al@gtx.com (Alan Filipski) (05/03/89)
In article <100780003@hpcvlx.HP.COM> everett@hpcvlx.HP.COM (Everett Kaser) writes:
->The concept of six legged animals (especially mammal-like animals) has a long
->tradition in fantasy and science fiction, and there are many examples of
->creatures in our world with more than four appendages. Most of them are
->insects, spiders, and crustaceans. After reading this book, I got to thinking
->about WHY there are no large six-"legged" animals on earth. I came up with
->two possible reasons: 1) there was one "creature", or a small group of
->"creatures" who were the ancestors of ALL large reptilian/mamallian animals
->on earth today, and THEY just happened to be of the four-legged variety, or
Maybe pectoral/pelvic fins => 4 legs, whereas fundamental insect design
just doesn't scale up well for other reasons (like respiration and
having an exoskeleton).
->2) six legs/arms take 1/2 again as much energy to operate than four legs, and
->as such would be a significant survival hindrance for larger animals. Reason
->number 2 seems much more likely to me, and as such would seem to limit the
->likelihood of large six-legged "creatures" ever evolving on any world (with a
->few exceptions, granted).
One possible factor is that the more highly developed nervous systems of
vertebrates help compensate for the loss of stability in going from 6
to 4 (or even 2) feet. 6-legged creatures are almost always stable
(i.e. freeze them at some random point in mid-step and they don't fall
down). 4-leggers are unstable at many points of their gaits and require
much more active and rapid feedback control to keep going. We bipeds
are ridiculously unstable and require great nervous apparatus to keep
from falling down. Maybe having fewer legs requires more reliance on
higher motor centers rather than simple pattern generators.
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( Alan Filipski, GTX Corp, 8836 N. 23rd Avenue, Phoenix, Arizona 85021, USA )
( {allegra,decvax,hplabs,amdahl,nsc}!sun!sunburn!gtx!al (602)870-1696 )
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