Why Do Koalas Only Eat Eucalypt Leaves? (English Version)

Having dissected a few Koalas and being familiar with their gastrointestinal (GI) tracts I considered that if they ate anything more fermentable than eucalypt leaves (i.e. Grass, fruit etc.) then the gases produced from fermentation would remain immobile as they would not be able to overcome the backpressure created by the size of their enormous GI tract and its contents. This would cause extreme bloating and pressure incompatible with long-term survival.

Warning - contains picture of koala guts

Gum leaves are low in carbohydrates digestible by animals and kill some bacteria, this makes them very slow to ferment and consequently produce little gas during digestion. It takes about four days for food to pass through a koala, some bits take considerably longer.

Photo 1 -  Naming scheme used

Note: This koala was diagnosed by a veterinary surgeon as having an unrelated terminal disease and humanely put-down.

Koalas have a very unusual GI tract - They have, for their size, a relatively large small   intestine that carries the contents of their stomachs to a 'T-junction' with a large tube. One side of this tube gradually tapers off to end blindly and is commonly referred to as a 'caecum'. The other side, the large intestine, heads off in the usual direction towards the colon.

In order to simulate the eating of gas-producing carbohydrates, a tube was inserted to approximately halfway down the 'caecum' and the intestines were inflated as hard as I could blow - still nothing was coming out of the other end.

After recovering from almost fainting, I estimated the pressure to be about 2 PSI.  This may not seem very high but it is the equivalent of laying the animal on its back and placing a weight of about 25 kilograms on its chest. Enough to make breathing difficult and the heart unable to operate at full capacity.

The GI tract was then deflated leaving a small pocket of air (approx. 100ml) near the beginning of the large intestine and simulation of natural intestinal wall muscle movement was attempted. No amount or degree of manipulation or squeezing could induce that pocket of air to move anywhere. The only way the gas could be moved if the animal was alive would be by being dragged through during the normal and slow process of digestion and excretion.

Issues

1: This is a single Koala GI tract; it may be different from others.

2: It could be the case that excess gas could be driven up the small intestine and released through the mouth but I'm calling this very unlikely and difficult to test as a deceased animal's pyloric sphincter is likely to be functionally different from a live one.

Conclusions

Koala GI tracts are capable of being inflated to unhealthily high pressures without release of gases or digested material.

Eating by koalas of foods high in digestible carbohydrates would cause extreme bloating and intra-abdominal pressure incompatible with long-term survival.