On the timeline of the evolution of flight in birds, gliding seems like a logical first step. But new research suggests that some species could have made the jump straight to flapping flight without a gliding phase in the meantime, which could force a rewrite of our understanding of avian evolution.

Modern birds are believed to have evolved from certain types of dinosaurs, and the transitional species Archaeopteryx sits neatly in the middle. Living about 150 million years ago, this raven-sized creature had an odd mix of avian and reptilian features, sporting feathers and wings but also teeth and a tail. Recent studies have shown that it probably glided, or at most flew in a hopping manner like a pheasant. Others like Anchiornis are thought to have only been capable of gliding.

But a new study suggests that gliding doesn't necessarily need to be an intermediate step towards active flight involving flapping wings. The researchers focused on Caudipteryx, a larger, peacock-sized animal that is the earliest-known non-flying dinosaur to boast a pair of feathered "proto-wings."

Although it couldn't fly, Caudipteryx's wings might have flapped when it ran, which in turn could have led to the eventual evolution of active flight. To test the hypothesis, the researchers analyzed the mechanical effects of running on different parts of the animal's body. According to those calculations, if Caudipteryx was running at speeds between 2.5 and 5.8 m (8.2 and 19 ft) per second, the forced vibrations from the legs would have caused those proto-wings to flap. And that's pretty easy for a creature with an estimated top speed of up to 8 m (26.2 ft) per second.

To test it out in the real world, the team then built a life-sized Caudipteryx robot that could run at different speeds on a treadmill. And sure enough, the motion of running caused those wings to flap. This was later backed up in another test, where the researchers fitted artificial wings to the back of a young ostrich.

"Our work shows that the motion of flapping feathered wings was developed passively and naturally as the dinosaur ran on the ground," says Jing-Shan Zhao, lead author of the study.. "Although this flapping motion could not lift the dinosaur into the air at that time, the motion of flapping wings may have developed earlier than gliding."

The next steps, the team says, are to analyze how much lift and thrust the wings may have generated while flapping. The ostrich test can be seen in the video below.

The research was published in the journal PLOS Computational Biology.

Source: PLOS via Science Daily