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Modern Bipeds [was re: Biped advantage]

At 01:41 PM 11/14/96 -0500, David Peters wrote:
>[Kangaroos] are also former tree dwellers,
        This brings up one of George Olshevsky's points from an earlier
thread which I've been meaning to get back to.  Much is made by some of the
"fact" that bipedality in modern tetrapods seems to be correlated with
arborealality.  It really looks to me like this is not necessarily correlative.

        Modern _motile_ bipeds:
        (I have no delusions that this is comprehensive)
        1) Running lizards:  Facultative bipedality in cursorial locomotion,
seems to be an extreme case.  Are these running lizards arboreal?  Not that
it would seem to matter.  I'm sure that if they wandered around bipedally in
trees, George would have told us by now.  :)
        2) Apes (excluding humans):  Knuckle walking and occasional bipedal
movement seems related to arboreal adaptations (long arms, adaption of manus
for grasping).
        3) Humans:  We do not seem to be arboreal, although we can and do
climb trees.  We decended from arboreal ancestors, yet our closest living
relatives, some of which are not arboreal (gorillas), are not bipeds.
        4) Birds:  Are decendants of bipedal ancestors, and, unless you
choose to a priori assume George's "BCF" theory is correct, were bipedal
before developing flight.  This does not, however enlighten us as to whether
bipedalism evolved from an arboreal condition.  Shucks.
        5) Kangaroos, Wallabees, et al.  I assume that there is evidence
(given how often people bring it up) that these forms are decendants of
arboreal animals.  There are tree kangaroos, so at least here we seem to
have an arboreal biped.  To my untrained eye, however, all of the kangaroo's
locomotor adaptions seem to at least superficially parallel those of the
rabbit (excepting, of course, the long heavy tail), especially in the
disproportion of fore- and hind- limbs.  Both of these animals exhibit a
broadly similar locomotory style, and I cannot help but draw the tenative
conclusion that bipedality in kangaroo-type critters is realted to a a
leaping locomotor funciton rather than to arborality.

        It seems to me that yet again the modern analogs can be misleading.
We need to look at why the animal is bipedal, not just *that* it is bipedal.
I'd appreciate some insight from those of you out there who work on locomotion.

| Jonathan R. Wagner                    "You can clade if you want to,     |
| Department of Geosciences              You can leave your friends behind |
| Texas Tech University                  Because your friends don't clade  |
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|       *** wagner@ttu.edu ***           Then they're no friends of mine." |
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