Why do koalas have such smooth brains?
Koalas have remarkably smooth, small brains that occupy just 60% of their skull—an energy-saving adaptation to their low-calorie diet.
Unlike most mammals, koalas have smooth brains without the typical folds found in other species. This unusual adaptation is perfectly suited to their specialized diet of eucalyptus leaves, which are nutritionally poor but abundant in their habitat. By maintaining a smaller, simpler brain, koalas dramatically reduce their energy needs, allowing them to sleep up to 20 hours a day and survive on very little food.
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The koala (Phascolarctos cinereus) has one of the smallest brain-to-body mass ratios of any mammal. While most mammals have brains with gyri and sulci—folds that increase surface area for enhanced cognitive function—the koala's brain is smooth and featureless, a condition called lissencephaly. This distinctive structure evolved in direct response to their specialized diet of eucalyptus leaves, which are high in fiber and toxic compounds but extremely low in calories.Brain tissue is metabolically expensive to maintain, requiring substantial energy. To compensate for their nutrient-poor diet, koalas have evolved a brain that occupies only about 61% of their cranial cavity, with the remaining space filled with cerebrospinal fluid that provides physical cushioning. This reduction in brain mass significantly lowers the animal's basal metabolic rate, allowing them to survive on minimal energy intake.Koalas have also adapted behaviorally, remaining stationary or asleep for roughly 18 to 20 hours daily to conserve energy. Their digestive system is equally specialized, featuring a cecum up to 2 meters long that ferments tough eucalyptus leaves for nutrient extraction. Together, these physical and behavioral adaptations enable koalas to thrive in a nutritional niche that few other animals can exploit.
Verified Fact
FP-0002680 · Feb 17, 2026