Zoologger is ours weekly obelisk highlighting extraordinary pets – and also occasionally other organisms – from around the world


by Michael Marshall

Ostriches: record-breaking birds

Nature photo Library/Alamy stock Photo

Species: Struthio camelus

Habitat: most of Africa, particularly the reasonably flat bits

Some people just achieve too much. For instance, iron Maiden singer Bruce Dickinson is also a qualified airline pilot, a skilled fencer and a published novelist – i beg your pardon is frankly galling.

You are watching: What is the only animal with 4 knees


Ostriches space the Bruce Dickinsons that the bird world. Not only are they the largest living bird species, they additionally lay the largest eggs of any bird alive and also hold a Guinness world Record to the effect. And also they have the right to run faster.

However, what you might not know is the they room quite perhaps the only animal to have actually two kneecaps in each leg. This we’ve known because at least 1864, however why it must be so has remained a mystery ever since.

Funny bones

To find out, Sophie Regnault and she colleagues at the royal Veterinary university in London, check a solitary dead ostrich donated to the college. Castle alternately bent and straightened the ostrich’s knees, and also used one imaging technique called biplanar fluoroscopy to track just how the bones moved. Climate they constructed a simple model to understand just how the kneecaps impacted the leverage that the muscles managing the knee.


“The top kneecap looks comparable to the solitary bone in ourselves and other animals,” claims Regnault. However, “the reduced one is really closely attached to the lower leg bone… a little like the allude of your elbow.”

Typically, kneecaps improve the leverage the the knee extensor muscles, therefore they don’t need to produce as much force to straighten the knee. “It’s a little like putting the door handle more from the hinge,” states Regnault. “It requires less force to open up the door.”

But the ostrich kneecaps walk the opposite. “The top kneecap actually appears to decrease the leverage that the knee muscles, not boost them,” claims Regnault. This sounds choose a disadvantage, yet it appears to allow the ostrich come straighten its leg much more quickly in ~ the knee, albeit in ~ the price of needing an ext force to execute so. This might assist the pet to run quickly.

Meanwhile the reduced kneecap – the one most species don’t have actually – could be protecting the join tendons crossing the former of the knee.

Knees everywhere

It is tough to call if these explanations are correct, since the ostrich is unique and also we have no other pet with two kneecaps v which to to compare it. Regnault says there is an 1884 report of another bird, a red-breasted merganser, that also had 2 kneecaps (Proceedings that the unified States national Museum, vol 7, ns 324) – but it “doesn’t seem to have been confirmed by anyone”.

There is additionally evidence that some frogs might have two kneecaps, although do of cartilage quite than bone. But frogs live such different lifestyles come ostriches the they could not tell united state anything.

Bizarrely, many of the ostrich’s closest loved ones don’t have kneecaps in ~ all. In 2014 Regnault proved that emus and cassowaries, and also likely the extinct moa, all seem to absence kneecaps. However, kiwis and tinamou, which are also closely related, do.

“It’s a bit baffling why some types have them and also others don’t,” claims Regnault. She claims it could be precious looking at lizards, i m sorry haven’t been examined as very closely as birds, to check out if any of them have actually two kneecaps.

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Ultimately, there can not be a simple pattern. “Our studies imply that the bird, lizard and also mammal lineages stand for independent evolutions the the , so ns wouldn’t hold my breath for one solitary rule underlying why some pets have one or two kneecaps,” claims Regnault.

Journal reference: Journal of experimental Zoology component A: Ecological and also Integrative Physiology, DOI: 10.1002/jez.2082