Imagine being able to sniff or lick a chunk of cheese and immediately know its nutritional value. That’s what koalas do when they forage on their sole foodstuff: eucalyptus leaves toxic enough to kill most mammals. Now, researchers know which genes make these cute Australian icons such foodies.

After sequencing the genome of the koala ( Phascolarctos cinereus ) and determining which genes are active in the heart, liver, and other tissues, the researchers discovered that these marsupials have greatly expanded the number of bitter taste buds , they report today in Nature Genetics . In addition, the animals have an extra copy of a gene that helps them assess a leaf’s water content, and more “sweet” taste bud genes than most animals on specialized diets. These extra genes finely tune the koala’s ability to assess their snack’s nutritional value. Finally, their genomes pack in an unusually large number of genes for detoxifying the leaf’s toxins, the scientists report.

All these precautions help koalas get the most bang for the bite of food, which is important because eucalyptus leaves are not very high in calories. Yet even filling their gullets with the best leaves, koalas don’t have a lot of energy to do much more than eat: They can spend up to 22 hours a day resting or sleeping.

The genomic data also reveal how the koala immune system reacts to chlamydia, a sexually transmitted bacterium thought to be acquired from livestock brought in by Europeans just a few centuries ago. That information could help speed the development of a vaccine. Moreover, with the genome in hand, conservationists can better assess the genetic relatedness of koalas and try to protect the animal’s genetic diversity.