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Re: Feathers and flight; BCF



At 09:07 PM 13/02/1998 -0000, John Jackson wrote:
>Ronald Orenstein says flight may have evolved for short fluttering flights
>between branches.  However, this is not an easy first stage to achieve -
>such flights involve take-off, often direction changes, and efficient short
>landings with braking - all difficult.  All this does not make for a
>feasible first step.

Not necessarily.  Let's not forget the propulsive power of the hind limbs -
all my scenario involves is the ability to jump.  As I mentioned, a number
of living birds progress from branch to branch by strong upward leaps
without using their wings; a good example is the New Zealand Kokako, an
extremely poor flyer.  I have also watched birds-of-paradise do this
regularly in New Guinea.

Further, what I was actually proposing was not so much branch-to-branch
progress as the type of foraging ornithologists refer to as "hawking" -
basically, plucking food from a surface while airborne.  Let us assume, for
example, that a small bipedal theropod is able to haul itself into the
lower branches of a tree and proceed upwards by strong leaps from branch to
branch.  In doing so it can reach a wide range of invertebrates or small
vertebrates not accessible from the ground.  However, many of these may
perch on the tips of thin branches, or on leaves, that cannot support our
predator's weight.  Its only way of reaching them may by leaping upwards
from a stable perch, seizing the prey in mid-leap and then falling back to
its perch.  Certainly such a scenario involves certain abilities that have
nothing to do with flight - balance, ability to judge size and distance,
coordination.  However, anything that would allow it - as I said before -
to reach a little further, or to remain airborne even moments longer, might
greatly increase the success rate of these leaps for food.  Thus even
slight modifications to the arms that might assist this would be
selectively advantageous - overcoming the concern with, say, the gliding
hypothesis that the value of early stages of adaptation is hard to imagine.

Of course the same sort of behaviour could exist in a terrestrial animal,
especially for seizing flying insects or prey on low bushes - but I suspect
that the advantages are far greater, in terms of prey accessibility, in
trees.  Indeed most living birds that use hawking as a foraging strategy
are arboreal.

>BUT, long before these have been tuned to effective aerofoil shapes,
>AND the bone structure grown and adapted, problems such as the costs of the
>extra extra muscle, the feathers getting in the way of the claws and
>snagging on things would have been paid for, but most af all, they will
>have given DRAG.

Drag may be an unimportant factor in the kind of behaviour I am talking
about - in fact, if the arms were not deployed until the apex of a leap,
but held closely against the sides, drag would not be a problem on ascent,
but could be an advantage on descent where it might, admittedly minimally,
slow the fall and thus increase the ability of the animal to coordinate a
safe landing.  If you add to this the grasping ability of the arms, the
animal could use them to break its fall in case it missed its perch or made
a clumsy landing.

>However, drag is exactly what you want  when you are falling out of a tree.

Or from one branch to another, or back to a starting perch, as I have said.

>but believe me, any path to vertebrate flight other than
>parachute-glide-powered flight is a total non-starter since it does require
>far too many evolutionary changes to appear and be co-ordinated all at
>once.

Again, I disagree, and note that parachuting (or increased drag on descent)
may have nothing to do with gliding ability.  I think we need to consider
what flight is for - we all seem to assume it could only have evolved for
longer-distance movements, ie from tree to tree.  The kind of thing I am
talking about is useful for very short manoeuvres within a tree - we may be
talking, not about any increased distance, but merely increased stability
and control over a distance that could be achieved by a jump alone.

This is not to say gliding has no role to play; many birds, for example,
forage by proceeding up a tree, then gliding or flying down to the base of
the next.  Archaeopteryx may have done likewise - but this ability may have
developed only after the initial stages of arm modification for assisted
leaping, and may have driven the expansion in size of the wing area beyond
that needed for the kind of short leaps I am suggesting as the preliminary
stage.

Of course all this is speculative - but I suggest it as, first, something
living birds do, and, second, as a way out of the dilemma expressed very
well by Pat Shipman in "Taking Wing" that a trees-down theory necessarily
involves a glider (and thus seems unlikely for a bipedal theropod), so that
only the ground-up view works for the dinosaur-ancestry scenario.  my point
is that there are other things one can do in a tree - and do very usefully,
too - with proto-wings besides gliding, and that these might confer
selective foraging advantages even in the very early stages.

>There is an insulation paradox for evolving from cold to warm bloodied: 
>Warm bloods need it, but cold bloods cant's have it. 

Shipman cites work by Ruben suggesting that, as he puts it. "A fully
feathered Archaeopteryx could have been either ectothermic or endothermic".
 I am not saying I necessarily agree with this - only that the statement
above may not necessarily be true.  I would point out that a good many
living birds bask by raising the feathers on their backs or rumps, exposing
dark-pigmented skin to the sun.  Given this ability, I do not have great
difficulty imagining a bird that is less than a perfect endotherm, with
erectile contour feathers but. perhaps, limited or no  down, that could use
the position of the feathers for passive temperature regulation depending
on the ambient temperature and degree of insolation.
--
Ronald I. Orenstein                           Phone: (905) 820-7886
International Wildlife Coalition              Fax/Modem: (905) 569-0116
1825 Shady Creek Court                 
Mississauga, Ontario, Canada L5L 3W2          mailto:ornstn@inforamp.net