Baby koalas (joeys) drink mother's milk during their first 6-7 months of life. After 30 weeks, the mother produces a substance called pap. This substance is actually a specialised form of the mother's droppings which, having passed through her digestive system, give the joey the enzymes it needs to be able to start digesting the tough gum leaves, making an easier transition for the baby koala to start eating eucalyptus leaves.

Eucalyptus leaves are tough, toxic and low in nutrition, but the koala's digestive system is capable of removing the toxins, filtering them out by the liver. The caecum completes the process by changing the eucalyptus leaves into digestible nutrients. Without the transitional stage of feeding on pap, the joey cannot develop its digestive system properly.