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Research News: “Flapping first” hypothesis a new step in the evolution of flight

September 27, 2012

Have you ever watched birds flit to your feeders and wondered how they achieved the magnificent ability of flight?  The evolution of flight has been an intriguing puzzle ever since evolutionary biologists realized that modern birds evolved from dinosaurs.  An article published recently in PLoS ONE adds a new hypothesis to the recipe for avian flight.

An artist’s rendering of Deinonychus antirrhopus.

The new hypothesis adds to the three existing main theories about the evolution of flight in Archosaurs (the taxonomic group that includes crocodilians, dinosaurs, and birds).  One of the most puzzling parts about the evolution of flight is that all of the feathered dinosaurs that have so far been unearthed appear to be a link between scaly, ground-dwelling dinosaurs and our modern birds.  These dinos had feathered limbs that were clearly advantageous in some way, but the size and shape of their feathers and the musculature of their bodies would not have enabled them to actually fly. So the question is: what was the step (or steps!) in between ground-dwelling reptiles and airborne birds that pushed evolution toward flight?

The oldest of the theories that takes dinosaurs from the ground to the air is the cursorial or “running” model.  This model describes flight as evolving from the behavior of predatory terrestrial dinosaurs. In this model, an Archeopteryx-like dinosaur would jump while running in pursuit of prey and flap its feathered wings.  The theory behind this is that lift and glide would be achieved through the short jumps while the dinosaur would also expend less energy and reach higher speeds.  Eventually, flight would have evolved from these short gliding jumps, from the ground up.

The second major theory about the evolution of avian flight — the arboreal theory — suggests that birds evolved from tree-dwelling dinosaurs.  This theory states that Archeopteryx and its relatives lived in the trees and glided from tree to tree and down to the ground, using its feathered forelimbs for balance and lift.  This second model stands up better to the tests of feasibility.

A third theory came about in the early 2000s from University of Montana ornithologist, Ken Dial, who looked at behavior in modern birds and came up with the “wing assisted incline running” (WAIR) model.  Ken looked at Chukars, a species of partridge that appear to actually run vertically up tree trunks.  These birds create downforce by flapping their wings, which allows their feet to maintain greater traction on vertical or steeply inclined surfaces.  WAIR hypothesizes that early birds started out with WAIR behavior to navigate inclines to escape predators. This may have been a stepping stone toward full-fledged flight, as feathered dinosaurs moved up into trees.

For more information on these previously existing theories and more, go here.

The new hypothesis about the evolution of flight, like the Chukar research, also comes from observations of extant birds — in this case raptors.  The article, published by researchers from the Museum of the Rockies, Montana State University-Bozeman, and Brown University focuses on Deinonychus and other dromaeosaurids (such as the famous Velociraptors) — predatory dinosaurs that had a specialized enlarged “killing claw” that allowed them to take down prey much larger than themselves, just like many existing raptor species do.  The researchers hypothesize that once dromaeosaurids took down their large prey, they stood atop their prey, grasping it with the killing claw and using “stability flapping” to stay on top of the animal while it was still alive.  Once their claws are locked onto the prey, they are no longer useful in terms of stability, so flapping the feathered forelimbs would be essential to keep the dinosaur balanced on top while it devoured its meal.  Imagine standing on top of a round, wobbly rock – you would have to “flap” your arms to stay balanced.  Imagine how much more balance you could achieve if you had feathers to create a bit of lift and draft to keep you stable.

Watch modern raptors use stability flapping:

This Cooper’s Hawk would employ stability flapping while preying on the Mourning Dove. Photo by FeederWatcher David Smith of Grand Junction, CO.

Employing stability flapping does not mean that these dinosaurs had the actual capacity to fly, but this “flapping first” hypothesis shows that flapping could have been an intermediate step in the evolution of flight. Dinosaurs like Archeopteryx and Deinonychus have long puzzled researchers because they did not seem to possess the musculature that would create enough lift to actually fly, but they would have had the ability to use stability flapping while on top of prey.  Their short, broad, feathered forelimbs would not have been well suited for soaring or speed, but would have provided great maneuverability, agility, and lift — much like in modern Accipiters. The musculature, shape, and size of the forelimbs of these precursors to birds would have been a perfect fit for stability flapping, and this could be an essential missing step in the evolution of flight.

Deinonychus antirrhopus engaging in the prey restraint model of predation where stability flapping would have been used.

Since PLoS ONE is an open-access journal, you can check out the article for yourself and read about the “flapping first” hypothesis as well as some other fascinating new theories about predatory behavior in Deinonychus, Velociraptors, and other similar dinosaurs here.

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