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RE: Longer Tails in Birds: Functional "Re"Evolution



 
Jaime Headden wrote:

>True, in
>hypercursoprial birds like ostriches, there is a very short tail, but
>this is true in hypercursorial theropods, too ... reduction of the tail
>and elevation of the neck resulting in narrowing the turn radius and
>allow greater turning control. Nonetheless, comm ents please?

I was with you all the way until this last point.  Wouldn't a long, heavy
tail would be advantageous to a cursorial bird?  In neornithines,
the centre of mass is forward of the hips - and the evidence points to this
being a flight adaptation.  As it is, secondarily terrestrial cursorial
birds (neornithines - like ratites) have to avoid the problem of toppling
forward while walking or running by swinging the femur under the body so it
can provide more "ballast" in the hip region.  This is why cursorial birds
have such long legs: the femur is effectively decoupled from locomotion, and
the "effective hindlimb" begins at the knee.

(By the way, the above has been used by Jones et al. [2000] to argue that
_Caudipteryx_, which has proportionally long legs [comparable to ratites],
had avian-style locomotion, and was therefore most likely secondarily
flightless.  However, many juvenile non-avian theropods also have comparably
long hindlimbs relative to the trunk ...)

Secondary lengthening (specifically to give it greater mass) the tail would
alleviate the problem of reconfiguring the hindlimb elements in cursorial
avians.  But, feathers aren't that heavy ("as light as ...") and the
molecular software may not have been amenable to "re-lengthening" the caudal
skeleton.

Also, I'm curious what you mean by "hypercursorial".  Do you mean birds that
run fast, or run often?  The secretary bird (_Sagittarius serpentarius_) has
been described as cursorial; it has very long legs and spends most of its
time on the ground, where it hunts.  However, the secretary bird rarely
runs, and it flies up into trees to roost.  I've wondered if the long legs
are there just to make the bird taller - raising its sensory platform.
Ratites, on the other hand, being flightless, run fast and run often (with
the exception of the kiwis) - it's their only way of eluding predators.

As Jaime said, comments (and peanuts) welcome.


References

Jones, T.D., J.O. Farlow, J.A. Ruben, D.M. Henderson and W.J. Hillenius
(2000).  Cursoriality in bipedal archosaurs.  Nature 406: 716-718.


Tim




------------------------------------------------------------ 

Timothy J. Williams 

USDA-ARS Researcher 
Agronomy Hall 
Iowa State University 
Ames IA 50014 

Phone: 515 294 9233 
Fax:   515 294 3163