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Re: Arms into wings



In a message dated 3/3/99 8:53:02 PM, Dinogeorge writes:

<< Roadrunners are already quite capable fliers, so the point is moot: In this
instance, flight did indeed come first. What you want is a bird that exhibits
this kind of behavior but whose lineage is entirely flightless (not just
secondarily flightless)--and there is no such bird extant. >>

Absolutely not true that it is moot. The roadrunner's leaping, flapping
behavior is quite sufficient as a model of the brooding-to-flight transition
regardless of the roadrunner's past evolutionary history. The roadrunner
doesn't stop and think, "Gee now's my chance to use a leaping flapping
behavior that I acquired from flying ancestors." It just LEAPS. And if a
cursorial theropod did the same 160 million years ago, it didn't have to stop
and think, "Gee, I had better not try flapping in the middle of this leap,
because my ancestors couldn't fly." It just FLAPPED.

<<extant birds use flight feathers (as defined by their aerodynamic
properties) for brooding, not brooding feathers for flight. I'm quite sure
that many, many more birds use their wings for flying than for brooding. So it
is much more straightforward to regard brooding as a flight exaptation rather
than flight as a brooding exaptation.>> 

Aerodynamic properties are a poor choice for constraints on this scenario. You
are expressing a long-held tenet of arborialites that aerodynamic feather
shape is somehow a proof of flight-relatedness. But among small animals, it
really matters whether a gust of wind will blow you over, and whether your
integument catches in the brush as you run through it. Hence, a completely
non-flying early theropod would have needed a streamlined or "aerodynamic"
shape on any brooding feathers it might have carried around with it. Again,
for a model of how the "flight" feathers can be tucked away and carried
efficiently while on the ground, consult the roadrunner. I am sure caudipteryx
bore its "wing" feathers like a roadrunner, regardless of whether its
ancestors flew or not.

<<1. Can you exhibit a bird that uses its wings for brooding but not for
flying? 2. Can you list the characteristics or features of a wing that you
would consider to be obligatory for a brooding capability? 3. In other words,
what features of a wing are present solely because the wing is used as a
brooding organ, that presumably arose first, before the wing became a flight
organ?>>

Nice thoughtful set of questions, George. Answers (as best I can):

1. The great majority of birds use their feathers in brooding, for warming,
cooling and/or hiding their offspring. Among these are flightless cormorants,
with more or less aerodynamically intact wing feathers. And of course ratites
of various stripes. Despite the foregoing, I don't believe that there is any
need to provide such examples. None can exactly match the situation that
existed 100+ million years ago.

2. Brooding capability depends on environment. Hot sun requires a canopy. Rain
requires an umbrella. Frigid conditions require a blanket. Gales require a
wind-breaker. Hiding requires camouflage. Mobility requires a folding tent.
The fact that babies may trample the primary feathers (I've seen photos of
this in Mallards) requires a robust cleanable and repairable structure. Any
particular species' requirements may be drawn from among these.

3. With a wing used solely as a brooding organ, a case can be made, based on
the points above, that among many possible variations of brooding wing and
feather construction, the one that was the direct antecedent to flight
happened to require a suite of characteristics very similar to the
archaeopteryx-caudipteryx condition.

<<If you wish to argue that wings evolved for some other purpose and later
became exapted for flight, then the burden must be on you to present a
compelling argument why this counterintuitive scenario occurred.>>

I take comfort in the realization that I am in no danger of better-established
paleontologists running off with our idea and claiming it as their own before
it is published in the Dinofest 98 Proceedings volume. There are detailed
answers to some of your concerns forthcoming therein. Regarding
counterintuitive -- thanks, I'd rather not think of my arguments as intuition
either. But perhaps you should spend some time watching a mother hen with her
chicks. Maybe then you would feel that attaching great significance to
brooding would seem -- what's the opposite of "counterintuitive?"

Tom Hopp