Food Selection
Eucalyptus
As a herbivore, the Koala’s diet is often thought to contain only the leaves of the Eucalyptus tree. However, koalas are also known to forage on other gum trees, including Acacia, Allocasuarina, Callitris, Leptospermum, and Melaleuca. The koala’s low energy requirements allow it to consume solely (or, mostly) eucalyptus.
Despite its low nutritive value, the eucalyptus tree has over 600 species, of which the koala prefers approximately 30. The species of eucalyptus koalas prefer tend to contain high amounts of protein and low promotions of lignin and fibre, such as E. microcorys, E. tereticornis, and E. camaldulensis. These three species comprise about 20 percent of their diet. The benefits of eucalyptus foilage is its high water content which allows the koala to not descend the home tree to drink as often. In fact, the daily turnover rate for water is roughly 70-90 millilitres/kilogram of its body weight. While the female koala, smaller than the male, can essentially meet all their water requirements by acquiring it from foliage, the males require additional water from the ground.
Despite its low nutritive value, the eucalyptus tree has over 600 species, of which the koala prefers approximately 30. The species of eucalyptus koalas prefer tend to contain high amounts of protein and low promotions of lignin and fibre, such as E. microcorys, E. tereticornis, and E. camaldulensis. These three species comprise about 20 percent of their diet. The benefits of eucalyptus foilage is its high water content which allows the koala to not descend the home tree to drink as often. In fact, the daily turnover rate for water is roughly 70-90 millilitres/kilogram of its body weight. While the female koala, smaller than the male, can essentially meet all their water requirements by acquiring it from foliage, the males require additional water from the ground.
Since Eucalyptus has low levels of starch, amylase activity is quite low in the salivary glands, and the saliva contains a high buffering capacity due to high levels of bicarbonate and phosphate levels. As well, the sublingual and mandibular glands have rather large flow rates since koala’s are adapted to perform evaporative cooling through panting and licking themselves (Hume, 1999).
All species of eucalyptus contain terpenes. This volatile unsaturated hydrocarbon is found in the essential oils produced by plants, and can be potentially toxic, requiring metabolism to be removed from the koala’s system. A study by Boyle et al. (2000)), found that male koalas show an increase in the amount of highly oxidized metabolites compared to other marsupials, which indicates a “progression from a generalist feeder to a specialist feeder.” Eucalyptus typically grows in soil that is low in nutrients, leading to the consequently low level of nutrition available in the leaves and the synthesis of carbon-based defensive compounds known as phenols and terpenes (Hume, 1999). The utilization of this plant is also limited by secondary metabolites such as hydrolysable tannins, cyanogenic glycosides, essential oils, and low-molecular weight phenols are. These metabolites make it more difficult for cell wall carbohydrates, such as lignin, and condensed tannins (complexed with proteins) to be digested by microbes. Since the eucalyptus is an evergreen covered in oil glands (characteristic of its genus), the essential oil it produces can be extracted and function as a powerful disinfectant. While it is toxic in significant quantities, Marsupial herbivores such as the koala are quite tolerant to this oil, as well as other toxins in the leaf which the koala can detect the scent of, allowing it to make its leaf choice (Eschler et al. 2000). The essential oil is also toxic to bacteria and resultantly can reduce the efficiency of digestion for other dietary compounds including the polysaccharides which comprise the cell wall (Hume, 1999).
The caloric content, according to bomb and adiabatic calorimeter for Eucalyptus saligna is determined to be 4127 calories/g (Rodriguez, 1973). As well, the plant has a biomass (oven-dry kilograms) that is equivalent to the weight of the tree above ground, including leaves and twigs, estimated in a regression equation: 0.2510 D^2. Its pole weight or volume, in dry kilograms (x 10^-3 m^3) is roughly 0.291D^2 (Timyan 1996). References:
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According to another study by Hume and Esson (1993), the most influential factor in leaf preference for koalas is the ratio of nitrogen to fiber, and nitrogen to condensed tannins. It was also discovered that leaves with less that half its contents being water, and 2% essential oils, were rejected (1993). Ultimately, foliage selection for the koala is grounded in whether the leaves contain the minimum levels of water, nitrogen, essential oils, and little condensed tannins (1993).
The preferred species of Eucalyptus among koalas is shown to be one containing more volatile monoterpenes than sesquiterpenes, the latter of which are harder to detoxify (Hume, 1999). Koalas were found also to have anywhere from nine to twenty times higher catalase activities than "grazer” marsupials, and this catalase is pivotal in protecting the oxidation of hemoglobin. During times of sickness, Koalas activate the emetic system to limit their intake of secondary metabolites, specifically the species E. ovata and E. viminalis which are particularly high in these contents. Normally, the koala consumes 22-36g/kg^0.75 of E. ovata per day, and 14-46 g/kg^0.75 of E. viminalis according to Hume (1999). Compared to other marsupials, the koala demonstrates perhaps the most significant ability to utilize Eucalyptus as its primary and often, only, nutrient source. In a study by Stalenberg et al. (2014), research found that the density of koala populations varies with variation in the nutritional quality of the eucalyptus leaves they consume. The study sampled trees to test how leaf chemistry affects low-density, free-living koalas in New South Wales, Australia. The researchers determined an “interaction between toxins and nutrients in leaves at a small spatial scale” such that the koalas prefer leaves with a higher concentration of nitrogen and less secondary metabolites.
The data showed that in the eucalyptus leaves there was approximately 0.061 in terms of available nitrogen, 62.79 in DMD, and 3.77 in secondary metabolites. In the first graph one can examine the proportions of the three chemicals which comprise the total foliar nitrogen concentration (% dry matter) in different species of eucalyptus. The total nitrogen content is made up of fibre-bound, tania-bound, and available nitrogen. The second graph looks the mean concentrations of total secondary metabolites and total FPC (formylated phloroglucinol compounds) concentrations in four species of eucalyptus. In terms of nitrogen connect available, the leaves of E. bosistoana and cpellocarpa had the highest levels, while conversely two other species had the lowest |