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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?
is the specific fluid process that increases drag as the membrane surface
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
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,
> long wide tail is not much good for generating lift.
Uhm, it can function as a low aspect ratio wing, generating lift by means
paired longitudinal counter-rotating vortices that are shed near the front
then move slightly inboard as they flow parallel over and along the top of
long axis of the tail. It's a process that can create a substantially
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
Iowa State University
Ames IA 50014
Phone: 515 294 9233
Fax: 515 294 3163
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