Sunday, January 8, 2012

The Why of Fly - The Origin and Evolution of Flight in Birds: Part 1

By Gavin Leighton

The most salient trait of birds is their ability to fly. Flight allows peregrine falcons to dive at just over 200 miles/hour, flight allows for migration, and flight allows birds like albatrosses to make vast expanses of ocean their home. Indeed, the evolution of flight was a critical event (and maybe the critical event) in the evolution of birds. Given the importance of flight for the class Aves, evolutionary biologists have long studied the origins of flight, and researchers are beginning to delineate how and why birds evolved flight.
Peregrine Falcon, Point Reyes, CA.  Dec. 2011 [Photo by Lukas Musher]
How birds evolved wings with flight feathers is an interesting story, especially since flight has evolved independently at least four times (insects, bats, pterosaurs, and birds). To begin I will very briefly outline the major physiological steps that produced wings that could produce self-powered flight. In the second part of the series, I will try to describe the selective mechanisms that favored the following physiological changes.

Birds’ closest ancestors were theropod dinosaurs (1). We all know what theropods dinosaurs are thanks to Jurassic Park (and as far as I’m concerned, only one Jurassic Park movie was ever made and it came out in 1993). As depicted in Jurassic park dinosaurs such as Velociraptor (see below), which is actually much smaller than depicted in the movie , and Tyrannosaurus, which may have been more of a scavenger than a predator, are both examples of theropod dinosaurs. In general, theropods were bipedal dinosaurs with strong hindlimbs and a long tail that was likely used for balance. From theropods we get birds, here is a woefully brief description of how.
An artist's rendition of velociraptor [Wikipedia]
To get from velociraptor to a contemporary bird, the first thing we need is feathers. Feathers were traditionally thought to only occur in birds, but a cascade of recent evidence demonstrates that many theropod dinosaurs had feathers as well (2) – I know what you are thinking, and no, Tyrannosaurus Rex could not fly. The theropod feathers were not used for flight, so other explanations must be invoked. The two common explanations are that the dinosaur “protofeathers” were used for signaling during sexual selection (due to the types of melanin researchers have found in fossils we know the dinosaur feathers were black, white, or red-brown type colors [3]); or, for facilitated thermoregulation. These two ideas are not mutually exclusive and hopefully future work will indicate the selective pressures that favored the evolution of the first feathers.
An artist's rendition of anchiornis [National Geographic]

At this point we have feathers on a roughly bi-pedal animal, but we still aren’t close to powered flight. To get powered flight we must move from the proto-feathers (see below) here is a very simple model of how feathers evolved) that resemble the downy feathers of chicks, to asymmetrical flight feathers. To get to contemporary flight feathers, feathers passed through intermediate stages, and an excellent review of this process was “Which came first, the feather or the bird?” by Prum and Brush (2003 – see citation 2). One of the main intermediate stages is the symmetrical flight feather (3). The symmetrical flight feather, compared to the asymmetrical flight feather, is actually worse for flying due to physics that will not be described here. Despite problems with a symmetric feather, components of the symmetrical flight feather were useful. For example, there are tiny hooks called barbules that help smooth the feather to maintain a feather’s airfoil shape. The evolution of barbules almost certainly occurred with or after the evolution of the symmetrical flight feather, thus making the symmetrical flight feather a useful stage to focus on. So, let’s skip ahead a bit and assume we have some sort of flight feathers on the arms, what comes next is the development of adequate wings.
Proto-feather evolution in chronological order. [Wikipedia]
The change from forelimbs with separated digits to the common bird wing involved numerous skeletal changes that I won’t describe.
Wings, such as the long wings allowing Black-footed Albatross to wander countless miles over vast expanses of ocean
evolved from the forelimbs of therapod dinosaurs. Pelagic out of Fort Bragg, CA. Aug. 2011. [Photo by Lukas Musher]

What I will mention as an aside is the downstream evolutionary consequences after wings evolved. After proto-birds lost any sort of forelimb dexterity, the duty of manipulating objects was transferred to the beak. This transfer likely explains the extraordinary diversity of bill forms, compared to other groups. So lets assume that the wing morphology is coming into place, even with wings, other physiological structures and adaptations were necessary to yield contemporary birds. Among the adaptations are hollow bone structures that largely reduce the weight of a bird (1), and save the bird significant amounts of energy during flight. And to actually get into flight, birds rely on their oversized breast muscles, which became so powerful that they need to be anchored to a keeled sternum.
Archeopteryx lithographica, Wyoming Dinosaur Center.  Taxonomically the first bird.  One of 8 specimens worldwide.
Note the details of feathers and other traits that link dinosaurs to birds including the furcula. [Photo by Lukas Musher]
All of these physiological adaptations compose the part of the story that describes “how” bird physiology changed over time to allow for powered flight. In addition to how birds anatomy changed, a complementary question is “why” birds are the way they are, and not otherwise. To answer why birds can fly, several ideas have been posited with varying degrees of support. In part 2 of this series, I will limn the most well established ideas for why birds evolved flight and some of the newer ideas as well. 

Gavin Leighton is a PhD candidate at the University of Miami, studying cooperative behavior in Socialble Weavers.  See more on Gavin at Guest Writers.

1 Kaiser, G. The inner bird: anatomy and evolution. 386 (2007).
2 Prum, R. & Brush, A. Which came first, the feather or the bird? Sci Am 288, 84-93 (2003).
3 Vinther, J., Briggs, D. E. G., Clarke, J., Mayr, G. & Prum, R. O. Structural coloration in a fossil feather. Biol Letters 6, 128-131, doi:10.1098/rsbl.2009.0524 (2010).

To read the Why of Fly - the Origin and Evolution of Flight in Birds: Part 2, click here.

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