Koala bear's nutritional symbiotic adaptation.
The koala bear, has an enlarged cecum, where symbiotic bacteria ferment finely shredded eucalyptus leaves. Herbivorous animals face a nutritional challenge : Much of the chemical energy in their diets is contained in the cellulose of plant cell walls, but animals do not produce enzymes that hydrolyze cellulose.
Many vertebrates solve this problem by housing large populations of symbiotic bacteria and protists in fermentation chambers in their alimentary canals. These microorganisms do have enzymes that can digest cellulose to simple sugars and other compounds that the animal can absorb. In many cases, the microorganisms can also use the sugars from digested cellulose along with minerals to make a variety of nutrients essential to the animal, such as vitamins and aminoacids.
The location of symbiotic microbes in the digestive tracts of herbivores varies, depending on the type of animal. The hoatzin, an herbivorous bird that lives in South American rain forests, has a large, muscular crop that houses symbiotic microorganisms. Hard ridges in the wall of the crop grind plant leaves into small fragments, and the microorganisms break down cellulose. Many herbivorous mammals, including horses, house symbiotic microorganisms in a large cecum, the pouch where the small and large intestine connect. The symbiotic bacteria of rabbits and some rodents live in the large intestine as well as in the cecum. Since most nutrients are absorbed in the small intestine, nourishing by–products of fermentation by bacteria in the large intestine are initially lost with the feces.