Digestion and nitrogen metabolism in the koala, Phascolarctos cinereus
Harrop, CJF & Degabriele, R 1976, Australian Journal of Zoology, vol. 24, no. 1, pp. 201-215.
The koala’s digestive tract is shown here to be well-suited to the consumption of fibrous Eucalyptus leaves. The caecum, the sac at the beginning of the large intestine that stores food in herbivores and allows bacteria to break down cellulose, was large, as would be expected for a specialised mammalian herbivore. Koalas maintained a positive but low nitrogen level over the experimental period, with digestibility of nitrogen lower in winter, requiring more nitrogen to be consumed. This was demonstrated by mean nitrogen balances and apparent digestible nitrogen intakes not being significantly different between summer and winter, and the daily dietary nitrogen intakes differing significantly between seasons (0.426 g/kg W0.75 in summer and 0.493 g/kg W0.75 in winter).
Absolute lengths, relative lengths and flat widths of sections of the digestive tract were recorded during necropsies of three koalas, with the caecum observed as the dominant feature of their digestive tracts, taking up 19-23% of the post-gastric intestinal length. Dry matter intake and faecal output were measured to determine apparent digestibility of the Eucalyptus punctate provided, and the nitrogen content of the leaves, faeces and urine were also recorded to calculate nitrogen intake and output. The animals maintained a positive nitrogen balance throughout the entire experiment, with the exception of one week for one koala, suggesting that they were able to take sufficient nitrogen from their diet. Higher dietary nitrogen intakes and similar apparent digestible nitrogen intakes in winter, however, demonstrated that more foliage was needed to be consumed in winter to maintain the same nitrogen level as in summer. This effect was likely due to the quality of the leaves offered, although could possibly be due to a seasonal difference in metabolic efficiency.
The necropsies revealed that the caecum of the koala is considerably larger than other species previously studied, including rabbits and brushtail possums of a similar size, both in terms of absolute length and proportion of post-gastric intestinal length. In addition, previous studies had suggested a lower daily dry matter intake than this study demonstrated, however the apparent digestion was consistent with other studies of both koalas and other herbivores. Koala daily digestible nitrogen intake and urinary nitrogen output were also comparable with other herbivorous species such as kangaroos and sheep.
Greater knowledge of the anatomy and metabolism of the koala informs better care and treatment of koalas in captivity and understanding of koala behaviour in the wild, particularly with regards to diet and foraging.
Summarised by Laura Wait
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