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Feathers and flight, section 3
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 Origin of Powered Flight in Birds
Most people feel sure that protofeathers must already have been evolved
before birds attempted flight. The question of the origin of flight is
thus independent of the origin of feathers, because flight evolved in
bats and pterosaurs without feathers. There have been three important
hypotheses for the origin of bird flight, and I shall add a fourth.
The Arboreal Hypothesis
The arboreal hypothesis suggests that a ground-running biped first
became adapted to life in trees, where it took to leaping from branch
to branch, then parachuting. Later it developed flapping flight.
Feathers became aerodynamically important at the jumping stage and
evolved directly into flight feathers. The arboreal theory is the most
favored at the moment, but it does have some difficulties.
Like all little theropods, Archaeopteryx was bipedal, with legs and
feet that were well adapted for ground running. Bipedality is a rather
poor preadaptation for living in trees, and Archaeopteryx had long,
erect hind limbs that were particularly ill-suited to climbing tree
trunks (some arboreal theorists suggest that it climbed sloping
branches instead!). Archaeopteryx, with long, erect limbs, a
comparatively short trunk, and bipedal locomotion, was exactly the
opposite in body plan of all living mammals and reptiles that jump
and glide from tree to tree.
The claws on the hands of Archaeopteryx were long, thin, and sharp.
They look like very effective tearing and slicing weapons, but were
far too sharp and pointed to have been useful for climbing either trees
or rocks. The claws on its feet have been compared with the claws on
the feet of perching birds, but they were also very like the talons
of an eagle or a theropod dinosaur, which shows only that they were
equally well adapted for clutching branches or prey, or both, and we
cannot tell which. Certainly bigger theropod dinosaurs with perching
claws did not climb trees.
Altogether, the arboreal hypothesis is not unreasonable, but it does
require a lot of special conditions. It looks vulnerable to a better
suggestion that would explain more of the evidence.
The Cursorial Hypothesis
Perhaps some adaptations in a ground-dwelling protobird could provide
some of the anatomy and behavior necessary for flight, such as lengthening
the forearms, especially the hands, placing long, strong feathers in
those areas, and evolving powerful arm movements. An early version of
the cursorial hypothesis suggested that a fast-running reptile might
evolve long scales on the arms. In this theory, the scales generated
lift as the arms were actively flapped on the run. The animal could
now take long leaps, perhaps encouraging the scales to evolve into
feathers and the leaps to evolve into powered flapping flight.
We know now that feathers did not evolve from scales, and in any case
this idea does not work mechanically. Any lift generated by a flapping
arm detracts from the ground traction given by the feet, and acceleration
is lost. A racing car is held down on the track by its airfoils for
good traction, and an aircraft cannot be driven through its wheels on
the takeoff run. A protobird that flapped its arms on the run would
increase drag: the faster the running, the greater the drag. Only a
very small amount of thrust would have been generated by the arms in
the early stages of the process. Running takes a lot of energy, and
it is not clear why leaping would have benefited the animal.
Both the arboreal and cursorial hypotheses must face the problem
of changing from parachuting flight (from a jump off a branch or a
leap into the air from the ground) to true powered, flapping flight.
Flapping arms or protowings - in fact, any feathers at all on wings or
tail - increase drag. Aerodynamically, the transition from gliding to
flapping is difficult: there is only a narrow theoretical window through
which the transition could have been made. This transition would have
been very difficult for Archaeopteryx because it had such a long, bony
tail with long feathers on it. Such a tail (an obvious display structure
in my opinion) adds much more drag than it adds lift. Therefore the
arboreal and cursorial hypotheses are not impossible, but they invite
a better idea.
The Running Raptor
More sophisticated recent versions of the cursorial hypothesis are
much better: they are mechanically sounder and include behavior that
involved strong, synchronized arm strokes and the evolution of strong
John Ostrom suggested that the protobird was a fast-running hunter,
perhaps using its arms to strike down insects that it disturbed.
Such an action could encourage the evolution of the muscles and the
joint movements that would approximate a wing-stroke. No living bird
catches insects this way, however, probably because a feathered wing
generates an air blast, while a well-designed fly-swatter has holes
in it to avoid blowing away the prey. Some egrets scare fish into
motion by wing flapping, but not at a run, and the wings are not used
Gerald Caple and his colleagues at Northern Arizona University
suggested instead that a protobird hunted by running fast and leaping
after flying or jumping insects it disturbed. To catch an elusive dodging
prey while its feet were off the ground, a protobird would have to be
able to adjust its body attitude in the air and then regain a stable
position for landing. Such adjustments could be made aerodynamically
by generating a small amount of lift or drag at appropriate points on
the body surface. Calculations suggest that a small amount of lift at
the tips of the arms would have a large effect on the body as a whole,
and if the right arm movements were added, the effect would be greater
still. The protobird would now be well on the way to flapping takeoff,
and the flights would be gradually prolonged until complete control
had been reached.
But this proposed activity would consume a lot of energy. No predator
today, bird or otherwise, runs at high speed to flush out insects it
can leap after. Furthermore, effective attitude control for a leaping
animal is not possible below a critical airspeed that is highest in
the earliest stages of the process, when the protowings are just
beginning to generate lift. The required speed might have been 10
meters per second, over 25 mph, far too much for any reasonable
protobird. The idea cannot explain the evolution of flight on its
own, but it may have components that could apply to later stages in
the evolution of flapping flight.
A new and powerful argument for the evolution of flight among
fast-running protobirds is related to CarrierUs Constraint. Powered
flapping flight demands a sustained high-energy output, so animals
that operate it have to have excellent respiratory and circulatory
systems. Flying insects pump air in and out of their spiracles in
synchrony with their wingbeats. In flying vertebrates, the muscles
that flap the wings are anchored on the rib cage, and expand and
contract the chest cavity with each wingbeat. Fruit bats, vampire
bats, and pigeons take exactly one breath per wingbeat, big geese
take one breath every three wingbeats, and pheasants and ducks take
one breath every five wingbeats. Perhaps, then, a bipedal reptile
running rapidly on the ground with erect limbs would already have
its breathing synchronized with its running, and it would have a high
metabolic rate and the capacity for sustained power output. Such a
preadaptation for powered flight would be more likely to occur in a
fast, bipedal runner than in a jumping, quadrupedal tree-dweller.
To be continued......