the nitrogen from both sources was high, with a mean value of 76% for the pollen and 73% for the mealworms. This was much higher than would have been inferred from the common practice of measuring the percentage of empty pollen grains in fecal samples. The truly digestible maintenance nitrogen requirements of C. nanus on pollen were exceptionally low: 2.6 mg N d-1 compared with 9.5 mg N d-1 on mealworms. The value for pollen is the lowest yet recorded for any mammal. The difference between the requirements of C. nanus on the two diets appeared to be related to the composition of the mealworm and pollen protein. The biological value of the pollen nitrogen was exceptionally high for a plant protein, at 72%, whereas the biological value of the mealworm nitrogen was only 42%. This suggests that the amino acid composition of the pollen corresponded more closely to the requirements of C. nanus than the composition of the mealworm protein did. Pollen is an excellent source of nitrogen for C. nanus, and it should be considered as a potential nitrogen source for other flower-feeding animals.