Koalas are marsupials (mammals with pouches to carry young). They live in the eucalyptus forests of Australia, relying on the trees for both habitat and food. When not sleeping, they are eating. Adult koalas feed almost entirely on eucalyptus leaves, eating a pound (450 grams) or more of the gray-green foliage daily. The leaves contain so few calories and protein, they often spend over 20 hours each day resting or sleeping; they simply haven’t the energy to do more.

What makes the koala’s diet unusual is the fact that not only do eucalyptus leaves have very little nutritional value, but they are also toxic, as well as fibrous and hard to digest. The stringy eucalyptus leaves contain cyanide compounds that render the plant not only inedible, but deadly, to basically every other living thing.

Specialized microbes found in koalas’ digestive tracts help flush the toxins out quickly, so they can eat their way through pounds of leaves without getting sick. Researchers discovered the microbes were able to break down poisonous compounds. They also noted that koalas would sniff leaves, then choose to eat some, while discarding others. This led to the finding that koalas are able to sense how toxic or nutritious each leaf is based on the smell, thanks to extra scent receptors in that cute little nose.

Gut microbes are passed down from mother to baby koala through pap, a specialized form of droppings, which allows baby koalas to safely transition from milk to eucalyptus leaves.

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