Koalas have a tiny, almost useless finger flexor muscle that primates lack.

Koalas live in trees, just like many primates do. To climb and hold onto branches, animals need strong forelimb digital flexors—the forearm muscles that curl the fingers and claws inward. Primates have been studied closely, but scientists lacked modern anatomical details about how these muscles work in koalas. To fix this, researchers examined the digital flexor muscles and nerves in four koala forearms. They found that a deep muscle called the flexor digitorum profundus is very large and shaped like a U. Two separate nerves, the median and ulnar nerves, control it. In contrast, the superficial flexor muscle is small and wrapped inside the deep muscle. Strangely, the nerve branch feeding this smaller muscle branches off near the wrist and loops backward up the arm to reach it. Overall, koalas evolved a completely different muscle setup than primates to grip tree branches.

Why it matters

This matters because it shows that koalas and primates evolved to climb trees in completely different ways. Koalas rely on a single, massive flexor muscle for gripping, while primates use a more balanced system. Understanding these differences helps biologists see how evolution can solve the same problem—living in trees—with very different body plans.

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