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Digestion, nutrition & metabolism

Levels of trace elements in the liver and diet of free-living koalas, Phascolarctos cinereus (Goldfuss)

McOrist, S & Thomas, KW 1984, Journal of Wildlife Diseases, vol. 20, no. 3, pp. 220-225.

In free-living koalas in Victoria, reference values were determined for a number of different trace elements in the liver and blood plasma. The findings were comprehensive, ultimately revealing suboptimal copper levels in both the plasma and liver, especially in older koalas. No significant trends with age were identified for cobalt, zinc or manganese levels.

  Twenty-eight koalas from East Gippsland and French Island, of which only seven were alive, were submitted for examination, where several samples were taken from the liver of each individual. Samples were tested for trace metal concentration, measured via absorption spectroscopy. Eucalypt foliage samples were taken from the two study sites and tested for trace element content. Post-mortem inspections showed that none of the koalas exhibited any conditions likely to cause trace element deficiencies. Liver examinations revealed the existence of liver cell granules, where a general trend of increasing granule number with higher age class was recognised. Mean concentration (mmol/kg) of copper, manganese, zinc and cobalt was shown to be 0.25, 0.2, 2.97 and 2.81 respectively, with copper concentration comparable between the sexes. Copper concentrations also appeared to be inversely correlated with age.

  Given the post-mortem results, the concentrations of trace elements established here are unlikely to have been influenced by conditions such as liver disease or parasitism. This is further confirmed by the finding that liver trace element levels are similar to those reported in other hindgut fermenters, including the rat, rabbit and horse. Additionally, the trend of decreasing liver copper concentration with age is consistent with trends in both the rabbit and rat. When compared to other hindgut fermenters, however, mean copper levels in the plasma are noticeably lower in the koala. This may be explained by the low copper levels present in diet of koalas, with data showing that copper levels in the foliage sampled from the study sites are low. Such findings can possibly be attributed to relatively low copper concentrations in the soil at both sites. Build-up of hepatocyte granules with age is a likely consequence of the koala’s Eucalyptus diet, through which a significant quantity of volatile oils are ingested. Accordingly, it was proposed that granule formation results from the detoxification of such oils.

  This study assessed the relationship between diet and trace element composition and concentration in the liver of free-ranging koalas. Findings indicated inadequate copper levels, particularly in elderly koalas; however, this is likely a manifestation of poor diet. As such, further research is required to compare these findings to koalas in alternate study areas where soil and foliage quality is higher.

 

Summarised by Julian Radford-Smith

 

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