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summer science quiz #2

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  • summer science quiz #2

    Dear List-members,

    I welcome the introduction of these summer science quiz questions. They are
    definitely encouraging a fruitful discussion.

    I will state up front that I fall into the "dumb luck" camp on this
    discussion.

    In this discussion we need to be careful to distinguish between what
    architectures of animals actually exist and what architectures are possible.
    There is a big difference between the two and by no means are all the
    possible modes of locomotion represented in the fauna of this planet.

    Therefore making the conclusion that bi-laterally symmetrical animals are
    faster than assymetric and radially symmetric animals because that is what
    we see in the animal kingdom is valid (with perhaps the exception of the
    squid).

    But to draw the further conclusion that therefore bi-laterally symmetric
    architecture always yields faster locomotion does not follow. One can
    imagine a rabbit-like animal which possesses a single front limb instead of
    two (a three legged rabbit!). This front limb may be placed along the
    midline of the animal and the locomotion would be much the same as the
    common four-legged rabbit. With the same maneouvrability.

    One could also image an scorpion-like animal with 3 legs on one side and 2
    on the other. The timing of the legs would be somewhat different perhaps,
    but no biomechanical restrictions would stop this animal from being as fast
    or maneouvrable as its 6 legged cousin.

    These thought experiement animals aren't even particularly different from
    the existing fauna. And I would suggest that there are plenty of other
    odd-legged schemes that the bioengineer could dream up that would hold their
    own just fine in a foot race with even-legged competitors.

    Therefore, as unsatisfying as it is to an engineer, it is probably just dumb
    luck that the even-legged animals on this planet are the fastest. And also
    dumb luck that we even-leggeders are in the oppressive majority.

    To paraphrase George Orwell: "Even legs good, odd legs bad".

    Tom Jenkyn

    -------------------------------------------------------------
    Thomas Jenkyn, PhD (Bioengineering)
    Post-doctoral Research Fellow
    Orthopaedic Biomechanics Lab
    Mayo Clinic/ Mayo Foundation
    200 First St. SW
    Rochester, MN 55905

    Tel: 507-284-2262
    Fax: 507-266-2227
    Email:
    -------------------------------------------------------------


    Some comments on the origins and utility of bi-lateral symmetry
    (i.e., even numbers of limbs) in animals.

    >My solution: Bilateral symmetry in the animal body plan produces the even
    >number of limbs, but those animals that develop with radial symmetry (like
    >the starfish) can have odd numbers of limbs. Why does bilateral symmetry
    >predominate in the animal kingdom would perhaps be a better question. My
    >answer to that question is: pure dumb luck.

    Dumb luck is possible, but before we accept this non-functional
    explanation, we should consider whether bi-lateral symmetry might
    have a functional role that lead to its being selected for (rather
    than merely tolerated) in the evolutionary history of animals.

    Bi-lateral symmetry is locomotor appendages is not universal, cf.
    star fish, and snails (the latter have only one foot, an odd number).
    But it is very common (including fish, which don't have legs, but do
    have bi-laterally symmetric fins, and even have bi-laterally
    symmetric muscles attached to their otherwise unitary tails). One
    thing that distinguishes bi-laterally symmetric animals from
    asymmetric ones is that the former move faster (relative to the
    viscosity of their media). Bi-lateral asymmetry in quick-moving
    animals is useful simply because it makes for more efficient
    locomotion when speed matters. Star fish are in no hurry, but if
    they had evolved in the direction of faster locomotion, their
    asymmetric bodies would have imposed rather severe constraints on how
    the increase in speed could be accomplished (i.e., consider what odd
    interlimb coordination patterns would be required for a starfish to
    "run"). Bi-laterally symmetric animals, such as fish or humans, have
    a great advantage when it comes to organizing and achieving
    locomotion.

    I don't think the "choice" was dumb luck.

    *****************************************
    Thomas A. Stoffregen, Associate Professor
    Department of Psychology
    University of Cincinnati
    P. O. Box 210376
    Cincinnati, OH 45221-0376
    USA

    Office: 513-556-5535; Lab: 513-556-5682; Fax: 513-556-1904

    URL: http://ucaswww.mcm.uc.edu/psychology/Faculty/Stoffreg.htm
    Lab: http://homepages.uc.edu/~stoffrta/psl/index.html

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