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On 4/23/2011 3:30 PM, Vivian Allen wrote:
On 23 Apr 2011, at 19:47, Don Ohmes<email@example.com> wrote:
Perhaps it might seem overly broad to use the term "locomotion" in
regard to very short limbs which are used to assist the snout in
entering and retreating from nooks and crannies that are just large
enough the head (or even slightly too small in the case of an
'expandable' substrate), but it is nonetheless technically correct.
I think most people in biomechanics would agree that for something to
be considered locomotion you have to move the centre of mass a
reasonable distance. Otherwise opening a door would count as
locomotion. Although your definition makes your earlier statement a
bit easier to understand. I would go out a limb and say that I am
99.9% certain that an animal of the relative limb proportions of these
things was not adapted to use it's pectoral limb for terrestrial
locomotion (in the sense of actually effecting translation of the
whole body centre of mass). They are far, far too short.
Thanks for the response. I appreciate it.
In the case I specify, which can be viewed as "an animal pushing it's
head, neck and/or shoulders into a burrow, hollow log or perhaps a
crevice in the interior of a large carcass while in search/pursuit of
prey" -- translation of center-mass over a span of even a centimeter or
two per "step" can be an effective (read "reasonable") distance.
This is especially true when extricating the head and neck from a tight
spot in the case where the hindlimbs have little or no traction, and
(logically) often with bulk-enhancing prey in jaws.
Modern animals use their hind limbs to vigorously push their heads into
tight places, but their forelimbs and shoulders seem to do much of the
work in (successful) extrications, and failure can be critical to
continued survival (personal obs.*).
It follows that evolutionary significance accrues to any increase in the
ability to push into, pull away from, and/or expand the diameter of
whatever crevice, hole or cranny that hunger has induced an animal to
poke it's head into.
Granted, the examples I have personally observed are mostly quadrupedal
mammals, but the same evolutionary principles apply, and the mechanics
relative to bipeds seem do-able to me.
*Example observations -- dog-in-an-armadillo-burrow,
bear-in-a-garbage-can, possum-in-a-mayonnaise-jar... :)