Diet

The koala is a very picky eater.  Its diet consists primarily of eucalypts leaves, although the animal is known to occasionally eat buds, fruit, or bark.  Even though the koala's diet consists mostly of eucalypts leaves, the animal is still picky in which types of eucalypts leaves it will eat.  The koala is the only mammal known to eucalypts because the leaves contain toxins.

Also, the eucalypts leaves are hard to digest and provide very little nutrition to the animal.  This causes the koala to sleep 18-22 hours a day to conserve the little energy that is obtained from the leaves.  The adult koala eats around 1 pound of leaves each day.  The female koalas must consume more leaves when they are lactating to maintain higher energy levels (Krockenberger).    

Baby koalas, called joeys, drink milk from their mother when they are first born.  The joeys drink the milk until they are about 6 months old.  At that point, the mother koala starts to produce a substance called pap, a specialized runny form of feces that distributes important digestive enzymes to the joey.       

After eating pap for a while, the joey will be able to shift to a diet of eucalypts leaves like the adults.  

Digestion 

The koala's digestive system is adapted to its unique diet.  The digestive system can detoxify the poisons found in the eucalypts leaves.  The koalas have a special structure, called a caecum, where digestion of fiber takes place.  Other mammals have caecums, but the caecum of the koala is enlarged to better absorb the fiber obtained through the diet.  Since the koala cannot directly digest the fiber, there are millions of bacteria living in their digestive tract to break it down for them, making it easier to absorb the nutrients.  Koalas can also absorb a great deal of water from the eucalypts leaves, allowing for them to drink water only in times of drought.

How does it eat?

Teeth

The teeth of the koala are adapted to its diet, as well.  The koala, like the human, has four types of teeth: incisors, canines, premolars, and molars.  The number, arrangement, and shape of the teeth differ from humans, however.  The incisors are used to manipulate the branch as the koala eats.  The sharp premolars are used to rip the leaves off of the tree.  The leaves are then transported by the tongue to a large gap called the diastema.  From the diastema, the leaves are moved to the molars, where mastication takes place. 

It is worth mentioning that koalas experience a great deal of tooth wear as they age.  This is caused by the abrasive fiber that the eucalypts leaves contain.  Older animals must therefore eat more to get the same nutrition.  By eating more, the animals are able to get a normal amount of finer particles that are necessary for digestion.  This is such a common phenomenon that at one point, scientists measured the age of a koala based on their amount of tooth wear.  More recently, studies have shown that as tooth wear increases, koalas exhibit rumination-like behaviors (Logan).  The rumination allows for the animal to consume the larger particles, regurgitate, remasticate and reswallow the particles.  Only the fine particles can pass through the rumen into the rest of digestive system.