Re: origin of flight

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

>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.

This actually does not due justice to the arboreal hypothesis as espoused
by various workers since the 1970s.  In fact, they seek the origin of birds
from ground-running or ground-sprawlling forms (sphenosuchians and
megalancosaurs, respectively) which became adapted for life in trees.

>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

Now that's a variation I haven't heard yet...  I like it, though...  (Most
arboreal advocates say that the claws of Archie were too sharp for
predators, and more like climbers.)

>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.

Actually, the claws are Archie are not much like eagles, but more like
cats.  Allosaurus and Megalosaurus and other traditional "carnosaurs" (and,
of course, Dryptosaurus aquilunguis) actually have very eagle-like claws.

>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

No, we don't.

>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.

Leaping would benefit an animal with a dromaeosaurid-like (or cat-like)
predatory technique.

[deleted - reasonable comments about how Archie-style feathers add drag
more than lift.]

[deleted - discussion of how catching insects with wings doesn't work very
well.]
>
>A new and powerful argument for the evolution of flight among
>fast-running protobirds is related to Carrier's 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.

Two things.  One, "preadaptation" is an ugly (and more to the point,
nonsensical) word, which has been replaced with "exaptation".

Secondly, Archaeopteryx and its closest nonavian relatives, the
dromaeosaurids, do not show advances towards a more cursorial life-style.
In fact, their relatively short tibiae and metatarsi are more consistant
with a slower maximum speed than other, similar-sized theropods.

Looking forward to the rest of this.

                                
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.