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Habitat: , especially the reasonably flat bits

Some people just achieve too much. For instance, Iron Maiden singer Bruce Dickinson is also a qualified airline pilot, and a published novelist – which is frankly galling.

Ostriches are the Bruce Dickinsons of the bird world. Not only are they the largest living bird species, they also lay the largest eggs of any bird alive and hold to that effect. And they can run faster.

However, what you may not know is that they are quite possibly the only animal to have two kneecaps in each leg. This we’ve known , but why it should be so has remained a mystery ever since.

Funny bones

To find out,  and her colleagues at the Royal Veterinary College in London, examined a single dead ostrich donated to the college. They alternately 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.

“The upper kneecap looks similar to the single bone in ourselves and other animals,” says Regnault. However, “the lower one is very closely attached to the lower leg bone… a bit like the point of your elbow.”

Typically, kneecaps improve the leverage of the knee extensor muscles, so they don’t need to produce as much force to straighten the knee. “It’s a bit like putting the door handle further from the hinge,” says Regnault. “It requires less force to open the door.”

But the ostrich kneecaps did the opposite. “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, albeit at the cost of needing more force to do so. This might help the animal 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.

Knees everywhere

It is hard to tell if these explanations are correct, because the ostrich is unique and we have no other animal with two kneecaps with which to compare it. Regnault says there is an 1884 report of another bird, a red-breasted merganser, that also had two kneecaps () – but it “doesn’t seem to have been confirmed by anyone”.

There is also evidence that some frogs might have two kneecaps, although made of cartilage rather than bone. But frogs live such different lifestyles to ostriches that they might not tell us anything.

Bizarrely, many of the ostrich’s closest relatives don’t have kneecaps at all. In 2014 Regnault showed that emus and cassowaries, and likely the extinct moa, . However, kiwis and tinamou, which are also closely related, do.

“It’s a bit baffling why some species have them and others don’t,” says Regnault. She says it might be worth looking at lizards, which haven’t been examined as closely as birds, to see if any of them have two kneecaps.

Ultimately, there might not be a simple pattern. “Our studies suggest that the bird, lizard and mammal lineages represent independent evolutions of the [kneecap], so I wouldn’t hold my breath for one single rule underlying why some animals have one or two kneecaps,” says Regnault.

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