Foliage of Eucalyptus punctata and the maintenance nitrogen requirements of koalas, Phascolarctos cinereus
Cork, SJ 1986, Australian Journal of Zoology, vol. 34, pp. 17-23.
In an analysis of foliage fed to captive koalas, winter foliage was found to have a lower nitrogen content than summer foliage; however, this was compensated for by the koalas eating more during winter. Nitrogen balance in the koalas was, therefore, not significantly different between the seasons. Similarly, the excretion of nitrogen did not change seasonally but the non-dietary losses of nitrogen in the faeces were higher in the winter compared to summer.
Eight male koalas fed a sole diet of Eucalyptus punctata foliage were studied. The nitrogen composition of food, faeces, and urine were deduced. Undigested dietary nitrogen was determined by analysis of nitrogen content of isolated faecal cell-wall constituents that had not been digested. Approximations of digestible and dietary nitrogen requirements to maintain equilibrium were calculated as 271 mg . kg -0.75. d-1 and 283 mg . kg -0.75. d-1. This was similar to other previous reports for marsupials; however, comparatively low nitrogen excretion through urine was recorded. Further, in accordance with the standard metabolic rate of the koala, the faecal non-dietary nitrogen losses were found to be relatively large. Additional findings from the study include that faecal excretion of nitrogen was correlated to nitrogen intake, but no association was detected between the nitrogen excretion in the urine and the nitrogen intake.
Koalas exclusively feed on Eucalyptus tree foliage, which is characterised by its low nitrogen composition and minimal nutritional benefit. The high tannin concentration in this foliage may influence an animal’s nitrogen requirements through protein digestion interference. A deficiency of nitrogen in the foliage of Eucalyptus may be a limiting factor in koala distribution. The current paper aimed to investigate nitrogen metabolism in koalas. Koalas in the study had a lower urinary nitrogen excretion when compared to the majority of marsupials. This suggests that this excretion has an endogenous element that is related to the standard metabolic rate. The nitrogen maintenance requirements of koalas were greater compared to marsupials with diets that are fibrous, which is a product of the high non-dietary nitrogen loss in the faeces. The high concentrations of tannin and lignin in the Eucalyptus leaves are postulated to lead to the removal of epithelial cells and enzyme secretion into the gut which is then correlated to high protein losses in the faeces. Furthermore, there is little hindgut fermentation of microbes in koalas, reducing microbial protein production. The decreased nitrogen excretion in the urine may compensate for the high faecal losses and can be considered an adaptation to the diet.
The present study concludes that the high comparative faecal loss of endogenous nitrogen is a result of a diet primarily consisting of Eucalyptus foliage with the reduced standard metabolic rate being an adaptation that is critical to compensate such a diet. Further research into the possibility of nitrogen shortage limiting populations is warranted.
Summarised by Alexander Murdoch
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