The eucalyptus leaves that form the major portion of the koala's diet are high in water content, allowing females to meet their water needs without seeking other water sources. Larger males, with a higher need for water, meet their water requirements by drinking water from hollows in trees or dropping to the ground to find water, according to Wikipedia.
Koalas live a sedentary lifestyle that limits their need for water. Their diet requires they sleep most of the time in order to digest their food. On warm days, they seek the shade of lower branches near the trunk of the tree, but if the weather is cool, they climb higher in order to absorb heat from the sun.
Their sedentary nature makes living in trees advantageous in protecting them from predators. While they do come down for water or to move to a tree, they are adapted for living in the trees that provide their food and most of their water. Their reduced tail, rounded bottom and curved spine make life in the trees ideal for these animals.
While legal hunting for koalas for their fur ended in approximately 1930, humans continue to be their biggest threat. Removal of eucalyptus forests, automobiles and pollution each provide a threat to their survival.