Koalas require a lot of sleep because their food gives them very little energy and eucalyptus requires a lot of energy to digest. Basically most of their energy is spent on digesting the eucalyptus they eat.

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Koalas are marsupials that carry their young, called joeys, in a pouch

Baby koalas eat poop to help transition from drinking milk to eating eucalyptus leaves

Koalas require a lot of sleep because their food gives them very little energy and eucalyptus requires a lot of energy to digest