Small dinosaurs used the first feathered wings to clamber up steep slopes, according to a controversial theory that could explain the origin of flight in birds.
The wings, rather than being used as aids for gliding or jumping through the air, are thought to have increased the grip of the dinosaur's feet on the ground. Scientists have tested the idea on running partridges, which frequently flap their wings frantically to run head-long up vertical surfaces. They believe their theory could explain the evolution of feathered flight.
There is wide agreement that some dinosaurs grew feathers, probably for insulation, and that these were later adapted by the earliest ancestors of the birds for flying, but how powered flight came about is still open to question. One theory is that feathered forelimbs were used by tree-dwelling dinosaurs to glide from one branch to another. Another idea is that they were used to keep a jumping dinosaur in the air for longer.
Professor Kenneth Dial, of the University of Montana in Missoula, has found evidence to back up his idea that the first feathered wings were designed to increase the traction between the animal's feet and the ground.
In trying to explain the evolution of flight, scientists have to address the problem of what benefit the partially evolved wing, which would be of little use for flying, might have conferred on an animal. "It turns out that the proto-wings – precursors of wings that birds have today – actually acted more like a spoiler on the back of a race car to keep the animal sure-footed even while climbing up nearly vertical surfaces," Professor Dial said.
To test his idea, Professor Dial filmed partridges scrambling up slopes to see how they used their wings to increase ground traction, which was measured by instruments that detected the g-forces of a bird's body on the floor. The study, published in the journal Science, found that newly hatched chicks still unable to fly could nevertheless use their stubby, undeveloped wings to climb up a 50-degree incline. Slightly older birds used their flapping wings to "walk" up vertical slopes, while fully grown adults could even grip overhanging surfaces – defying gravity using the downward movement of their wings.
Professor Dial said the way the partridge used its wings to surmount difficult obstacles was a remnant of a similar behaviour invented by the early ancestors of birds, to escape predators. "A significant portion of the wing-beat cycle involves ... forces that push the bird toward the inclined substrate, permitting animals to run vertically," Professor Dial said. "In the proto-bird, this behaviour would have represented the intermediate stage in the development of flight-capable, aerodynamic wings."
The earliest ancestors of birds are believed to have evolved from feathered dinosaurs 225 million years ago.