The ostrich is an odd bird, namely because it is the only animal in the world that has a double kneecap. But what purpose does this extra kneecap serve? Scientists simply do not know and have chalked it up to an evolutionary mystery. However, a new study of the ostrich’s double kneecap looked to crack the mystery, giving researchers an understanding of configurations found in different animal kneecaps, which could help inform surgical interventions, the design of prosthetics, and even develop robots with better joints, according to the Society for Experimental Biology.
Sophie Regnault, a PhD student with the Royal Veterinary College in the UK, said the upper kneecap of an ostrich looks quite similar to the single kneecap found in almost all other species. However, she noted that the lower kneecap looked more like a fixed bony process such as that found in the human elbow.
During the study, Regnault and her team built a 3D model of the bones and kneecaps found in the leg of an ostrich using CT scans and fluoroscopy, which is known as X-ray reconstruction of moving morphology (XROMM). Then the team put the leg in motion, which allowed them to animate the model and demonstrate how the patellae move in 3D.
According to study results, the double kneecap appeared to work counter-intuitively, decreasing the knee’s extensor muscles mechanical advantage. In other species, including humans, the effects were mixed. The mechanical advantage increased at some angles of the knee-joint and decreased at others.
It was difficult to identify what effect the double kneecap had on the ostrich’s running performance. Regnault and her team speculate that the extra kneecap allows these birds to extend their knees faster.
Regnault said she and her team are still not certain why the ostrich evolved a second kneecap. The reason may have been simply to protect the tendon due to the fact these birds are fast runners. However, she said there are other potential reasons that have not yet been studied.
By Trixie Dillwood