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



On 11/10/2011 12:42 AM, Tim Williams wrote:

The hallux could easily have become better
adapted for grasping branches, without interfering with a
ground-foraging lifestyle.

Special pleading, and you ignore the arithmetic.

There is no evidence it can be done starting w/ a generic theropod pes w/out first passing through a perching-only phase, and absolutely zero support for an enhanced survival value accruing to claw-climbing animals that roost in trees, but forage exclusively on the ground.

Unless we continue to pretend that all trees have only small branches and small animals w/ claws are insecure in trees, and fall over when they sleep, like humans do. And that giant dinosaurs thoroughly comb the tree-tops at night, searching for snacks that apparently too deaf to hear them coming and flee...

But even under such unreasonable assumptions -- of the total variance possible in pes design per replication, there is a limited pool of fortuitous design variants, relative to "perching" -- of that pool, there is only a subset of variants that do not in any way interfere w/ terrestrial locomotion.

The increase in survival probability would need to be very high indeed for such unlikely changes to occur and be conserved in the face of the thousands of high-stress steps taken daily while foraging by not only the animals considered, but their ancestors as well.

Given that you cannot demonstrate such an increase in survival, even hypothetically, we are more or less engaging in mysticism to consider lack of change in the hallux as evidence roosting did not occur in pre-birds.

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.

Bad analogy.

As previously stated -- presumably turkeys (American-style) are descended from perching ancestors.

As previously stated -- adapting a perching foot to terrestrial foraging in an animal w/ full flight powers does not parallel the process of creating essentially the same design in an animal that begins w/ a generic theropod pes and has limited (or no) active flight talents.

Previously mplied -- the turkey pes is moving toward the terrestrial condition, not away.

Even so -- although turkeys run well for birds, they would be quickly eaten into oblivion should their wings be clipped, or even be similar to Archie + immediate ancestors.

So explain how you know that there are no trade-offs between terrestrial-only and perching + terrestrial?

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.

Sigh. They are modern birds. They ALL descend from perching ancestors. And are very tasty. And make a bad analogy...

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.

Sure it is. Claws, Tim, they had claws...

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.

I am sorry, but the above is quite ridiculous. Not likely even a long and unimpeded fall would kill or even injure an Archie-type. And how likely is a fall, unlikely in itself, to be unimpeded?

Even as an old man on crutches, I have no problems staying in an appropriately sized/shaped tree -- the problem is getting out. Be nice to have glider wings.

When young, 'no hands, mom' was just plain fun -- when an adult, claw-climbing was sometimes a daily activity and falling was a risk that never manifested itself, even when in trees deliberately chosen for difficulty.

To posit that the vastly superior Archie-type body would be insecure in a tree is beyond drama...

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

The mechanics (vernacular sense) of foraging in trees logically promote perching (i.e., the ability to grasp small branches). Especially flight-foraging, as landing on a small branch is tricky.

Roosting, not so much -- if small branches are a problem, you just avoid them...

You stated (iirc) that flight precedes perching in the record. That is not surprising.