[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: Re - Feathers and Flight
While I note that Kevin Padian (whom I, too, wish would contribute more)
thinks that we armchair theorists can only get so far with our ideas before
"real" science takes over, I thought I would brave the whirwind and throw in
a few thoughts.
>And yes, evolutionary hypotheses (like the=20
>display hypothesis, for example) must at least be compatible with hard=20
>evidence if they are to be considered seriously. As it happens, the=20
>display hypothesis is. It doesn=D5t ADD anything (maybe) but that's another
>question. Certainly our scenario is not denied, and it brings with it
>functional explanations for features that are otherwise puzzling.
I think this is the answer to Kevin's point about hypotheses being
"testable". The fact is that we can never "prove" which of the various
scenarios for Archaeopteryx behaviour is the correct one merelyby examining
its physical adaptations - at best we can only eliminate some unlikely ones.
Kevin warns against extrapolating too closely from living animals. This is
certainly valid; we know that quite a few living creatures possess unique
behavioural adaptations, and if they had become extinct prior to our
studying them we might conclude that such adaptations were "unlikely" for a
fossil because they were not observable in living forms.
Nonetheless, I think that obervations of living creatures (and other, more
closer studies such as biomechanical analyses) can be very useful for
fossils. For one thing, there is nothing better for showing that a view
that a certain behaviour "could not" have been used than by demonstrating
that a living creature does just that. For example, the argument that
Archaeopteryx could not have been arboreal because the habitat around
Solnhofen was not forest can be disproved by showing that there are living
birds of similar habitats (eg chachalacas) with close relatives (and,
probably, ancestors) in tall forest.
More to the point, I would argue that observations of living forms can help
decide which of a number of hypotheses is the most parsimonious. Thus,
although no living bird I know of uses its wings as an insect trap, many use
them for display (including flightless species like the kagu). Therefore,
between these two hypotheses for the function of Archaeopteryx wings
(especially if flight be discounted), display is a more parsimonious
hypothesis because we know that wings of this type can, and are, used in
this way. Of course this only refers to what Archeopteryx did with its
wings, not what its ancestors did.
>Examples: why Archaeopteryx evolved the worst possible tail for flying=20
>with; why it retained claws on the ends of the wings where the longest
>primaries would be expected, etc. No phylogeny here, but *explanation.*=20
Though I'm a fan of the display hypothesis I am not sure about this
statement; after all, except for its feathers the tail of Archaeopteryx
represents a therapod, not a bird, character state so was not "evolved" but
retained. One would hardly expect the first flying bird to be
aerodynamically perfect in all respects.
>I agree that behavioral interpretations are often difficult to establish=20
>beyond all doubt (example of an exception: the dinosaur stampede in=20
>Australia, which *proves* rapid running beyond any doubt). But let's=20
>test the display hypothesis as far as we can.
As noted, display is no less difficult to establish than any other behaviour
(prey-catching, tree-climbing etc.)
>suggestion that they were all (originally =3D primitively) visual,
>displaying creatures (Jacques Gauthier said it, not me). Lizards do it,=20
>birds do it, they all have color vision, and they are daylight,=20
Actually, cannot some "hard" evidence for this be derived from casts of the
size and proportion of the optic lobes?
In fact most tetrapods that are not actually blind have SOME visual
displays, including nocturnal species.
. Let's look at a bird that has feathers used ONLY=20
>for thermoregulation, not for flight, not for display: an Emperor penguin
>chick. That's a fair test, it seems to me, and it has an answer: down.=20
I'm not so sure here. Emperor penguin feathers are hardly just insulation
but insulation under the most extreme temperature regime experienced by any
nesting bird. To suggest that characteristics of their feathers are
necessary indicators of thermoregulatory function strikes me as a little
like saying that because a light sweater isn't like a down alpine jacket it
couldn't possily be intended to keep you warm.
As it happens, of course, down is common to many nestling birds; a more
reasonable question might be why birds do not retain it throughout life. Of
course we don't know what the structure of contour feathers in protobirds
was like; maybe they WERE down-like.
Of course none of this explains the development of remiges and rectrices,
which certainly are not thermoregulatory structures; but we are left with
the question of which came first in the evolution of the feather. Did
contour feathers evolve first, perhaps for thermoregulation, and later
specialize on the limbs for display and other functions, or vice versa? In
other words, even if the display hypothesis is true, at what point in
feather evolution did it "kick in" - is it the reason for their original
evolution or their later modification?
>Kevin explains very lucidly Jeremy Rayner's problem: why the running=20
>predator would be at the very edge of its envelope before it could=20
>get off the ground: exhaustion at take-off is not a good way to learn=20
>to fly. The beauty of the display-and-fighting idea is that the=20
>lift-off takes place at ZERO ground velocity: it's all *lift*, and=20
>short-lived lift at that, at this stage, ideal pre-flight training=20
>conditions, with forward speed coming later. Our display-and-fighting
>hypothesis can provide Jeremy and Kevin with a flying bird from the
>ground-based predatory animal they both prefer as its ancestor -=20
>they just can't have it running flat-out to take off.=20
I find this an extremely attractive aspect of the display hypothesis, and I
agree that it does not contradict the arm-stroke work. There is no reason
to assume that the skeletal structure and mechanics of the arm evolved for
the same reason as the flight feather structure and arrangement; in fact the
mobility of the arm, even if it developed for prey capture, might have made
it particularly suitable as a site for display structures that needed to be
manipulated ion a certain way for maximum effect.
>Richard Cowen - speaking for Jere, too, I hope.
And may I add my thanks for posting your most interesting article.
PS: I've had the feeling that some of my messages to this list have been
disappearing into an electronic void - so if someone gets this, please let
Ronald I. Orenstein Phone: (905) 820-7886 (home)
International Wildlife Coalition Fax/Modem: (905) 569-0116=
Home: 1825 Shady Creek Court Messages: (416) 368-4661
Mississauga, Ontario, Canada L5L 3W2 Internet: firstname.lastname@example.org
Office: 130 Adelaide Street W., Suite 1940 Compuserve ID: 72037,2513
Toronto, Ontario Canada M5H 3P5 =20