Koalas Sleep 22 Hours Per Day

A koala's famously drowsy lifestyle is a remarkable evolutionary solution to a difficult dietary problem. Their sole food source, the leaves of the eucalyptus tree, presents a unique challenge: it is extremely fibrous, low in nutrients, and laced with toxic (Review) compounds that are poisonous to most other animals. To survive on such a demanding diet, the koala's body has developed a highly specialized digestive system, including an unusually long cecum, to slowly break down the tough fibers and extract every possible calorie.

This intense digestive and detoxification process is so metabolically expensive that it consumes the vast majority of the koala's daily energy budget. The liver works overtime to neutralize the eucalyptus toxins, a task that requires an enormous amount of power. Consequently, there is very little energy left over for physical activity or even maintaining a large, energy-hungry brain; in fact, the koala's brain is one of the smallest relative to body size of any mammal. This extreme energy-saving strategy is why these marsupials can be observed resting or sleeping for 18 to 22 hours a day, a necessary trade-off for thriving on a food source that no one else wants.