Photo of koala's digestive tract adapted from diagram at The Koala Page.


The ability of koalas to utilize Eucalyptus foliage as a sole source of nutrition makes them unique and amazing creatures. To cope with this unpromising and inhospitable diet, different aspects of the koala's morphology and physiology have become highly specializd. The digestive system of the koala is no exception. After food is broken down in the mouth, it enters the koala's highly specialized digestive system. The koala's low metabolism allows solutes and fine particles to remain in the gut for extended periods of time. A majority of the digestive process occurs in the koala's highly developed caecum, where fermentation and microbial breakdown supply the koala with a sufficient amount of energy to meet its reduced metabolic requirements (Ullrey et al., 1981). So, by combining reduced metabolic requirements with a slow metabolism and highly specialized morphological and physiological features, the koala has adapted to utilize Eucalyptus as a sole source of nutrition.

Caecum: Compared to it's body size, the caecum of the koala is strikingly enormous. Many studieds of the have closely examined the digestive system of the koala and noted the exteme development of the caecum . This marsupial herbivore is often referred to as having the greatest caecal development within the Mammalia. The koala's caecum is up to four times its body size. In addition, within the caecum and proximal colon of the koala, there are 8 to 14 longitudinal folds that augment the surface area enormously. The caecum also contains millions of bacteria which help break down the fibrous diet into substances that are much easier to absorb (Snipes et al., 1993). Eucalyptus leaves have high concentrations of tannins, which are compounds that readily form chemical complexes with proteins making them resistant to degradation within the gut of mammals. Researchers isolated an enterobacterium in the feces and caecal wall of a koala that is capable of degrading tannin-protein complexes. The bacterium is called T-PCDE, or tannin-protein-complex-degrading enterobacterium. If koalas didn't have an active mechanism to use tannin-bound protein, eucalypt leaves would offer them absolutely no nutritional value because of the relatively high tannin content and low protein content. This bacteria is thought to reduce the effect of the tannin, making more of the protein in the koalas diet available for digestion (R. Osawa et al., 1993).

Back to Hot Topics in Animal Physiology

This page was created as an assignment for Animal Physiology at Davidson College. If you have any questions, you can e-mail me at

Click here to return to the homepage.