Re: Feathers

[Tom Holtz <tholtz@geochange.er.usgs.gov>]

>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.

In fact, the opposite is true - chemically and developmentally, feathers
ARE modified scales.  As you note, not all the scales in a bird are
modified, but then again, not all mammal sweat glands have turned to
mammary glands, nor all mammal fur turned to vibrissae (whiskers).

>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.

[Note that many of us are desperately waiting for the Spanish team to
publish detailed descriptions of the "integumentary structures" in
Pelecanimimus...]

>This theory for the origin of feathers [thermoregulatoin] 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.

First off, the question of pterosaur fur is much debated (as per recent
discussions on the net).  Also, although flight feathers are fairly
susceptible to damage, contours feathers are less so.

>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 -25 C (-13 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.

Penguins are highly derived birds evolved from volant species.  There is no
evidence to support (and much evidence to suggest) that the adaptations of
penguin feathers evolved within the direct ancestors to penguins, and do
not represent the primitive bird condition.

>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.

This is true.

>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.

The above does not follow.  Here is an alternative suggestion - that
protofeathers evolved for thermoregulation, and then were modified for
flight in the immediate ancestors of Archaeopteryx.  This is consitent with
the evidence (and is the prefered hypothesis of most vertebrate
paleontologists, actually).

>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.

This idea has great merit.  However, it is not inconsistant with an
hypothesis of thermoregulation as well (much as the ears of elephants [and
the plates of Stegosaurus?] serve both for display and thermoregulation).

>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.

Also, given that the adaptations of the tail of Archie (much like
dromaeosaurids) allow it to be raised up vertically behind the head, the
radial feathers of the tail might make a very effective display.

>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.

Although this is possible, the explanation for the fainter impressions
might have more to do with variations in the substrate than in the bird.

                                
Thomas R. Holtz, Jr.                                   
tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile                  Phone:      703-648-5280
U.S. Geological Survey                                FAX:      703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092
U.S.A.