Re: Adaptive advantage (was Re: ABSRD BAND on Sinornithosaurus feathers)

[Ronald Orenstein <ornstn@home.com>]

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 
> to branch.

I was using "scansorial" in the sense I know it, which simply means 
"climbing".

>     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 
different, walks.

>     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.

Such as?


>     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, 
perch.

 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:ornstn@home.com