Ostriches are the only animals in the world to have a double kneecap, but its purpose remains an evolutionary mystery. A doctoral student from the Royal Veterinary College, London, England, Sophie Regnault, said that gaining an understanding about different kneecap configurations in different animals could help to inform surgical interventions, robots with better joints, and prosthetic design.
“In ostriches, the upper kneecap looks similar to the single kneecap in most other species, but the lower kneecap resembles a fixed bony process, like the point of your elbow,” said Regnault. “As far as we know, this double kneecap is unique to ostriches, with no evidence found even in extinct giant birds.”
From Regnault’s work, it appears that the ostrich’s double kneecap counterintuitively decreases the mechanical advantage of the knee extensor muscles, while in other species, including humans, it has more mixed effects: increasing mechanical advantage at some knee joint angles and decreasing it at others. The effect that this double kneecap has on the running performance of ostriches is hard to identify, but Regnault and her team have a few ideas. “We speculate that this might mean ostriches are able to extend their knees relatively faster than they would with one kneecap,” she said.
Using a combination of CT scans and fluoroscopy, known as x-ray reconstruction of moving morphology, on a real ostrich leg, Regnault and her team built a 3D model of the ostrich’s leg bones and kneecap. “We then moved the ostrich’s leg, allowing us to animate the CT bone models to show how the patellae are actually moving in 3D,” she explained.
While this research has highlighted one aspect of how the sesamoid bones function, their true purpose remains a mystery. “We are still not sure why ostriches might have evolved this second kneecap,” said Regnault. “It might help to protect the tendon of these heavy, fast-running birds, but there are other potential roles that we haven’t yet explored.”
Editor’s note: This story was adapted from materials provided by the Society for Experimental Biology.
