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Re: Progress in evolution and the origin of flight feathers
George O. quotes Rob Meyerson:
>> I have a small objection to the word "primitive"
> Let's see some synonyms for "primitive" as an antonym to "derived."
I have mixed feelings about the issue in question. I agree with Rob
that the word "primitive" conjures up (in the lay public even if not
even subconsciously amongst those in the field) some inappropriate
imagery. Off the top of my head, though, ancestral is the only other
antonym I can think of that receives common use. It does lose some of
the problems that "primitive" brings with it, but it suffers from other
problems when you're talking about character states in coeval
organisms. Aeh what can y' do?
Now since I've been having so much fun battling with George both out
here and in private, let me take issue with:
> Humans are the most complex organisms to ever have evolved on earth.
To be blunt, I think the above statement is rather vacuous. I argue
against such statements not because I think we have a need for
absolution but rather because I don't think it has any truth to it (or
rather, its truth depends so strongly upon the definition of
complexity that it seems it can only be true by definition -- hence
its vacuity). I don't particularly want to argue it here, though,
since I think there are plenty of other places where the discussion is
more appropriate (sci.bio.evolution comes to mind, though it's been a
while since I've had time to read that group...).
So let's go back to something more dinosaurian. George asks:
> Didn't someone (Ostrom?) once suggest archaeopterygid flight
> feathers as insect traps?
Yes. However, it's my impression that nobody takes the idea seriously
any more. However, the idea has gotten a fair amount of press, so
let's turn to another idea that Richard Cowen (another prominent
lurker) gave us last winter and which I think could stand some more
consideration. I'm only excerpting a piece of what Richard sent last
January. If you want more, look through the jan95 archive (archives
that old were kept by month rather than by day, so be prepared to sift
through a lot...) or better yet look for Richard's book:
This is a slightly edited version of a section from "History of
Life" by Richard Cowen, published by Blackwell Science, 1994.
Copyright Richard Cowen.
The Display and Fighting Hypothesis
Either the arboreal or the cursorial hypothesis would work, and work
much more easily, if a protobird already had long, strong feathers
in the right places and already had powerful arm movements. Jere
Lipps and I suggest that display was involved in the evolution of
flapping flight as well as in the evolution of feathers. Display
provided long, strong feathers on arms and tail. Successful display
was increased by lengthening the arms, especially the hand, and by
actively waving them, perhaps flapping them vigorously. Flapping in
display would have encouraged the evolution of powerful pectoral
But a threat display must not be seen as an empty bluff. Fighting is
the last resort. Living birds often fight on the ground, even those
that fly well. Wings are no longer clawed but are still used as
weapons in forward and downward smashes (steamer ducks are
particularly deadly at this). Beaks and feet can be used as weapons
too and are most effective when used in a downward or forward
A strong wing flap, directed forward and downward, is also the power
stroke that gives lift to a bird in takeoff. Lipps and I suggest
that strong wing flapping is a simple extension of display flapping,
encouraged by fighting behavior. Powerful flapping used to deliver
forearm smashes could have lifted the bird off the ground, allowing
it also to rake its opponent from above with its hind claws. The
more rapidly the wings could be lifted for another blow, the more
effective the fighting. This would rapidly encourage an effective
wing-lifting motion that minimized air resistance, so the wing
action would then be almost identical to a takeoff stroke.
A variant of our idea has also been proposed by Kevin Padian, who
prefers to think of the wing stroke evolving from the arm strike
used by a theropod in predation. It's not clear how this could have
led to whole-body takeoff, however.
A few living birds use their wings extensively as weapons. The
steamer ducks of the South Atlantic are large, powerful birds with
heavy, bright orange, horny knobs on the wings of both sexes. These
are used by both sexes in display and fighting. Steamer ducks
(especially males) fight a lot among themselves for mates and
territory, and they often kill other species of water birds, holding
them by the neck and beating them to death with the wing knobs. Some
species of steamer duck are flightless; in other species, the males
are often too massive to fly, even though juveniles and females can
fly well. Selection has favored fighting ability over flying ability
for many steamer ducks. Flight is perhaps less important for them
than for many birds, because they live in shoreline habitats where
food is plentiful all year round.
Archaeopteryx fits our display-and-fighting hypothesis well. It was
well adapted for display. Like any small theropod, it was well
adapted for fighting with its teeth and the strong claws on hands
and feet. Archaeopteryx did not have long primary feathers on its
fingers, probably because they would have hidden the claws in
display and would most likely have broken in a fight.
Did Archaeopteryx Fly?
Archaeopteryx, then, was a fierce little fast-running, displaying
bird, which probably spent its life scurrying around the Solnhofen
shore, hunting for small prey such as crustaceans, reptiles, and
mammals. In hunting style, Archaeopteryx was probably much like the
roadrunner of the dry country of the American Southwest, but its
ecological setting was closer to that of a steamer duck - a
shoreline with year-round food supply. Archaeopteryx did not compete
in the air with the pterosaurs that are also found in the Solnhofen
From Display to Flight
In our theory, display and fighting were simple selective agents
that encouraged the evolutionary transition from small dinosaurs to
birds. The idea fits with our current knowledge of the biology and
behavior of living birds. Display, and fighting if necessary, is
very important, even within a species. Bald eagles and frigate birds
often try to rob other birds of food instead of catching prey
themselves. Because the penalty for wing injury is high, many birds
can be intimidated by display into giving up their catch rather than
fighting to defend it.
Display and fighting in birds, whether for territory, dominance, or
food, takes a lot of energy, but only for brief periods or seasons,
and it provides an enormous payoff in survival and selection.
Sexual display in most living birds must be done correctly, or no
mating takes place. New behaviors are quick to evolve, and they are
evolutionarily cheap, because they usually do not require any
important morphological changes in their early stages. Bowerbirds,
for example, show distinct behavioral differences in display between
closely related species.
The display hypothesis suggests that a protobird gained flight
behavior, anatomy, and experience at low ground speed and low
height, ideal preflight training. The selective payoff for
successful mastery of the flight motions gave significant
advantages, even before flight itself was possible. From that point,
the many advantages of flight were added to those of social or
Lipps and I envisage Archaeopteryx as a small, fierce predator,
capable of liftoff but not true flight. Once liftoff was achieved,
flapping flight quickly followed. There is no need to suggest any
difficult evolutionary sequence to complete the final transition to
full powered flight. In more advanced birds than Archaeopteryx, the
pulley system of the shoulder evolved for quick wing upstrokes,
while the wishbone evolved into a spring. The breastbone evolved as
the anchor for the flight muscles. The forearms became longer,
lighter, and more fragile in bone structure, becoming specialized as
wings, and losing the finger claws. The feathers became more
aerodynamically suited to powerful swishes through air. Meanwhile,
the feet and beak became the dominant fighting weapons, as in most
living birds today.
The display hypothesis for the origin of flight is particularly
attractive because it demands few of the assumptions required by the
arboreal or cursorial hypotheses, yet it is fully compatible with
the morphology of Archaeopteryx and the biology of living birds.
NOTE: If you want to quote this hypothesis, I'd suggest that you use
the phrase Cowen and Lipps in Cowen (1994), and refer to my
textbook. I hope that Jere and I will be able to get the
full-jargon version out in a journal some day. Meanwhile, Jere and I
are responsible for the idea, and you have to blame me for the
detailed wording that I prepared for my text.