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Ingestion and excretion of Eucalyptus punctata D. C. and its essential oil by the koala, Phascolarctos cinereus (Goldfuss)

Eberhard, IH, McNamara, J, Pearse, RJ & Southwell, IA 1975, Australian Journal of Zoology, vol. 23, pp. 169-179.

This study describes a feeding trial during which ingestion and excretion by four captive koalas fed an exclusive diet of Eucalyptus punctata punctata was examined. Findings showed that 60% of leaf dry matter was digestible and of the volatile oil ingested, an insignificant proportion was excreted, with 7-30% passing through to the faeces and 1% in the urine.

  During the trial, each koala spent a total of six days in a metabolism cage, where faecal and urinary excretion was measured. Urine, faeces and leaf samples were steam-distilled to obtain their oils and gas-liquid chromatography was performed with infrared spectra measured. The mean weight of organic matter ingested (g/24h) and faeces excreted by all four koalas was 440 and 166 respectively. Steam distillation showed that mean oil yields differed significantly between leaf, faeces and urine samples (1.3%, 0.59% and 0.063% respectively). Infrared spectra revealed the relative proportions of alcohol, carbonyl and hydrocarbon components. Results showed that carbonyl content did not impact the amount of plant matter ingested. Gas-liquid chromatography analysis indicated that, compared to the faecal oil, leaf oil differed in the percentage composition of a number of components. Additionally, chromatograms showed three oil components that were present in the faeces, but not in the ingested material. Infrared spectra of faecal oils highlighted the removal of a large proportion of carbonyl compounds and a reduction in cineole (eucalyptol).

  As, on average, only a small proportion of the ingested oil (15%) was shown to be excreted, this suggests that a significant portion is absorbed during digestion. Transformation within the gut is likely to have led to the occurrence of novel components in the faecal oil. A further explanation is that these components arose after absorption by the liver and subsequent excretion via the bile duct. Toxins are converted into non-toxic glucuronides in the liver, where excretion to the intestine via the bile, followed by hydrolysis and further metabolism, is likely to have created new components. Throughout the trial, koalas ingested, on average, 0.1g of cineole per kg of bodyweight per day; considerably lower than the calculated lethal dose of 1.2g for the brushtail possum. Other research reports, however, that koalas excrete 2-3 times more glucuronic acid than possums,  alluding to a significantly greater potential for detoxification. In turn, it is possible that the high glucuronic acid content in the urine of the koalas explains the detoxification of over 50% of oil ingested. The effect of essential oils on caecal flora was also considered. As components with high germicidal values were shown to be largely absent from faecal oils, highly selective absorption or transformation is likely to take place throughout digestion. This suggests that essential oils may play an antibacterial role.

  This study has provided important information regarding the essential oil composition of excreta and ingesta, giving insight into the significance of volatile oils for koalas. Additional research on wild populations concerning favoured and avoided species, their volatile chemical composition and their impact on physiology is recommended.

 

Summarised by Julian Radford-Smith

 

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