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Re: Adaptive advantage (was Re: ABSRD BAND on Sinornithosaurus feathers)

James R. Cunningham wrote:

Why do membrane surfaces, when positioned far from the body, promote drag? What
is the specific fluid process that increases drag as the membrane surface moves
further from the body?

Off the top of my head (and in brief), distal positioning of the membrane/feathered surface allows a wider arc of movement (increased excursion). Perhaps "promoting drag" is the not the best way of saying it. The drag allows increase maneuverability and governs the attitude of the leap. This might be very mportant for a predator jumping from a branch to the ground, and which wants to land as close as possible to the prey - and stay upright! (Cats apparently do not need drag-inducing surfaces, but they have four legs to land on, not just two. Stability and orientation is more imporant for a biped.) Since it is discontinuous with the rest of the body, the distally-positioned "gliding" surface does not increase lift. The patagia of flying lizards (_Draco_) and flying mammals (_Petaurus_, _Pteromys_, _Glaucomys_, _Cynocephalus_ etc) are continuous with the body wall, allowing them to act as a single continuous surface to catch air and promote lift (like a diamond kite).

For a more elegant explanation (and the behavioral and evolutionary implications), check out:

Emerson, S.B. and Koehl, M.A.R. (1991). The interaction of behavioral and morphological change in the evolution of a novel locomotor type: "flying" frogs. Evolution 44(8): 1931-1946.

Garner, J.P., Taylor, G.K., and Thomas, A.L.R. (1999). On the origins of birds: The sequence of character acquisition in the evolution of avian flight. Proceedings of the Royal Society Biological Sciences Series B. 266 (1425): 1259-1266.

Putting Garner et al.'s model of the "Pouncing Proavis" with Chatterjee's tree-climbing "protodromaeosaur" and the evidence for arboreal adaptations from _Microraptor_, I think we have a pretty neat model for the origin of flight in theropods.

The "climb-leap-flap" arboreal model is not too different from the "run-jump-flap" cursorial model of Caple, Balda and Willis. Incipient flight structures on the ends of the arms were suggested by this trio as stabilizers during lunges at flying insects. As well as preventing the proavian from toppling forward during these brief aerial forays, the "proto-wings" helped control the direction of the leap - and later (as the flight surface expanded) the duration of the leap as well. The downward-and-forward movement of the arms used for these lunges was the precursor to flapping flight.

I like this idea, proposed nearly 20 years ago (and it is consistent with the arrangement of feathers in _Archaeopteryx_ and _Caudipteryx_.) But, personally I have objections to the "ground-up" evolution of flight. I am very comfortable with small theropods being semi-arboreal, so I tend to favor a "trees-down" origin of flight - the initial stages being assisted by gravity, rather than in defiance of it. I also don't think _Archaeopteryx_, or its flightless ancestors, were suited for snatching insects out of the air. First (as noted by Greg Paul in _PDW_) there is the energy cost required for leaping into the air (against the force of gravity) in an attempt to outmaneuver fast (very fast) darting insects in order to catch and eat them. Tough way to make a living. Secondly, if the proavians were specialized insect-catchers, why are _Archaeopteryx_'s jaws so narrow? A wider gape is better suited for catching flying insects - like the wide beaks of many modern insect-eating birds (the frogmouths come to mind, but there are others).

> Also its tail is long and covered in a pairwise
> arrangement of retrices. Although the tail feathers are asymmetrical, the
> long wide tail is not much good for generating lift.

Uhm, it can function as a low aspect ratio wing, generating lift by means of the
paired longitudinal counter-rotating vortices that are shed near the front and
then move slightly inboard as they flow parallel over and along the top of the
long axis of the tail. It's a process that can create a substantially higher
maximum lift coefficient than a high aspect ratio wing can, and it can also
achieve a higher deck angle. Though it IS a rather draggy process.

Thanks for the explanation James. I quizzed an acquaintance of mine (an ornithology student) about the purpose of a long, heavy tail with a pairwise arrangement of asymmetric feathers. I left him scratching his head.



Timothy J. Williams

USDA/ARS Researcher
Agronomy Hall
Iowa State University
Ames IA 50014

Phone: 515 294 9233
Fax:   515 294 3163

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