The Koala is the only mammal, other than the Greater Glider and Ringtail Possum, which can survive on a diet of eucalyptus leaves. Eucalyptus leaves are very fibrous and low in nutrition, and to most animals are extremely poisonous. To cope with such a diet, nature has equipped Koalas with specialised adaptations.
A very slow metabolic rate allows Koalas to retain food within their digestive system for a relatively long period of time, maximising the amount of energy able to be extracted. At the same time, this slow metabolic rate minimises energy requirements. Koalas also sleep somewhere between 18 and 22 hours each day in order to conserve energy.
The Koala's digestive system is especially adapted to detoxify the poisonous chemicals in the leaves. The toxins are thought to be produced by the gum trees as a protection against leaf-eating animals like insects. Trees which grow on less fertile soils seem to have more toxins than those growing on good soils. This could be one reason why Koalas will eat only certain types of eucalypts, and why they will sometimes even avoid them when they are growing on certain soils.
Photos by August, Hsueh-Cheng Ho
Koalas have a special fibre-digesting organ called a caecum. Other animals, such as humans also have a caecum, but the Koala's is very long (200 cms). The caecum contains millions of bacteria which break down the fibre into substances which are easier to absorb. Even so, the Koala is still only able to absorb 25 per cent of fibre eaten. Water is also absorbed from the gumleaves, so that Koalas rarely need to drink, although they can do so if necessary, such as in times of drought when the water content of the leaves is reduced.
P.Schouten, From 'Koalas, the little Australians we'd all hate to lose' Bill Phillips AGPS
Each Koala eats approximately 200 to 500 grams of leaves per day. The teeth are adapted to deal with their specialised diet. The sharp front incisors nip the leaves from the tree. The molars, or back teeth are shaped to allow the Koala to cut and shear the leaves rather than just crush them. A gap between the incisors and the molars, called a 'diastema', allows the tongue to move the mass of leaves around the mouth efficiently.
Koalas are very fussy eaters and have strong preferences for different types of gum leaves. In Australia there are over 600 types of eucalypts, but Koalas will not eat a large proportion of these. Within a particular area, as few as one, and generally no more than two or three species of eucalypt will be regularly browsed. (we call these "primary browse trees") while a variety of other species, including some non-eucalypts, appear to be browsed occasionally or used for just sitting or sleeping in.
Different species of eucalypts grow in different parts of Australia, so a Koala in Victoria would have a very different diet from one in Queensland. Also, just think how boring it would be to eat the same thing every day. Koalas like a change, too, and sometimes they will eat from other trees such as wattle, tea tree or paperbark.