[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
RE: Longer Tails in Birds: Functional "Re"Evolution
Okay, I admit to being confusing in my last post on this subject ... I
forgot to add a condition in there to delegate a change of aspect. See
below for a clarification....
Tim Williams (TiJaWi@agron.iastate.edu) wrote:
<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.>
A long, heavy tail is not a good thing for a cursor to have. It adds
weight and decreases the ability to affect a tight turning radius when in
pursuit or fleeing, the only ecological reasons to _be_ cursorial.
Similarly, maneuvering fliers are forest dwellers, typically, and possess
large, broad tails ... but these are not typically elongated as in the
case of some birds *Geococcyx*, the road-runner, or other similar cuculids
and passeriform birds. The long tail is an effective balancing structure.
One sees it most in _perching_ birds, for which the ability to effectively
balance out the body is neccessary when balanced on a branch, and then
sleeping without the fear of falling off. The special hallux modification
of the tendon would do no good if the body was front heavy.
In cursorial birds, as in ornithomimid dinosaurs, have very long legs,
reduction of pedal digits, and _very_ short tails. The greater mobility of
the neck provided by the heterocoelous vertebrae and the shorter trunk are
modified in running by being oriented subvertically, in this case
improving the turning radius. Hypoercursors reduce tails ... which seems
odd in the case of the road-runner (not a hypercursor, but similar in
regards to being a running, predominantly-terrestrial bird) ... but this
may be as a result of less time to develop a shorter tail. The road-runner
may be a more recent variation on the cuckoo bauplan which is nearly
identical to that of the passerine. In passerines, elongation of the tail
is marked as a perching adaptation, as I see it, and thus they are not
contiguous. However, my point was to illustrate that the feathers may be
used to modify the need of a long, bony tail to indicate the use of "tail"
before did not take this into account. The road-runner just seems to be an
animal on the verge of transforming this. One reason the tail may stay is
that the bird does not make effective use of the tail in cursorial
locomotion as does an ostrich: during running, the roadrunner does not
elevate the front of the body, but instead "lays it all out" and extends
the neck to balance the body again. I have yet to see a satisfactory
explanation of this compared to ratite cursors. Note that in the gruiform
phorusrhacids, the "ratite" style body appears to show that they, too, ran
as in ostriches.
<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.>
By hypercursorial, I mean the difference between a dog and a horse in
running style and leg anatomy. The horse developes elongated distal limb
bones and a fusion of the metatarsals, they become cannon-ate, and
locomotion is run on the ends of the toes, not the extent of the toes, as
in typical cursors. Dogs will place the whole of the paw on the ground
during terrestrial contact, but in ungulate cursors it is only the last
phalanx per digit, covered in the keratinous hoof. Hypercursors also
shorten the femur relative to the elongated ilium (iliac process of the
innominate in mammology), so these qualities are osteologically correlated.
Jaime A. Headden
Little steps are often the hardest to take. We are too used to making leaps
in the face of adversity, that a simple skip is so hard to do. We should all
learn to walk soft, walk small, see the world around us rather than zoom by it.
Do You Yahoo!?
Send FREE Valentine eCards with Yahoo! Greetings!