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Evolution of Flight: Did Early Birds Run and Flap Before They Flew?

By Veronique Greenwood
Jun 24, 2011 10:03 PMNov 20, 2019 2:00 AM


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Flapping while running up a ramp takes far less energy than flight at the same angle.

What's the News: How did birds get their wings? And how did they start using them to fly? These questions have bedeviled evolutionary biologists for more than a century, and with flight's origins long buried, a lot of careful measurements of how modern birds work combined with clever guesswork has resulted in several fiercely differing theories

. The two major camps have proto-birds either dropping from trees or running along the ground before finally taking to the air. A new study

lends credence to the idea that flapping wings while running could have been involved by showing that it requires much less energy than flying while still helping birds get over obstacles. This suggests that it could have been an easy way for proto-birds to start going through the motions. How the Heck:

  • The researchers had noticed that young birds running up ramps and other obstacles flap their wings strongly, gaining speed and balance. The team wondered how much energy the process took: as the behavior gets birds over obstacles as effectively as actual flight, if it took less investment of energy, it could have been a flight stepping stone accessible to early birds.

  • They trained adult pigeons to run up nearly vertical ramps as well as fly to perches from the ground, requiring a similar angle of ascent, and then implanted sensors in their flight muscles. When they recorded how much energy each took, they found that running took less than 10% of the energy of flight. This confirmed that flapping while running up obstacles could have been a plausible intermediate stage in flight's evolution, and could have been performed even with fairly small muscles, such as those in early birds.

What's the Context:

  • The two primary competing theories, arboreal (tree-dropping) and cursorial (running along the ground), imply totally different lifestyles for early birds. Thus, a lot of the debate over these models, which were first suggested in the 1880s, revolves around what bird-like fossils like the feathered Archaeopteryx reveal in terms of tree-climbing or running ability. But interpretations of the structure of its bones, claws, feathers, and probable musculature differ considerably.

  • The evolution of feathers has likewise been drawn into the fray, as when they arose, and which ones, pinions or secondaries, came first, matters in models of early flight (see here for a detailed discussion of both models and the evolution of feathers).

  • Ken Dial, the senior author of the new study, came up with the idea that studying how young birds learn to fly could give scientists insight into how flight evolved in the first place. While there's a whiff of recapitulation theory there (the now-discounted idea that a creature's development recapitulates its evolution), in terms of physics, the pre-flight motions of young birds are one of the few ways scientists can get a look at what might have happened at a point when creatures had wings but did not yet fly.

  • Though the involvement of running seems to place this work in the cursorial camp, Dial, citing problems with both the major models, has said that he sees his work striking off in another direction. "I would argue that studying the proto-wings of juvenile, still-developing birds might provide a useful avenue by which to approach the problem of the earliest flyers," he says (via CABINET). "Rather than concentrating on the arboreal/cursorial divide, I think it might be more fruitful to focus on questions like these, such as how does the flight stroke itself develop, what structures make it possible, and what purposes can they serve at differing levels of efficiency."

The Future Holds: As long as time travel is impossible, studying the physics of modern birds and comparing them with fossils is the most effective way to learn about the evolution of flight. Expect more experiments dealing with the mechanics of flight, as researchers focus more and more on detailed aerodynamics in sussing out flight's origins. Reference: Jackson, B. E., Tobalske, B. W. and Dial, K. P. (2011) The broad range of contractile behaviour of the avian pectoralis: functional and evolutionary implications

. J. Exp. Biol. 214, 2354-2361. Image credit: Dial, K.P., J. Exp. Biol.

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