Dear all Quiz-lovers,

Thanks for the overwhelming response to Quiz #2. I just hope the
BIOMCH-L moderators (thanks especially to Krystyna!) will accept our
gratitude and continue to tolerate us.

It being Friday, I feel obliged to summarize (endevouring wherever
possible to classify and condense), knowing that I am bound to offend
either by omission or false attribution. Nevertheless here goes with the
suggested replies to the question posed 2350 years ago by Aristotle
(don't blame me!) as to why animals have an even number of feet...

Many animals (starfish) in fact have and odd number of feet (Greiner,
Sotelo, de Lussanet, St. Laurent, Pell) - good point and surely
Aristotle must have known this?
Kangaroos, T. Rex (Hoff, Lloyd, Baker, Pell) - well, the tail is not
really a foot, but fair enough...
Evolutionary inheritance/bilateral is no better than radial
symmetry/Cell division/dumb luck (Ferris, Cramp, Min Te Choo, Jenkyn) -
meaning that symmetry happened early and we just got stuck with it...
not very satisfying, methinks, but might be correct...
Injured crabs (Pell) - this is interesting, because Dr. Pell points
out that nobody has reported the results of removing only one leg (I am
very close to Baltimore... more fava beans, anyone?... I'm not sure that
I could stop at one)
Wheels/Three & five-legged (Manx) rabbits - the Reliant Robins
(apologies to non-Brits) of the animal kingdom perhaps? (Voo, Hu, St.
Laurent, Jenkyn, Smith):
Gazelle 'stotting' on five 'limbs' (Pell) - this surely is a curious
behavior: "How the Cheetah Lost Its Stotts," New Scientist (1986)
Redundancy in the face of injury or use of one limb for
scratching/Two-legged dogs better than three-legged (Cooney, Roe, Riley)
- I confess find this explanation plausible
Sphinx (Helenberger, Legreneur, Ceriani) and what I will call
"middle-leg syndrome" (various mostly unsavoury suggestions) - Oedipus'
response to the Theban riddle ("What is it that walks upon four legs,
two legs and three legs, and is weakest when it has most?"): "The man
because he crawls on all fours as an infant, stands firmly on his two
feet in his youth, and leans upon a staff in his old age."
is perhaps the
most famous quiz in all history, but, if Levi-Strauss is to be believed,
speaks more to the problem of incest than locomotion:

.. and finally:

Simplified neuromuscular control/Coupled oscillators/Change in
direction/GRForces equal in two legs/Office chairs(!)/Lateral
stability/Direction of Travel/Left-right/Best foot forward (Baker,
Riley, Baker, Burkholder, Smith, Cardinal, Lavrov)
Efficiency/angular momentum/speed (Stoffregen, Ariel)

A quiz is surely only a quiz if there is a solution, so in the good
tradition of 'The Weakest Link', I am going to say a Robinsonian
"Goodbye" to all but those last two suggestions, which basically agree
with Aristotle's assessment, in which he implicitly addresses the
problem of the kangaroo and Tyranosaurus/Oedipus Rex:

"But every limbed animal has necessarily an even number of such limbs.
For those which only jump and so move from place to place do not need
limbs for this movement at least, but those which not only jump but also
need to walk, finding that movement not sufficient for their purposes,
evidently either are better able to progress with even limbs or cannot
otherwise progress at all every animal which has limbs must have an even
us for as this kind of movement is effected by part of the body at a
time, and not by the whole at once as in the movement of leaping, some
of the limbs must in turn remain at rest, and others be moved, and the
animal must act in each of these cases with opposite limbs, shifting
the weight from the limbs that are being moved to those at rest. And so
nothing can walk on three limbs or on one; in the latter case it has no
support at all on which to rest the body's weight, in the former only in
respect of one pair of opposites, and so it must necessarily fall in
endeavouring so to move."

Once more, as your humble servant, I await the flames, but please be
quick, because I will have another for you on Monday...

Dr. Chris Kirtley MD PhD
Associate Professor
HomeCare Technologies for the 21st Century (Whitaker Foundation)
NIDRR Rehabilitation Engineering Research Center on TeleRehabilitation
Dept. of Biomedical Engineering, Pangborn 105B
Catholic University of America
620 Michigan Ave NE
Washington, DC 20064
Tel. 202-319-6247, fax 202-319-4287

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