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

Having dissected a few Koalas and being familiar with their gastrointestinal (GI) tracts I hypothesized that if they ate anything less bactericidal and with more digestible carbohydrates 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 gastric distension and intra-abdominal pressure incompatible with long-term survival.

Warning - contains picture of koala GI tract

Gum leaves are low in carbohydrates digestible by animals and have bactericidal components that make them very slow to ferment. The mean retention rate for food ingested by koalas in the wild is about four days with some components taking considerably longer.

Photo 1 - Naming scheme used

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

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 ingestion of gas-producing carbohydrates, the small intestine was severed close to its distal end and a tube was inserted to approximately halfway down the 'caecum'. The distal end of the small intestine was the clamped around the tube to prevent leakage.

Air was blown into the tube causing the 'caecum' to distend around the tube opening. As more air was blown in it further distended towards the blind end. Upon reaching full distension at the blind end, as more air was blown in, the distension of the 'caecum' continued towards the large intestine, eventually reaching the juncture of the large intestine and the distal colon.

At this point I could blow no harder, was well into the presyncope zone and no gas or digesta appeared to be entering the distal colon.

Being a healthy adult male that regularly exercizes my lungs to their full capacity I estimated the pressure in the GI tract to be slightly above 2 PSI (pounds per square inch). 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. Breathing would

be very laboured and the heart wouldn't be anywhere near able to operate at full capacity.

The GI tract was then deflated leaving a small pocket of air (approx. 100ml) in the proximal region of the large intestine and simulation of peristalsis 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 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 digesta.

Ingestion by koalas of foods high in digestible carbohydrates would cause gastric distension and intra-abdominal pressure incompatible with long-term survival.