Australia has some 650 species of eucalypt, but koalas are choosy eaters, and feed on only around 30 species, with just a handful, including river red gum, gray gum and manna gum, preferred. They have occasionally been known to eat non-eucalypt leaves, including acacia, mistletoe and box. Koalas eat about 1.3 to 1.8 lb (600 to 800 g) of leaves a day.
Koalas reach their food by climbing high up smooth, vertical eucalypt trunks, gripping with their sharp foreclaws and using their powerful front legs to pull themselves up, while bringing their hind legs up to their front. Although they generally move slowly and laboriously to conserve energy, they are capable of surprising agility and can leap 6 ft (2 m) from trunk to trunk.
Eucalyptus leaves are low in nutrients, contain a large proportion of indigestible cellulose and lignin, and are full of toxic chemicals. Koalas have evolved a number of ways to cope with this poor diet. They avoid the most toxic species, and vary their choice of food tree throughout the year as toxin levels vary seasonally in some species. The koala's liver is capable of detoxifying and excreting some poisonous compounds from those species they do eat. Koalas have large cheek pouches, to handle large amounts of poor quality forage, and well-developed teeth, which include a single premolar and four molars in each jaw, which grind the fibrous leaves to a fine paste. This is then digested by microbial fermentation in the animal's unusually long cecum (a blind sac in the digestive tract, between the junction of the small and large intestines). A koala's cecum can be more than 75 in (2 m) long.
The low energy yield of the koala's diet explains their slow, sedentary lifestyle. However, it is a myth that koalas are drugged by the poisonous compounds in eucalyptus leaves. What is true is that koala have one of the smallest brains of all marsupials relative to body size—only 0.2% of body weight— and this has been explained as a further response to diet, since the brain is one of the most energy-consuming of the body's organs.
Koalas obtain most of their water from leaves, but occasionally drink at streams, and in captivity often choose to drink fresh water.
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