Why do ostriches have two kneecaps?

BBC Wildlife contributor Stuart Blackman discusses this strange adaptation of ostriches. 

South African Ostrich, Struthio camelus australis, walking through grassland.

Ostrich walking through grassland in Namibia © Mint Images RF / Getty

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Ostriches are large, powerful, charismatic birds with numerous claims to fame: they are the largest living member of the avian family, are the fastest animals on two legs and are the only animals to bear twice the regulation number of kneecaps.

Kneecaps are sesamoid bones, meaning that they grow within a tendon, probably to protect the sinews from general wear and tear.

It’s not clear why ostriches have two per knee. Strangely, for an animal that is so reliant on ground speed, the ostrich’s plethora of patellae seems to reduce the effective power of its leg muscles.

It’s possible, though, that this is offset by an increase in the speed at which the leg can be extended during running, which might result in a higher running speed overall. Ostriches’ close relatives, emus and cassowaries, lack kneecaps entirely, but that’s another mystery altogether.

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