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Feathers and flight, section 2
This is a slightly edited version of a section from "History of Life"
by Richard Cowen, published by Blackwell Science, 1994.
Copyright Richard Cowen.
Archaeopteryx was the first feathered creature as well as the first bird.
Its few fossils must serve as the basis for discussing three important
questions: the origin of feathers, the origin of birds, and the origin of
flight in birds. The origin of birds clearly lies within small theropods.
But how did feathers evolve, and when and how did flight evolve in birds?
The Origin of Feathers
Feathers in living birds originate in a deep skin layer under the outer
layer that forms scales. Evolutionarily, then, feathers probably arose
under and between reptile scales, not as modified scales. Many birds have
scales on their lower legs and feet where feathers are not developed,
and penguins have such short feathers on parts of their wings that the skin
there is scaly for all practical purposes. So there is no real anatomical
problem in imagining the evolution of feathers on a reptilian skin. But
feathers are completely novel structures, and any reasonable explanation
of their origin has to take this into account. They evolved in the first
birds to replace scales as the primary skin covering. The problem is to
reconstruct why this happened.
Feathers may have evolved directly for flight. If so, they evolved in a
reptile that was already launching itself into the air, presumably as a
tree-dwelling jumper and perhaps parachuter. In this hypothesis,
feathers were first an aid to parachuting and then a way to achieve
flapping flight. This is a difficult process to imagine. Why feathers?
The scales of gliding reptiles do not project beyond the boundary layer
of air around the body, so they do not generate any lift. It is not
clear that protofeathers would have been any improvement. The
bone-supported skin membranes of parachuting reptiles are a much easier
and cheaper way to evolve an airfoil than any conceivable airfoil made
of protofeathers. Bats and pterosaurs evolved flapping flight without
feathers. Perhaps feathers evolved for some other function and were later
modified for flight.
Feathers may have evolved to aid thermoregulation. Small theropods
probably had a high metabolic rate and may have been warm-blooded.
Very small theropods would have needed additional insulation to keep
their bodies at even temperature. A few small reptiles today use long
scales to help trap a layer of air between the environment and the body
surface to cut down temperature fluctuations, usually as a heat shield
against the sun. It would not matter whether protofeathers were used to
conserve heat in cold periods, or to keep heat out in hot periods, or
both. In either case their insulation would have been useful. The
skin musculature would have been able to raise and lower protofeathers,
allowing free flow of air to the skin when necessary.
This theory for the origin of feathers is probably the most widely
accepted one today, but it does have problems. Again, why feathers?
Feathers are more complex to grow, more difficult to maintain in good
condition, more liable to damage, and more difficult to replace than fur.
Every other creature that has evolved a thermoregulatory coat, from bats
to bees and from caterpillars to pterosaurs, has some kind of furry cover.
There is no apparent reason for evolving feathers rather than fur even
for heat shielding.
Even within birds, down feathers are much better for retaining heat
than the contour feathers that are preadaptive to flight. An adult
emperor penguin has very efficient thermoregulatory feathers, but they
must also be water-resistant and hydrodynamically efficient. But an
emperor penguin chick does not fly, swim, or even walk very much.
Its primary need is to survive in the dark on the Antarctic ice cap
without a nest, in temperatures that average around P25 C (P13 F), and
in winds of 40 meters per second (100 mph). Its first feathers are molted
and replaced before it needs them for any other function, so they can be
the most efficient feathers evolved for thermoregulation alone. The
emperor penguin chick has down feathers. They are nothing like flight
feathers, display feathers, or the feathers of Archaeopteryx, and they
are developed equally over the body except for the wings and feet, where
they are shorter than normal rather than longer.
Thermoregulation cannot account for the length or the distribution of
the earliest known feathers, those of Archaeopteryx. Thermoregulation
would require feathers developed equally well over the whole body,
whereas Archaeopteryx had its longest, strongest feathers on the wings
and tail. Thermoregulation can be achieved perfectly well with short
feathers; it does not require the long feathers of Archaeopteryx.
So it is difficult to suggest that feathers evolved for thermoregulation
without also arguing that the feathers of Archaeopteryx had already been
evolved for some other function or functions and then modified. And once
that argument is made, the hypothesis of thermoregulation becomes
untestable on present evidence. It would be better to think of another
equally simple explanation of the feather pattern of Archaeopteryx.
I naturally prefer an idea that I developed jointly with my colleague
Jere Lipps of the University of California, Berkeley. In living birds,
feathers are for flying, for insulation, but also for camouflage and/or
display. Lipps and I suggest that feathers evolved first for display.
The display may have been between females or between males for dominance
in mating systems (sexual selection), or between individuals for territory
or food (social selection), or directed toward members of other species
in defense of territory or food.
Living reptiles and birds often display for one or all of these reasons,
using color, motion, and posture as visual signals to an opponent.
Display is often used to increase apparent body size; the smaller the
animal, the more effectively a slight addition to its outline would
increase its apparent size. Lipps and I therefore propose that replacing
scales with erectile, colored feathers would give such a selective
advantage to a small displaying theropod that it would encourage a
rapid transition from a scaly skin to a coat of feathers. Display would
be most effective on movable appendages, such as forearms and tail.
Display on the legs would not be so visible or effective. Forearm display
by a small theropod would also have drawn particular attention to the
powerful weapons it carried there, its front claws.
The display hypothesis explains more features of Archaeopteryx than
other hypotheses, with fewer assumptions. It explains completely the
feather pattern of Archaeopteryx. It explains why the feather impressions
are so faint on the smallest specimen of Archaeopteryx, which may not
have reached full adult size or status. This specimen is only about half
the size of the others and has no wishbone preserved, possibly because
it had not yet ossified. The display hypothesis assumes only that display
was important to Archaeopteryx; it assumes nothing special about its
habits, habitat, or body temperature.
To be continued.......