Koalas primarily feed on eucalyptus leaves, which make up the majority of their diet. These leaves are low in calories and nutrient-rich, so koalas must consume a large quantity to meet their dietary needs. They have specialized digestive systems containing bacteria that help break down the tough fibrous leaves and detoxify the compounds within. Eucalyptus leaves also have a high water content, which helps keep koalas hydrated. Despite their reliance on these specific leaves, koalas are picky eaters, often favoring only certain eucalyptus species for their nutritional benefits and taste.