[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Bats and Archaeopteryx

>Also, have there been any studies done comparing the flight energy 
>needs/consumption between bats and birds? Don't bats require/expend more 
>energy than birds for flight?

I have no readily available references with that information, but
based on what I've read about bat metabolism and flight, I'd be
surprised if it was that far different.  Might even be less.

>RE. Archaeopteryx: Hmmm, let me try to rephrase my questions on this. My 
>question is simply one of whether the evolutionary pressure from an 
>arborial gliding existence would tend toward a bird-like wing development 
>or a flying squirrel-like skin-flap development. Extant arboreal gliders 
>all use the skin flap method. We have no modern analog to show that a 
>bird-like wing can be derived from this (or that a bird-like wing will 
>even evolve in an arboreal species). None of the modern gliding 
>animals is even in an interim state of development <towards> the wing 
>structure that birds have. 

Yes and no.  Evolution works with whatever is around.  I think there is
little question that feathers evolved first as a scale-derived analog of
hair, for thermal regulation.  Given a small, feathery dinosaur bounding
about in the trees, it would seem logical to me that extending the
feathers would be easier than developing a skin flap - the skin would
be covered with more feathers, and heavier overall than just extended
feathers.  It is true we have no analog of that today, but then, neither
do we have many animals with feathers instead of hair to begin with, and
none live in trees.

But there are other analogs we can look for - given enough time and
opportunity, I expect evolution will "try" everything.  I don't recall
the name of it, but there is a snake that is much the same evolutionary
pattern as modern gliders, but rather than developing a skin flap _or_
feathers, it is developing a sophisticated airfoil shaped body.  It
varies the angle of attack all up and down its body to produce some
stupifying glides that look for all the world like it's slithering or
sidewinding in mid-air, and I've read of at least one report that its
been seen using thermals to gain altitude.  It is still just a glider,
and not even as good at it as some of the flying squirrels, but it is
definitely on that path.  I could envision a flying snake, flattened
body cocked into an asymetrical airfoil shape, beating the air with
some segments to generate lift...might never happen, of course, or it
might find thermal regulation for such activity would require some kind
of outer coat, which would then limit the areas into which it could

>The tendency in extant arborial glider development has been to 
>develop characteristics that are already present in the animal (ie. 
>webbing between limbs/body or fingers, etc.). In which case, it seems 
>reasonable to assume that even if proto-Archie took to the trees at some 
>point, that the wing-type seen had already begun to develop somewhere 

Not a wing, but an arm with feathers.  If this theory is right, we
should be able to find transitional forms with very narrow wings -
but Archaeopteryx is hysterically rare - and notorious for being
misidentified.  An ancestor like that could easily lie around in a
museum for decades mislabelled as "small dinosaur" until someone
notices the fuzzy outlines on the arm bones and makes the connection.
It would be even less obvious than Archaeopteryx, and it took quite
a while for him, too.

>else. Archaeopteryx itself may have been a tree dweller (it probably 
>could have functioned equally well on the ground AND in the trees 
>since there are benefits to be derived in both environments [and 
>therefore a great pressure to develop the wing further]).

Quite likely.

>Another question is in regard of the tucked wing position. Extant 
>tree-dwelling gliders sprawl themselves out to bring their center of 
>gravity closer to the tree. They don't tuck their front limbs close to 
>the body. Tucking isn't likely to occur as long as you are climbing the 
>trunk of the tree. However, once you have started to develop a tucked 
>limb position, you can still hop up into the lower branches. Now, correct 
>me if I am wrong (or my facts out-of-date), but I seem to recall that 
>Archaeopteryx limbs were pretty-much tucked in (like a birds) but with a 
>greater range of motion. This would lead me to suspect that they had at 
>least evolved out of the trees, and (if Archies were tree dwelling) moved 
>there subsequent to the development of the wing-limb.

We know that archie was a ground-dwelling dinosaur at some point, because
of its extraordinary resemblence to them - so close that at least one
archie was so misidentified for years.  But I'm not sure you don't have
the cart before the horse.  Most squirrels climb with legs _under_ the
body, not splayed, and so do flying squirrels.  Splaying is a preparation
for flight maneuver.

>Now, IF the structures were already present prior to tree dwelling, they 
>most likely were used to the greatest advantage (which would include 
>stabilizers for sharp turns, added lift on jumps/hops, etc. All of which 
>are perfectly good flight precursers) while on the ground. Once in the 
>trees, the evolutionary advantage may have been ENHANCED and thus develop 
>into what we see today, but the beginnings, I believe, started on the ground.

It don't doubt that many of the structures started out on the ground.  It
would be easy to imagine a feathered cursorial dinosaur, using winglike
structures for stablization in running and for _negative_ lift, that
gradually developed an arboreal lifestyle for hunting or escape from
predators.  It could have evolved the whole schmeer on the ground and
then just refined a few changes to instinctive operation (positive
instead of negative lift) for gliding or drop-hunting, where it might
hunt by swooping down from a tree and galloping after prey.  But the main
point here, I think, is that the trees were necessary - a pure cursorial
model places too much of a premium on negative lift for better speed and
too much of a penalty on speed loss during initial take-off.

Larry Smith