Caudipteryx Flapped its “Wings” as it Ran

Scientists from Tsinghua University (Beijing), in collaboration with colleagues from the Chinese Academy of Sciences have suggested that the way in which some Theropod dinosaurs ran caused their feathered arms to move up and down.  Involuntary wing flapping might have been the first stage in the evolution of powered, active flight.  This is the conclusion reached in a new scientific paper published in the academic journal “PLOS – Computational Biology”, after a series of highly innovative experiments that involved building a robotic dinosaur and strapping artificial wings to young ostriches.

Modelling the Running Action of Caudipteryx

Calculating the flapping of the wings (Caudipteryx).

Mechanically modelling the running action of the basal feathered dinosaur Caudipteryx.

Picture Credit: PLOS – Computational Biology

Ground Up or Tree Down

Most scientists now accept that the Dinosauria is divided into two divisions, the avian dinosaurs – the birds and the non-avian dinosaurs, essentially all the other dinosaurs.  In addition, it is also widely believed that a type of maniraptoran dinosaur (a clade that contains true birds and those dinosaurs closely related to birds), evolved into our feathered friends.  Trouble is, how did powered flight, a trait very closely associated with most birds alive today come about?  Were some dinosaurs arboreal, clambering amongst the branches of trees and they then evolved the ability to glide and finally powered flight came about in what is described as a “tree down” approach.  Or, were fast-running, cursorial dinosaurs learning to leap into the air and over many generations, feathered arms became longer and stronger and the lift generated led to the evolution of volant dinosaurs and subsequently the birds?  This theory is termed “ground up”.

The debate has persisted for more than a hundred years.

Proavis – A Hypothetical Attempt to Assess “Ground Up” – Fast Running Led to the Evolution of Powered Flight

A model of the hypothetical transitional animal Proavis.

A model of the hypothetical animal Proavis.  An early attempt to examine how fast-running bipedal animals might have evolved into birds.

Picture Credit: Grant Museum of Zoology

Taking a Mechanical Approach

The researchers adopted a mechanical approach to this evolutionary conundrum.  They looked at one of the most basal, non-flying feathered dinosaurs known – Caudipteryx and assessed whether if this dinosaur ran fast enough, its running gait might have caused its feathered arms to flap involuntarily.  In theory, if the arms were strong enough, the wings and their feathers large enough, flapping whilst running fast could generate lift and if the lift to body weight ratios were right, then the dinosaur could take to the air.  In essence, passive wing flapping may have been an evolutionary precursor to later active wing flapping and powered flight.

An Illustration of Caudipteryx


A basal feathered dinosaur that could not fly, but could it flap?

Picture Credit: Everything Dinosaur

An assessment of the fossilised bones of the pheasant-sized Caudipteryx led the researchers to determine that Caudipteryx had a top speed of 8 metres per second (28.8 kmh or 18 mph).  However, simulations using mechanical and computer models suggested that at even lower speeds from 2.5 to 5.8 metres per second, the gait of Caudipteryx would have created strong enough vibrations through its body to cause the wings to flap.

Testing the Physical Movement of Artificial Wings on Young Ostriches

Young ostriches fitted with artificial wings.

Testing the movement of artificial wings in young ostrich locomotion study.

Picture Credit: PLOS – Computational Biology

A Life-size Robotic Caudipteryx

To test their calculations, the scientists built a life-size, robotic Caudipteryx and tested its running performance on a treadmill.  Several young ostriches were kitted out with artificial wings equipped with sensors that could detect lift and forward thrust, or any coefficient drag.  These birds were then put on the treadmill to see how they would perform.  In addition, five different sizes of feathers on the wings were tried, the larger feathers producing more results akin to the development of powered flight.

Five Different Wing Sizes and Feathers were Tested

Wing and feather variations used in the locomotion experiment.

Five different wing and feather combinations were tested.

Picture Credit: PLOS – Computational Biology

Professor John Hutchinson (Royal Veterinary College, London), an expert on animal locomotion, although not directly involved in the research, described this physical modelling approach as “ambitious and creative”, but questioned the paper’s main findings.  The study, for example, may have oversimplified the biology, reducing a living organism to a series of springs and constituent parts with individual mass, subsequently compiled to produce a single result.  Caudipteryx could have ran with its arms held very close to its body, helping it to reduce air resistance as it moved quickly, but also negating some of the lift and thrust that might have been generated by its feathered forelimbs.

Despite his reservations, Professor Hutchinson sees this study has helping to “lay groundwork that could be built upon and tested more rigorously.”

It seems that for the time being, the debate between “tree down” and “ground up” remains unresolved and it is not certain how much of a role passive arm flapping as a result of terrestrial locomotion influenced the evolution of active wing flapping, the precursor to a truly aerial existence.

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