Eucalypt leaves are not an ideal food source for several reasons – they contain toxins (essential oils and phenolics); they are low in protein; they contain a lot of plant cell walls, fiber, and cellulose, making them physically tough; and they are high in tannins that make them hard to digest. Nevertheless, there are many Eucalyptus trees in Australia, and koalas have evolved a specialized digestive system that allows them to survive exclusively on their leaves. Koalas have strong molars with ridges that cut and shear the leaves as efficiently as the finest scissors. The small fragments of leaves pass into the stomach where some nutrients, water and toxins are absorbed. The koala’s liver is able to isolate the toxins and the kidneys excrete them.
The remaining food passes from the stomach to an elongated sac called the caecum where it is further broken down by a population of specialized bacteria whose most important contribution is to help to ferment fiber, providing the koala with an essential source of energy. The bacteria are acquired from the mother at around the time the young koala, affectionately called a “joey,” starts eating leaves and exits the mother’s pouch (at about 22–30 weeks of age). The transfer of bacteria from mother to joey occurs by a process called “pap feeding,” where the mother produces special pasty faeces that are made up of caecal contents, which the joey consumes, and incorporates into its digestive system.
This adult male koala was photographed at Cape Otway, in Victoria, Australia feeding on the leaves of a coastal manna gum tree (Eucalyptus viminalis).
Ben Moore, PhD, Hawkesbury Institute for the Environment, Western Sydney University, Australia
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