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Re: Speed in giants and cursors



Quoting GSP1954@aol.com:

> In his recent comments John H. made some statements that warrent a 
brief
>
> reply and comments, some of which are also in reply to other assorted
> comments on the lists and in the media.


Well stated, and I am glad that Greg is participating in this 
discussion, because he has a lot of observant comments to make.  He's 
clearly thought this over carefully, and that's why I respect his 
opinion.


> The ability to achieve a suspended phase run is primitive and normal 
for
>
> amniotes that live on flat land and have well developed legs. Among
> living
> animals it is a rare and derived condition to not be able to run,
> terrestrial
> turtles and elephants being the chief examples. Among extinct amniotes
> unambiguous nonrunners include sauropods, stegosaurs, unitatheres, and
> giant
> slothes.

A good start, although our research on elephants actually draws 
the "not running" dogma into question.  It becomes a matter of 
definition (biomechanical vs. kinematic). Research in the last 40 yrs 
has shown that the biomechanical (spring-mass running vs. inverted 
pendulum walking) definition is more rigorous because it is based on 
real physical mechanisms (exchange of energies) rather than superficial 
kinematics (aerial phase).

Furthermore, what we said in the paper has become a bit confused by 
media and other coverage.  Our models ruled out 45mph speeds on the 
basis of what muscles could generate/support, but because of the many 
unknowns we could not conclusively rule out slow running.  We showed 
that a very crouched posture was a bad strategy for a tyrannosaur in 
any case, not because of size strictly speaking, but because of moment 
arms (mechanical advantage).  10-25mph was a very rough range for what 
our models suggested was perhaps feasible, and higher speeds within 
that range would surely be running, not walking, by any definition.  I 
certainly do not think a tyrannosaur would be walking at 25mph.

So in some ways Greg and I are not so much in disagreement as one might 
think.  We should avoid polarizing the issue into simple "run/not run" 
dichotomies, just as we should avoid excluding relevant lines of 
evidence.  Our model is just one of many relevant lines of evidence, 
from trackways to anatomy and other biomechanical techniques.

I agree with Greg about the extinct unambiguous runners/nonrunners, 
more or less, although there are plenty of unanswered questions for 
those animals.

The key question in our paper focuses on the _ambiguous_ runners, which 
I would include all large (~Tyrannosaurus-sized) dinosaurs in.  I think 
most researchers recognize that the issue is not completely closed, and 
that's why we wrote the paper.  Even if we're wrong, our approach could 
(and I would argue, must!) be used to explain how tyrannosaurs ran 
fast.  It is flexible; that's what makes models useful despite their 
frustrating complexities.


> Contrary to common claims (incl a Harvard researcher on NPR), 
elephants
> do
> not establish that large animals cannot run for two reasons: they 
cannot
> run
> at any size, and they cannot run because they are not designed to. If
> elephants could run when they are the size of horses and lost the
> ability as
> they grew up that would be one thing. But they are as unable to run 
when
>
> young as when grown up.


Agreed, it is more complex than just body mass, and if you read 
our/Andy Biewener's arguments carefully enough this is clear.  Again, I 
am not convinced that elephants do not fit a mechanistic definition of 
running, but would rather not comment more on that right now.  There 
are some interesting scaling patterns that develop as elephants grow, 
and these patterns suggest that relative locomotor performance does 
decrease.

Greg is right that baby elephants do not have an aerial phase.  Hatari 
aside, I have worked hands-on with 48 elephants over the last 4 years 
and have a good understanding what they can/can't do.  But I also am 
very hesitant to use elephants as the single reference point for the 
locomotion of large animals.  They are important, but we should not 
predicate our understanding of size and locomotion on just elephants, 
ankles or other parts therein.


>As I've explained in
> the
> literature many a time, elephants of all sizes cannot run because they
> lack
> the anatomical adaptations to do so, most especially the long feet 
with
>
> flexible ankles needed to push off into a suspended phase.

I'm not at all convinced by this anatomical explanation.  It really has 
not been tested; just asserted by analogy and correlation.  It is a 
functional morphology "just-so-story."  It has not met the current 
standards for examining animal locomotion.  That doesn't mean it's 
wrong, though, but it cannot be assumed to be right.

I think the real answer is much more complex than a flexible/non-
flexible ankle and running/not running dichotomy.  Time will tell, and 
the best way to tell is by working with animals thru experimentation, 
modeling, and other lines of evidence.  The more that lines of evidence 
agree, the better our scientific confidence should be.


>If all
> elephant
> sized animals with flexible ankles could not run then the hypothesis
> that
> giants cannot run would be supported. But all are extinct (your giant
> theropods, ceratopsids, iguanodonts/hadrosaurs, titanotheres,
> indricotheres,
> recently extinct horned rhinos that reached 5 tonnes), so it is not
> possible
> to so directly test the question and resolve it in either direction. I
> believe John said on NPR that elephants don't leap because they are 
too
> big.

I don't think that's exactly what I said.  The real explanation is 
probably far more complex: an issue of muscle moments, bone strength, 
and whole-body dynamics.  I don't think we have a good answer yet why 
elephants do not jump, but I don't think body mass helps (or is 
insignificant) either.

Greg goes on to recount examples of animal speeds, and while some of 
these examples are documented well enough for me to put some stock in 
them, I think (and it seems that he agrees, given his wise cynicism 
about speed reports for some animals) that we still know precious 
little about the limits on speeds in living animals of many sizes, 
anatomies, and phylogenetic relationships.  The literature is full of 
specious anecdotal evidence, and there are very high standards in 
biomechanics today that anecdotes (and many previous studies 
of "cursorial" anatomy) do not measure up to.  Plenty of work for 
people to do, especially new students.

We need to advance the field more by integrating as many lines of 
evidence as we can, from studying living animals to digging up cool 
fossils (including tracks) and interpreting living/extinct animals with 
models, experiments, or whatnot.  While our paper does not claim to be 
the final word, we hope that it in some way moves the field forward a 
bit, as it seems that methods and evidence have been somewhat stagnant 
(with notable exceptions) since Alexander's early work.  Dialogue, 
especially disagreement, is important during this still youthful stage 
of the marriage of evolution and biomechanics.

Sincerely,
John