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Re: Avian flight stroke origin



Nice paper, seemingly strongly harming the trees-down hypothesis. If
we have not positive argument to expect climbing abilities, and winged
maniraptorans had to wait in the floor till some bird evolved the
necessary power to propel them in the air, what served the feathery
wing for?
Of course, there are lots of alternatives discussed in this very list,
as they being used as ornament for courtship and sexual selection,
which can be supported because of the much enlarged nature of these
feathers, which remind the long neural spines of pelycosaurs (and
surely many dinosaurs), whose growth allometry has been recently
hypothesized to reflect sexual selection instead of natural selection,
on the basis of theoretical arguments recently expressed by Mark
Witton and his collaborators. Other hypotheses imply appearing larger
to predators, etc.
Sorry if the argument is old in the list or elsewhere, but it is not
at least as far as I know. I was recently reading The Selfish Gene, by
Dawkins, and found some interesting considerations by ethologist
Zahavi regarding the possibility of males trying to fool females
during courtship. If the females view large muscles as desirable (for
they permit faster fleeing or better chances at fighting predators,
indicate good feeding state and survivorship until adultness, greater
possibilities of reproduction becuase of fighting or intimidating
other males), males may fool them by creating false muscles which
generate the same interest from the female. But then, according to
Zahavi, females better favoured by natural selection would be those
which learn to discern real muscles from false muscles, which in turn
will lead only males with truly large muscles to be selected. Later,
Dawkins indicated that these signaling processes may also work in
animal communication out of courtship.
It seems to me that, other mammals and some anurans, birds are the
only animals which fight largely using their wings and/or limbs. It is
equally parsimonious to accept other maniraptorans used or not their
extremities in fighting, and the presence of likely fighting
adaptations in the spikes of some iguanodontians seem to suggest they
were used more widely among dinosaurs. What if the long feathers give
the impression of a thick, strong extremity (affecting both fore and
hindlimb in relatively basal proavians), being useful as "false
muscles", which can either intimidate other males and/or impress
females? An interesting thing is that, if females finally discovered
the way by which discerning real from fake muscles, there was a
selective pressure upon males to develop true muscles. And, in the
case of the forelimb, the pressure may have resulted in the
enlargement of the pectoralis muscle, which may have permited strong
force at hitting (I remember reading elsewhere that a goose is capable
of fending off a similarly sized fox with the strenght of these blows;
besides, would you feel confortable fighting a guy with larger
pectorals than you?). No need to loose the "fake muscles" if there are
true muscles also.
When the blows were not hard enough, the presence of large, basal
maniraptoran-like claws, which seem to direct cranially, may have
increased the pain in the rivals, or even the effects. Stronger arms
may have made these distal bones more prone to breaking at impact,
which may relate to the reason by which the claws regressed in taxa
with stronger pectorals. Blunt force may have made the desirable
effects of the claws not so needed. Finally, in the
intraspecific-fighting part of this selective scenario, the long
feathers, both in body and on the wing, may have permitted to
relatively protect the body from the blows of other males, either with
claws or not. Wings and pectoral muscles can thus appear by a linked
reason, in different moments, as predicted by Zahavi's hypothesis, and
thus represent (required) preaptations.
This scenario can prove to be false if it is noted that the earliest
birds with thick pectorals or supracoracoid muscles do not have wings
able to deliver blows. From a parsimony viewpoint, it would not be
most parsimonious if most basal clades of Recent birds seem to
scarcely use their wings in intraspecific fighting (actually, this is
apparently present in ducks and fowls, and may be relativley
restricted to Galloanserae). I do not quite much see how this
mechanism leads to an increase in the excursion range of the forelimb,
necessary to flight, but a larger excursion range may increase the
range to which the blow can be delivered. Neither do I know if the
reduction of the unguals occurs after or at the time of the increse of
power of the pectoralis muscle.

Oh well, following with the spirit of the list, it seems that many
possibilities are there to explain the evolution of the "flying-like"
adaptations as adaptations for other things instead of flying.

Cheers,
Augusto.

2011/8/9 bh480@scn.org <bh480@scn.org>:
> From: Ben Creisler
> bh480@scn.org
>
> New in PLoS ONE (free pdf, of course):
>
> Dececchi, T.A. & Larsson, H.C.E. (2011)
> Assessing Arboreal Adaptations of Bird Antecedents: Testing the Ecological
> Setting of the Origin of the Avian Flight Stroke.
> PLoS ONE 6(8): e22292.
> doi:10.1371/journal.pone.0022292
> http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0022292
>
> The origin of avian flight is a classic macroevolutionary transition with
> research spanning over a century. Two competing models explaining this
> locomotory transition have been discussed for decades: ground up versus
> trees down. Although it is impossible to directly test either of these
> theories, it is possible to test one of the requirements for the trees-down
> model, that of an arboreal paravian. We test for arboreality in non-avian
> theropods and early birds with comparisons to extant avian, mammalian, and
> reptilian scansors and climbers using a comprehensive set of morphological
> characters. Non-avian theropods, including the small, feathered
> deinonychosaurs, and Archaeopteryx, consistently and significantly cluster
> with fully terrestrial extant mammals and ground-based birds, such as
> ratites. Basal birds, more advanced than Archaeopteryx, cluster with extant
> perching ground-foraging birds. Evolutionary trends immediately prior to
> the origin of birds indicate skeletal adaptations opposite that expected
> for arboreal climbers. Results reject an arboreal capacity for the avian
> stem lineage, thus lending no support for the trees-down model. Support for
> a fully terrestrial ecology and origin of the avian flight stroke has broad
> implications for the origin of powered flight for this clade. A terrestrial
> origin for the avian flight stroke challenges the need for an intermediate
> gliding phase, presents the best resolved series of the evolution of
> vertebrate powered flight, and may differ fundamentally from the origin of
> bat and pterosaur flight, whose antecedents have been postulated to have
> been arboreal and gliding.
>
>
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