RESEARCH: Why do ostriches have two legs but four knees

Why do ostriches, the largest living bird species, with the largest eggs of any bird, have two legs but four knees? Specifically, ostriches have four kneecaps, and therefore four knees. Emus and cassowaries have no kneecaps.

Sophie Regnault, and her research colleagues at the Royal Veterinary College in London, examined a dead ostrich donated to the college.

They bent and straightened the ostrich’s knees, and used an imaging technique called biplanar fluoroscopy to track how the bones moved. Then they built a simple model to understand how the kneecaps affected the leverage of the muscles controlling the knee.

Kneecaps help animals to flex the extensor muscles in their knees, so that they don’t need to exert force to straighten their knees.

However, ostriches have an upper kneecap and a lower kneecap on each leg that act differently from the kneecaps of other animals.

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African Harrier-Hawk

The African Harrier-Hawk (Polyboroides typus) is a medium-sized raptor, or a bird of prey

The African Harrier-Hawk has pale grey upperparts, head and chest. Its belly is white with fine dark barring. Its wings are pale grey with a black edge with a narrow white line. Its tail is black with a single broad white band. Young African Harrier-Hawks are brown instead of grey, with dark brown replacing the black parts.

It has double-jointed knees, which enable it to reach into holes and cracks in trees, to search for prey. It can climb trees.

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