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.

 

The upper kneecap looks similar to a human’s kneecap, but the lower kneecap is very closely attached to the lower leg bone… “a bit like the point of your elbow,” says Renault.

“The upper kneecap actually seems to decrease the leverage of the knee muscles, not increase them,” says Regnault. This sounds like a disadvantage, but it seems to allow the ostrich to straighten its leg more quickly at the knee, but needing more force to do so. This might help the ostrich to run quickly.

Meanwhile the lower kneecap – the one most species don’t have – might be protecting the joined tendons crossing the front of the knee.

 

Journal reference: Journal of Experimental Zoology Part A: Ecological and Integrative Physiology, DOI: 10.1002/jez.2082

 

Photographer: Martina Nicolls

Martina Nicolls: SIMILAR BUT DIFFERENT IN THE ANIMAL KINGDOM