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Re: Avian flight stroke origin



Jason Brougham <jaseb@amnh.org> wrote:

> My apologies for being unclear. I did not mean that Archaeopteryx may have
> stood in a tree like Penelope in that it would grip the tree with its
> halluces. I meant that it would stand upright, without aid of hand claws.


Yes, _Archaeopteryx_ may have stood in trees.  My argument is that
there is nothing in the anatomy that says it did so - at least not
habitually.  _Archaeopteryx_ might indeed have had an ecology similar
to a turkey (aside from the flight capabilities).  But turkeys are
fundamentally terrestrial birds, even though they occasionally venture
into trees.


> However, on the point of halluces I still do believe that animals without
> halluces, and galliform birds with poor perching ability, still do quite
> well in trees and roost in them routinely.
>
> In the wild turkey Meleagris, climbing in trees can be accomplished
> without use of a gripping hallux. In the image below we see a wild turkey
> climbing a narrow branch to reach fruit, and both halluces are free and
> not gripping anything:
>
> http://www.flickr.com/photos/jrnikon/3240582435/sizes/l/in/photostream/


Sure, turkeys do okay in trees... But turkeys don't spend much time in
trees, whereas other gallinaceous birds do, such as the cracids.  In
phasianids, the proportions of the first digit vary according to how
terrestrial/arboreal the bird is.  Turkeys (meleagridids) feed and
nest on the ground, and the first digit is quite elevated, due largely
to the shortened metatarsal I.  Nevertheless, the hallux of turkeys is
still reversed, and capable of some opposability.


An animal that spends much of its time in trees will acquire or retain
arboreal adaptations.  In the case of terrestrial gallinaceous birds
like turkeys, the hallux appears to have been secondarily elevated.


Certain birds can still sit or roost in trees even without a perching
foot.  Cormorants can, using their totipalmate feet - the anterior
digits wrap over the branch, the same way they do a rock.  But
cormorants are not in any sense arboreal.  Certain tinamous can roost
in trees - but they don't actually perch, but sit on their tarsi using
highly modified scales on the plantar tarsal surface.  Again, tinamous
are not in any sense arboreal.


> Goats, as we know, also climb trees without benefit of a hallux:
>
> http://www.odditycentral.com/videos/the-tree-goats-of-morocco.html


Ah, I wondered when we'd get to "But goats can climb trees!"  :-)


What we have here is the distinction between "capable of" and "adapted
for".  At any given time on planet Earth, the vast majority of goats
are not climbing trees, and have no desire to.  Just because an animal
*can* climb a tree under certain circumstances does not make it
arboreal.  Goats are *not* adapted for an arboreal lifestyle.





Cheers

Tim