Koalas are iconic Australian marsupials recognized for their distinctive appearance and specialized dietary habits. Their existence is intricately linked to a particular type of tree, and this unique diet has shaped their biology, leading to remarkable adaptations that allow them to thrive on a food source most other animals cannot consume.
Eucalyptus: The Staple Diet
Eucalyptus leaves make up almost 90% of a koala’s diet. These leaves are abundant across the koala’s native habitats in eastern Australia, providing a readily available food source. While plentiful, eucalyptus leaves are highly fibrous, low in nutritional value, and contain compounds toxic to most other animals.
Koalas consume 200 to 500 grams of leaves daily. The high water content, often up to 50%, provides most of their hydration, reducing their need to drink directly. Koalas have evolved specific characteristics to process this challenging, low-energy food.
Selecting the Right Eucalyptus
Koalas are highly selective, choosing only a few species from hundreds of eucalyptus varieties. From over 900 species, they typically consume leaves from 50 to 60, focusing on 10 to 12 locally. This selection is influenced by toxic secondary metabolites like phenolics, terpenes, and cyanogenic glycosides.
Preferences vary by geographic region and individual, reflecting local tree availability and chemical composition. Koalas prefer leaves based on specific chemical compounds, also considering water, protein, and fiber content. Common preferred species include:
- River Red Gum (Eucalyptus camaldulensis)
- Forest Red Gum (Eucalyptus tereticornis)
- Swamp Mahogany (Eucalyptus robusta)
- Tallowwood (Eucalyptus microcorys)
- Manna Gum (Eucalyptus viminalis)
Koalas sniff leaves before eating, a behavior thought to help them assess toxicity levels, which fluctuate with season and climate.
Specialized Digestion for a Leafy Diet
Koalas possess unique digestive adaptations to manage their challenging eucalyptus diet. A central feature is their extraordinarily long caecum, a pouch-like organ up to 200 centimeters in length, proportionally the largest among all animals. This specialized organ functions as a fermentation chamber, harboring diverse microbes and bacteria.
These microorganisms break down tough cellulose fibers and neutralize toxic compounds. Young koalas acquire essential gut microbes by ingesting “pap,” a specialized faecal substance from their mothers. Koalas also produce liver enzymes, such as cytochrome P450, which aid in detoxifying harmful chemicals. Their slow metabolic rate allows food to remain in their digestive system for extended periods, maximizing nutrient extraction despite only absorbing about 25% of the fiber eaten. This slow metabolism also contributes to their tendency to sleep 18 to 22 hours a day.
Occasional Dietary Additions
While eucalyptus leaves dominate the koala’s diet, they may occasionally consume other plant materials. These additions are rare and typically occur when preferred eucalyptus species are scarce. Koalas have been observed eating leaves from other tree genera, including Lophostemon, Melaleuca, and Corymbia, which are related to eucalyptus.
They might also consume leaves from species such as Casuarina, Acacia, or Banksia. Beyond leaves, koalas ingest small amounts of bark, stems, buds, and sometimes soil, potentially for essential minerals. These occasional dietary additions do not diminish that eucalyptus remains their primary food source.