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Re: Perching, climbing, roosting was Re: 11th specimen of Archaeopteryx



Don Ohmes <d_ohmes@yahoo.com> wrote:

> From an evolutionary perspective, the idea that merely roosting in a tree
> would counteract a terrestrial ground-foraging lifestyle and alter
> foot-design in a clawed animal that can already climb is the special
> pleading.


It wouldn't "counteract".  The hallux could easily have become better
adapted for grasping branches, without interfering with a
ground-foraging lifestyle.  Modern ground-foraging birds show us how
it can be done (turkeys, peafowl, secretary bird, etc etc). There is
no reason to assume that ground-foraging and roosting impose competing
selective pressures on the foot.  Turkeys etc do both quite well.


As I see it, you're essentially saying "roosting behavior doesn't
require any specific adaptations, so we would not expect to see any
special adaptations in roosting theropods."  The difficulty with this
line of argument is that extant ground-foraging birds that do roost
(when asleep) typically use a reversed hallux that is suitable for
perching.  I suspect that confuciusornithids could (and did) roost,
because the hallux seems low enough and reversed enough to effectively
oppose the other three toes (especially in _Changchengornis_).  But in
more basal forms, I don't see much evidence at all for spending long
periods of time in the crown of trees - active or otherwise.


> Ditto the claim that re-arranging foot design to satisfy somewhat
> anthropomorphic assumptions about roosting stresses is a trivial matter,
> design-wise...


I don't see how these assumptions are anthropomorphic.  Some
"re-designing" of the foot was achieved outside the bird clade - look
at the foot of therizinosaurids or _Balaur_ to see how the hallux can
be put to good use.


> Is the flamingo's foot shaped while it sleeps, or while it is foraging?


I'm afraid I don't understand the relevance of the flamingo.  AFAIK,
it doesn't perch or roost in trees.  It spends its days (and nights)
on terra firma.


> The foot is already optimized to standing around.
>
> "There is no direct evidence of roosting in (...)" is a correct statement.
> Claiming that simple roosting would necessarily be evident in the bones once
> the physical capacity to do so is established, is NOT supportable.


Again, roosting is not as "simple" as you make it sound.  It is not
simply standing/crouching/huddling/squatting on a bough or branch as
you would on the ground.  Even a small shift in position can be fatal,
so it helps to be adequately secured to the substrate (bough, branch,
whatever).  Both gravity and an uneven substrate are conspiring
against you.


> They were small animals and had clawed feet, not little roller bearings.
> Considering also their clawed hands and wings, the claim that they needed
> "securing to the substrate" appears to be special pleading as well.


And yet... while roosting, most modern birds that roost secure
themselves to the bough/branch using an opposable pes.  Such as your
good friend _Meleagris gallopavo_, the turkey.


> Foraging is out, because their feet are not adapted to foraging in trees.
> Daily foraging in trees will indeed quickly change the extremities, being a
> locomotor activity -- keyword = "activity"...


Briefly, climbing up trunks and foraging within the crown promotes
different adaptations.


> You have alluded to "other reasons" twice now, iirc. I am listening.


Sorry, I didn't mean to be cryptic.  I don't have an answer.  All I
was alluding to was that if proto-birds required elevation as a
prerequisite (or corequisite) to becoming airborne, then it may not be
necessary to put them in trees.  We may never know the precise
ecological circumstances that engendered aerial behavior (= incipient
flight).  I suspect an arboreal context is involved, because of the
changes going on in the feet of paravians... but maybe it's just my
theropod foot fetish that makes me think this way.






Cheers

Tim