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Re: Adaptive advantage (was Re: ABSRD BAND on Sinornithosaurus feathers)



At 09:14 AM 16/03/01 +0000, Scott Hartman wrote:
Theropods are the anti-scansors. Their limb proportions and acetabular articulation precludes this type of lifestyle. Archeopteryx takes these problems to truly amazing proportions, and was ill equipped for leaping about in trees.

I have been pointing out for some time now on this list that this simply is not true. There are living birds that are excellent climbers without having to resort to flight to help them do it - even if you leave aside specialized trunk-foragers like woodpeckers.


Citing birds (with grossly similar hindlimb morphology) as examples of facultative scansors is misleading. Birds probably could not function as scansors without the safety net of flight being present.
Notably, even though there have been small flightless birds, none of them were arboreal scansors.

This is not true either. The Kakapo is a flightless bird that is quite capable of climbing, using its beak as an assist. Furthermore, the only really small flightless birds in historic times have been rails and the Stephen Island Rock Wren, and of these most became extinct before anyone could study their behaviour and/or lived on treeless or near-treeless islands (eg the only living flightless bird to be really small, the Inaccessible Island Rail). Further, larger rails are quite capable of getting into some trees without flying, by walking up sloping limbs. The problem is that the range of behaviours of flightless birds from many families remains unknown to us because we wiped most of them out before we could learn about them.


And, as I have said before, some birds are adept at leaping from branch to branch with closed wings. The Kokako of New Zealand, a very poor flyer, tends to do this to gain altitude and glides downward to the next tree before beginning again. Birds of Paradise in New Guinea regularly climb trees in this way - I have watched them do this personally.

I see no reason why a long-legged theropod could not have done the same thing, if perhaps less efficiently - and remember that even though they lacked wings, they could use arms and mouth to grip limbs if necessary.

Which reminds me, of course, that hoatzin chicks are flightless arboreal scansors too.

I agree that insect predation seems an unlikely driving force for the evolution of flight, which is why I hypothesiszed that in fact predation of vertebrates resulting in a short-duration ballistic phase was a more likely pursuit (pun intended) for the antecessors of birds.

I have repeatedly suggested that an UPWARD leap to snatch prey from leaves or thin branches (eg insects, frogs, lizards could have been in the repertoire of early proto-flyers, and birds certainly do this today.


Energetic demands dramatically favore this interpretation of the insect version, since success with larger prey items far overshadows the cost of the behavior. Additionally, catastrophic failures (lack of stability resulting in being trampled by your prey) are far more likely when hunting vertebrates, providing a stronger selective impetus than hunting insects.

Falling out of a tree while trying to snatch a prey item in mid-leap can be pretty catastrophic too. Getting back to your original perch after an upward leap is a pretty important part of the process.


--
Ronald I. Orenstein Phone: (905) 820-7886
International Wildlife Coalition Fax/Modem: (905) 569-0116
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