One of the biggest debates in paleontology is how avian animals began to fly. The two major competing ideas are the descriptively named “ground-upward origin” hypothesis and the “trees-down scenario.” In a study released Thursday, both hypotheses were tested, not using fossils or previous research — but with a robot and a young, jogging ostrich. Flapping, the authors found, began with a whole lot of running around.

Flight is thought to have developed among dinosaurs, the original proto-birds. In the “trees-down scenario,” avian flight arises as the natural progression of these animals gliding down from trees with their feathered limbs. The “ground-upward origin,” meanwhile, suggests adaptations for flight occurred before avian dinosaurs made it into trees. In the new PLOS Computational Biology paper, Chinese scientists support the latter by showing that active flapping — flight as we imagine it — probably arose without a gliding phase. Gliding flight is presented as a separate form of flying that likely developed earlier in avian evolutionary history.

Flapping, they explain, eventually emerged as two-legged birds ran around with their feathered limbs trailing behind them. They support their idea with data on the Caudipteryx, a small theropod with a tail that ended in a fan of feathers. This dinosaur lived from 125 to 122 million years ago and is part of the clade of dinosaurs that scientists believe evolved to become modern birds.

The skeleton of the Caudipteryx. Wikimedia Commons

The experiments began with a mathematical model that incorporated measurements of the Caudipteryx’s body to evaluate the mechanics of how it ran. The calculations suggested that if the Caudipteryx ran at speeds between 2.5 and 5.8 meters per second, then forced vibrations — the movement of one object resulting from the movement of another object connected to it — likely caused its feathered “proto-wings” to flap. This echoed a previous study, which showed that Caudipteryx locomotion caused “only small amounts of lift and drag” but wasn’t enough to get the dinosaur airborne.

The authors of the new paper took this idea two steps further: First, they built a life-sized, roughly meter-long robot of the Caudipteryx. Its body was made from plastic and, because we don’t know what the feathers of the Caudipteryx were like, it was outfitted with real bird feathers. Sure enough, when this robot ran at different speeds, its wings flapped.

But in order to verify the results of the robot experiment, they fitted a juvenile ostrich with artificial wings of four different lengths. The ostrich, the scientists argue, is a “similar living bird to Caudipteryx.” When the ostrich ran, longer and larger wings provided a greater lift force, as the video at the top of this article shows.

The results, the scientists write, suggest that the flapping motion of avian flight was likely driven by the forced vibrations that happen during a run:

However, the lift obtained from the running-foot forced vibration shows that the longer and larger the wing was, the larger the lift would be. Therefore, forced vibrations may represent the earliest stages in the evolution of forelimb flapping in winged theropods. This suggests that flapping behavior evolved in non-volant theropods a long time ago, before they could actively fly.

And so, they argue, bipedal motion eventually produced the natural phenomenon of avian flight, setting it apart from the gliding seen in the mammals that lived alongside dinosaurs.

“Our work shows that the motion of flapping feathered wings was developed passively and naturally as the dinosaur ran on the ground,” co-author Jing-Shan Zhao, Ph.D., of Tsinghua University said Thursday. “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.”

Abstract:

The origin of avian flight is one of the most controversial debates in Paleontology. This paper investigates the wing performance of Caudipteryx, the most basal non-volant dinosaur with pennaceous feathered forelimbs by using modal effective mass theory. From a mechanical standpoint, the forced vibrations excited by hindlimb locomotion stimulate the movement of wings, creating a flapping-like motion in response. This shows that the origin of the avian flight stroke should lie in a completely natural process of active locomotion on the ground. In this regard, flapping in the history of evolution of avian flight should have already occurred when the dinosaurs were equipped with pennaceous remiges and rectrices. The forced vibrations provided the initial training for flapping the feathered wings of theropods similar to Caudipteryx.