In the photo – a koala (Phascolarctos cinereus) chews eucalyptus leaf in the zoo in Duisburg, Germany.
The koala bear diet consists almost exclusively of fresh eucalyptus leaves, so a rare zoo outside Australia can afford to keep these animals in captivity. Three koalas are living in Duisburg Zoo. Each of them eats 200-500 grams of leaves per day (depending on body weight, season, etc.). One eucalyptus grows right in the cage, the rest – on a plantation belonging to the zoo. Fresh branches of eucalyptus are also imported from Florida (USA) and kept in refrigerators.
In nature, the koala is used for food no more than 10% of eucalyptus species (about 700 of them are known). More than 20% of their diets are camaldulipte (Eucalyptus camaldulensis), E. microcorys, and E. tereticornis. However, despite their fastidiousness, sometimes koalas also eat tea tree leaves, acacia, allokazuarin, Callitris, and fine seeds. Some types of eucalyptus animals use it not for eating, but to sleep on them or to escape from overheating in heat (see a picture of Koala day on a tree).
Eucalyptus leaves contain a large number of secondary metabolites – phenolic compounds (such as tannins), terpenes – which can be lethal for most other mammals. Also, the leaves are very fibrous and low-calorie. So the koalas have few competitors for food resources.
The teeth and digestive system of koalas are perfectly adapted for such difficult food. They have sharp incisors that cut the leaves from the branch cutting the root teeth, and there is an interval (diastema) between the incisors and the root teeth that allows the tongue to effectively move the plant matter in the mouth. The blind gut of koalas is the longest relative to body size among mammals (of course, among those whose parameters have been studied). And the average retention time of substances passing through the gastrointestinal tract is the longest recorded for mammals. This allows for the effective digestion of food. The blind intestine is inhabited by bacteria that ferment porridge from the leaves (cellulose and lignin, the cleavage components of the cell wall), facilitating further digestion. Young koalas receive useful microflora by eating special fecal secretions from their mothers – perhaps that’s why the bag of koalas opens back for convenience.
Since the eucalyptus diet is low-calorie, koalas save energy and sleep up to 22 hours a day (so it was great luck for us to see koalas awake in the zoo: they ate, moved through the branches, looked at us). But they do not need to spend time looking for water (except for particularly hot weather): it is contained in sufficient quantities in the leaves of eucalyptus. Its reabsorption occurs in the distal (distant) part of the colon.
Koala is fussy not only in the choice of eucalyptus species (different species contain different amounts of toxic substances) but also in the choice of specific trees: eucalyptus leaves may contain different amounts of toxins depending on the soil on which the tree grows. Variety also manifests itself in the choice of leaves: koalas prefer leaves with a high ratio of nitrogen to fiber and tannins, containing at least 55% water. To choose the right leaf, the koala first sniffs and tastes it.
But how does the smell and taste of koala determine the content of the right substances and how does it neutralize toxins that are deadly to other mammals? The recent decoding of the koala genome has helped to answer these questions. Six genes of type 1 vomeronasal receptor (Vomeronasal receptor, V1R) were found, which are responsible for differentiating secondary plant metabolites. Duplication of the gene encoding Aquaporin 5 protein helps to recognize the “taste of water” and determine its content. Koalas have 24 genes of the TAS2R family (Taste receptor type 2) responsible for recognizing the bitter taste (which gives terpenes and phenols) – this is more than any other Australian marsupial!
Finally, the ability to detoxify substances in the eucalyptus leaves is due to genes encoding cytochrome P450 monooxygenase (CYP) – enzymes that oxidize toxic compounds, making them safe and easy to remove from the body. These genes are larger than many other animals. Their expression is especially high in the liver, where various substances are detoxified. Unfortunately, cytochromes also break down non-steroidal anti-inflammatory drugs, which are used to treat koalas in captivity or veterinary clinics after traffic accidents (wild koalas are often hit by cars), as well as antibiotics, which treat chlamydia, a common disease in wild koalas.