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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
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
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
1825 Shady Creek Court
Mississauga, Ontario, Canada L5L 3W2 mailto:email@example.com