How Do Koalas' Stomachs Help Them?

Koalas are one of only a few mammalian species that can eat eucalyptus. The leaves aren't very nutritious and are toxic to most animals. It's believed the trees developed these toxins to protect their leaves from leaf-eating insects and other animals -- but they are no match for koalas. The marsupials have a specialized digestive system able to extract nutrients from the tough plants while processing the toxins that make other animals sick. Koalas chew slowly, eating one leaf at a time and grinding it into the smallest particles so they can digest it. Because they absorb all the water they need from the leaves as well, they rarely drink.

All herbivores have larger digestive systems than carnivores, but koalas have the one of the largest digestive systems of any herbivore, relative to their size. The leaves they eat pass through their stomachs to their small intestines, which produce as much as 80 percent of the energy koalas need daily. From there, material passes into the fermentation chambers of the hindgut, including the caecum. At over 6 feet long, it's one of the longest caecums of any animal. Food particles may be retained in the hindgut for more than 100 hours, although larger particles move straight to the colon where water is reabsorbed before the waste material is passed as small, dry pellets.

Most herbivores, such as fellow Australians the kangaroos, have stomachs that are large and complex, with many chambers. Each chamber has different bacteria to assist the animals in digesting the tough fibers in the plants they eat. Koalas, however, have relatively small, simple stomachs that play only a small part in the overall digestive process. The stomach includes a gastric gland that secretes juices into the stomach to assist in breaking down food particles and extracting vital nutrients. Aside from that gland, koalas' stomachs aren't particularly distinctive compared with other mammals.

Initially, scientists believed koalas' hindguts played a key role in their digestive process. After study, it became clear that although the fermentation chambers in their hindguts are highly specialized, the animals get less than 10 percent of their daily energy needs from food processed there. Koalas retain food particles in their hindgut for 100 hours or more after eating, allowing the micro-organisms in their large caecum to break down fiber and extract nutrients, but the reasons for these extended processes are largely unknown.