Feathers and flight, section 2

[cowen@geology.ucdavis.edu]

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.......