They eat leaves of eucalyptus trees. Koalas do not drink often, they get most of their water from the eucalyptus leaves they eat. Eucalyptus leaves are poisonous, but koalas do not get poisoned. This is because they have certain bacteria in their digestive tract that can detoxify the poison in the leaves. Koalas, however, have to sleep long hours because eucalyptus leaves do not produce much energy and also because the digestive process takes a long time.