Koalas are classified as herbivores, meaning their diet consists exclusively of plant material. They are tree-dwelling marsupials native to Australia, and their entire existence is tied to consuming foliage. This dietary choice requires specialized biology to manage the nutritional and toxic challenges of their food source.
A Highly Specialized Folivore
Koalas are specific herbivores called folivores, which translates to “leaf-eaters.” Their diet is almost entirely restricted to the leaves of Eucalyptus trees, often called gum leaves. While there are over 600 species of Eucalyptus, a koala’s diet is highly selective, focusing on only a few dozen preferred species in any given region.
This selectivity is necessary because Eucalyptus leaves are a poor food source, characterized by low protein content and high levels of fiber. The leaves also contain various defensive compounds, including volatile oils and phenolic substances, that are toxic to most other animals. Koalas actively choose leaves with higher available nitrogen for protein, while avoiding those with higher concentrations of toxic chemicals. An adult koala consumes 200 to 500 grams of leaves daily to meet its energy and hydration needs.
Digestive Adaptations for a Toxic Diet
Survival on this toxic, low-quality diet is possible due to several biological adaptations in the koala’s digestive system.
Specialized Digestion
The most notable anatomical feature is the extremely long cecum, which can measure up to 200 centimeters in length. This large pouch acts as a specialized fermentation chamber, housing symbiotic bacteria and protozoa. These microbes are responsible for breaking down the tough, fibrous cellulose in the leaves into digestible nutrients that the koala’s own enzymes cannot process.
Detoxification and Metabolism
The koala possesses a highly efficient detoxification system centered in the liver. This organ produces specialized enzymes, such as cytochrome P450, which chemically neutralize and process the volatile Eucalyptus oils and phenolic compounds. The koala also maintains a low metabolic rate, which allows food to pass through the digestive tract slowly, maximizing the time available for both microbial fermentation and toxin processing. This low metabolism is why koalas rest or sleep for up to 22 hours per day, conserving the limited energy they extract from their challenging diet.
Koalas Are Not Bears
The common name “koala bear” is a misnomer that has persisted since early European settlers first observed the animal. Koalas are not related to bears, which belong to the order Carnivora and are placental mammals. Koalas are marsupials, a distinct infraclass of mammals whose closest living relatives are wombats.
Their scientific classification is Phascolarctos cinereus, and they belong to the order Diprotodontia. Marsupials are defined by their reproductive strategy, giving birth to underdeveloped young called joeys after a short gestation period. The newborn joey then crawls into its mother’s abdominal pouch, where it attaches to a teat and continues its development for the first six to seven months. This biological fact places the koala firmly in the marsupial lineage, far removed from the bear family.