Koalas eat primarily eucalyptus leaves, though they are unable to secrete the enzymes necessary to break down most of the fibrous material of the leaves. the most likely explanation for how they survive on this diet is that

Koalas eat primarily eucalyptus leaves, though they are unable to secrete the enzymes necessary to break down most of the fibrous material of the leaves. These fibres are mostly the cellulose that are extremely hard to digest. The koalas have a long intestinal tract. The intestine harbours bacteria that can slowly break down the cellulose present in the leaves to release water and sugar molecules. The length of the tract and the action of the bacteria make it possible for the koalas to digest their diet rich in eucalyptus leaves.