Koalas are among the few mammals that can survive on a diet of toxic eucalyptus leaves, thanks to specialized adaptations.

Eucalyptus leaves are notoriously low in protein and high in indigestible lignin and toxic phenolic compounds. For most mammals, consuming these leaves would cause liver failure or death due to terpenes and cyanide-like chemicals. Koalas belong to the family Phascolarctidae and have developed a highly specialized digestive system to exploit this niche diet.The koala's liver produces specific cytochrome P450 enzymes that break down toxins before they can enter the bloodstream. A 2018 study published in Nature Genetics revealed that koalas have a significant expansion of the CYP2C gene family. This genetic adaptation allows them to metabolize cineole and other monoterpenes found in eucalyptus species such as Eucalyptus punctata and Eucalyptus viminalis.To extract nutrients from fibrous leaves, koalas possess a caecum approximately 2 meters long—the largest relative to body size of any mammal. Within this organ, millions of specialized microbes ferment plant matter over 100 to 200 hours. This slow fermentation is necessary because eucalyptus provides minimal caloric value.Because detoxification carries a high metabolic cost, koalas have one of the lowest metabolic rates among herbivorous mammals. They spend roughly 75% of their time resting or sleeping to minimize energy expenditure. This evolutionary trade-off allows them to thrive in an environment where they face almost no competition for their primary food source.