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Re: Scaling problems in Hutchinson 2004

Michael Habib (habib@jhmi.edu) wrote:

<1) Presumably there _is_ a tradeoff between efficient support and
efficient running; ie. more vertical limbs are poor for running but good
for support stresses (because the strain is more axial in a more vertical
limb, see Matt Carrano's work), whereas bent limbs with gracile distal
elements are good for running but less efficient at supporting force
strains.  This does _not_ mean that a large animal cannot be built for
running, and in fact run very rapidly.  Rather, it suggests that IF a
large animal is not selected for running, then it may be selected for
efficient support, at the cost of running speed.  To put it another way:
perhaps a 40 ton sauropod could be built to run fast, but really, what is
the point?  Might as well reduce strain and put more energy into making,
say, a big belly.  To those on the list with expertise in this area: is
this reasonable?>

  Very much so. What has been missed by some in this discussion but not by
all is that as mass increases, the relative muscle mass to move the body
mass cannot be utilized for moving at the same speed, but the tradeoff
occurs to control the body at slower speeds and support the given mass. In
this manner, the 2/3 scaling rule holds true, even as the larger animals
scale down in speed, because the same relative muscle power as to keep the
body UP, whereas smaller animals do not suffer this problem. Paul mentions
Chihuahuas, but forgets to note that compared between Chihuahuas and
greyhouds, one animal is built for running, the other is a dwarf breed
that suffers from shortened legs, less efficient muscle mass to body mass
ratio, and shorter legs. In part of this, and cited again but not
mentioned by Paul, is that the mechanical attributes for which
small-bodied theropods ~50/500kg use to aid speed because elss effective
as mass increases, so that these same attributes if retained must either
be exapted, or lost, to favor other mechanical aids to its movement and
mass. Thus, the arctomet pes from tyrannosaur ancestors aids in its need
for control of turning, walking, stalking, etc, because of its LARGE size,
not because they are scaled-up ostrich-mimics. Other large bodied
theropods (Paul spends little time looking at the giant carnosaurs)
[>2000kg] use compressed metatarsals, and also show the same scaling
features from the smaller ancestors such as longer femora, shorter tibiae,
robust tibiae and metatarsals, broad pedal phalanges, etc., that are
required for support of mass, not neccessarily for quick transpoort OF it.

<I am not sure the muscle mass comparisons are that informative, even if
they are accurate.  Ratites also do not relate as well to say, a large
Tyrannosaur, as one might expect.>

  Given that Hutchinson studied the scaling between small-bodied and
large-bodied birds, and the relative energetics and expenditures of both
have been studied in depth by others, it is not surprising that the
adapted ostriches are better runners than the even larger moa (robust
indeces again with an hypothesized smaller femoral and pelvic muscle
mass), or smaller birds. Their anatomy was exapted even from basal
ratites, which are more sedentary. They also increased their leg mass
above that of, say, chickens, relative to body mass, and the limb scaling
corresponds. Paul has yet to respond adequately on Hutchinson's math,
which I myself cannot find fault on (though I have little head for it).


Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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