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



Jaime Headden wrote:

>  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.

Yes, but if true, it took a long time for dinosaurs to make this connection.
Ornithomimosaurs, troodontids, and hypsilophodontids - which are
traditionally regarded as the most cursorial of dinosaurs - all have long
tails.  The shortest tails (among bipedal dinosaurs) are seen in _Nomingia_,
oviraptorids, _Microraptor_, and basal avians - all of which (based on
hindlimb proportions) are far less cursorial.

The fairly long, stiffened tail of dromaeosaurids has been regarded as
actually *assisting* in executing rapid turns during pursuits.

>Similarly, maneuvering fliers are forest dwellers, typically, and
>possess large, broad tails ... 

I would argue that flight is not really comparable to terrestrial
locomotion, since the wings and tail of birds act in concert during flight -
effectively forming (in Gatesy's words) a single locomotor module.


>The long tail is an effective balancing structure.

I would agree.  It's purpose is to counterbalance the body anterior to the
hips.  The avian pygostyle shifts the centre of mass forward of the pelvis -
meaning that cursorial birds have to resort to recruiting the femur and
pelvis (see below) in pulling the centre of mass back in the direction of
the tail.

>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.

You know, when I picture a crow or a robin (or other arboreal perchers, such
as parrots), I have difficulty believing that the tail fan (and the
supporting pygostyle) really contributes all that much to balancing the bird
while perching.  Such a little tail (even with the tail-fan) at one end of
the bird, and such a BIG head at the other...  I think the grasping foot
(whether anisodactyl or zygodactyl) alone is sufficient to maintain a secure
footing when perching without any help from the tail.  

In any accomplished flier (passerines and psittacines included), the shape
and size of the tail is governed largely by the demands of flight.  The same
applies to the wing, for much the same reason: the wings and tail work
together during flight.

>  In cursorial birds, as in ornithomimid dinosaurs, have very long legs,
>reduction of pedal digits, and _very_ short tails. 

Ratites do have short tails, but the big, heavy pelvis is posteriorly
elongated - also seen in some large-bodied waterbirds (e.g. geese).  This
acts as a more effective balancing organ, and bypasses the need to re-evolve
a longer tail. 

> 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. 

I'm not quite certain what you mean by this.  I don't think tail *feathers*
serve any purpose at all in terrestrial and/or cursorial locomotion for any
bird.  I'm not an ornithologist, so I could be *very wrong* on this point.
However, ratites do show (1) a small head, (2) femora swung under the body
and (3) prominently expanded pubes and ischia.  (2) and (3) both compensate
(and correct) for the ancestral avian condition of shifting the centre of
mass forward of the hips.

> 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: 

Here I'm definitely confused.  Are you saying an ostrich does or doesn't use
its tail in cursorial locomotion?  I'd say it doesn't - but the posteriorly
elongated pelvic girdle serves much the same purpose as the counterbalancing
tail skeleton.  And, of course, the neck is held subvertically (as you note)
and the head is very small compared to flying birds, reducing the forward
weight.

>Hypercursors also
>shorten the femur relative to the elongated ilium (iliac process of the
>innominate in mammology), so these qualities are osteologically
>correlated.

Mammalian cursoriality is rather different to birds in this respect, since
in cursorial flightless birds, the femur no longer takes part in a true
locomotory function.  The distal hindlimb elements (tibiotarsus,
tarsometatarsus, pes) are what's left of the functional hindlimb during
cursorial locomotion.  That's why all ratites are such ungainly runners.


Tim



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

Timothy J. Williams 

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

Phone: 515 294 9233 
Fax:   515 294 3163