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Re: Adaptive advantage (was Re: ABSRD BAND on Sinornithosaurus feathers)
At 12:54 AM 17/03/01 +0000, Scott Hartman wrote:
Hoatzin chicks as scansors? That is news to me. I'd be interested in
a reference to hoatzin scansorial behavior, as every reference (and
nature film) I've seen shows them climbing slowly through the
trees. Laborious climbing is arboreal behavior, but is isn't scansorial
behavior. Scansors utilize a ballistic phase in locomotion from branch
I was using "scansorial" in the sense I know it, which simply means
The point you raise about lack of knowledge of recently extinct
flightless birds is true enough, but (even though I brought it up...) I
think the entire issue is a red herring, as is citing birds that exhibit
scansorial behavior without opening their wings. All birds have already
had millions of years of evolution that pre-adapts them to scansorial
behavior, namely they have had to adapt to the environmental
three-dimensionality common to flight and scansorial behavior. Since
flight is plesiomorphic to all extant groups of avian scansors
(obviously), they also all had the safety-net of flight _while_ they were
adapting to scansorial habbitats.
I disagree here. This is a genuine, known gait that allows an animal to
get around in trees without flying, and the "safety-net" need not be a
problem. Tree Kangaroos can jump 80 feet out of a tree to the ground
without harm, for example.
Finally, while hopping from branch to branch, avian scansors usually use a
symetrical gate, as do many terrestrial birds. A symetrical gate greatly
reduces the amount of yaw and roll created during locomotion.
Theropods adapting to scansoriality had none of these
benefits. There are no known non-avian theropod tracks showing
symetrical hopping gates, so suggesting that arboreal non-avian theropods
did would be speculative at best. (I am assuming mainstream theropod
phylogeny here, rather than BCF, which changes this arguement some.)
If the gait was used in trees, why would it leave tracks? The difference
between walking and hopping is really quite minimal; some birds do both,
and even among those that don't the difference is trivial; most Savannah
Sparrows hop, but the Ipswich Sparrow, which is only subspecifically
Moving the center of mass away from a substrate makes torosional
forces harder to counter, and theropods simply don't show adaptations to
counter those forces. They could have shortened their limbs, but in fact
the opposite trend is true of progressively more bird-like
theropods. Birds are a bad model for theropods in trees, and all other
scansors are even worse.
Have you ever analysed the gait of birds like guans in trees? Or couas, or
birds of paradise? This is a very sweeping statement!
I really don't object to small theropods getting into trees, I object
to them being well enough adapted arboreal scansors to have the optertunity
to evolve gliding behavior.
I've been through this one before, but I don't see that gliding behaviour
is a necessary precursor to flapping flight anyway (If you mean long-range
gliding as opposed to simple parachuting). If there was an advantage for
them to get from one branch to another, or to jump safely from a branch to
snatch a prey item, anything that added control or stability to that action
could be selected for.
I have repeatedly suggested that an UPWARD leap to snatch prey from
leaves or thin branches (eg insects, frogs, lizards could have been in
the repertoire of early proto-flyers, and birds certainly do this today.<<<
I agree that this is the most likely way that flight would evolve in
an arboreal scansor, but coelurosaurs consistently evolve characteristics
that would hinder this tye of behavior.
Of course, arboreal insect predation is also prone to catastrophic
failure, but no one has shown that there is an imediate benefit to
theropods with arm fringes in this type of behavior, while there is to
terrestrial vertebrate hunters.
There may very well be, if it gives even a momentary parachuting ability
that would permit the animal to return safely to its original, or a nearby,
Previous studies (such as Feduccia's) are misleading, in that they did
not consider raptorial claw types when making their comparison. Jason
Kenworhty's talk in Mexico City showed that the claws of Archaeopteryx are
more like those of raptorial birds than those of climbing or perching
birds. The reversed hallux of Archeopteryx is too small (and has too small
of flexor tubercles) to have been much use during perchng, and was more
likely used for manipulation of small prey. There continues to not be any
positive evidence of arboreal adaptation in Archeopteryx.
You don't need a reversed grasping hallux to sit in a tree. Tree ducks do
this quite well. And nobody ever looks at guans, which do not so much
perch as run along the upper surfaces of large limbs.
Ronald I. Orenstein Phone: (905) 820-7886
International Wildlife Coalition Fax/Modem: (905) 569-0116
1825 Shady Creek Court
Mississauga, Ontario, Canada L5L 3W2 mailto:email@example.com