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Fwd: Re: a little background



Thanks for this message, I appreciate people who try to stimulate dialogue in a mature manner rather than resort to smokescreens, straw men, or high schoolish nyah-nyah-nyahs. I try to respond to every e-mail I get, whether it's from a creationist or a ranter, but the ones that are the most fun to respond to are from people who calmly discuss methods, evidence, and questions.  I'll take my time with this one, then.

Date: Fri, 1 Mar 2002 15:20:50 GMT
From: Mike Taylor <mike@tecc.co.uk>
To: jrhutch@Stanford.EDU
CC: dinosaur@usc.edu
Subject: Re: a little background

>          Anyway, I always found Greg and others' arguments about
> fast tyrannosaurs very interesting, and at times convincing.  Greg
> has made the best (by far) arguments for fast-running tyrannosaurs,
> ever.

... and we need to find a way to make them true again :-)
[Beauty is truth and truth beauty, and running Tyrannosaurs are
beautiful, right?]


I get the feeling that this is the sentiment among many, many people who e-mail me.  It's like I've just called their best friend fat, dumb, stupid, and ugly or something.  An interesting sociological question why people take this so seriously!  But then, I do take my own research too seriously sometimes, so I can understand where they're coming from.


What intrigues me most is the idea of applying your methodology to
more extant species, so we can get a better idea of how well it
applies across the board.  Useful as the chicken and 'gator analyses
are, the straight line through two points seem to be a perfect fit :-)
(Just as I am really looking forward to Parrish & Stevens' new
dinomorph work which, as I understand it, aims to validate their
models of sauropod cervical anatomy by applying the same methods to
the necks of camels, giraffes etc.)

Yes.  We are working on many more species.  I did tinamous, iguanas, Compsognathus, Archaeopteryx, and a few others early on (will appear in another paper) and a quickie ostrich model too.  I'm building a better ostrich model; the key constraint on making models is that I need a freshly dead and previously athletic animal to dissect.  Similar plans for rhinos, elephants, etc. as soon as I get the right specimens, which takes time.  The model has a lot of anatomical details in it, especially for living animals which we can measure exactly, so it requires good specimens.


Specifically, I would be fascinated to see how ostriches come out.
(I've not read all of
http://tam.cornell.edu/students/garcia/.trex_www/naturepaper.html
yet, but a swift grep for "ostrich" reveals nothing.)  >From my totally
uninformed perspective, based on little more than looking at ostriches
in zoos, it seems to me that they have amazingly little in the way of
leg muscle, at least in the distal 3/4 of the leg -- yet they are
among the fastest of all terrestial animals.  Have you, by the
slightest chance, informally put an ostrich in your machine and
cranked the handle?

An early run worked out fine; had enough muscle to run.  Ostriches are ~10% body mass of leg muscle per leg on average, although ones bred for slaughter are bulkier and slower, with proportionately more leg muscle (and health problems!).


One more comment on ostriches, provoked in part by Tom's comment:

> Date: Fri, 1 Mar 2002 08:52:24 -0500
> From: "Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>
>
> One of the basic points coming out of various lines of research is
> that the mechanical world that 3 or 4 or 6 or more tonne animals
> live in is very different from the world that 100 kg or 500 kg or
> maybe even 1 tonne animals live in.

[Disclaimer: I am not a Real Scientist.  I am a computer scientist,
which is really a branch of art.  So I might be talking complete
nonsense.  Sorry if I am.]

Anyway -- just as the dynamics of insect flight are fundamentally
different from those of bird flight (dominated more by friction drag
than form drag), so I found myself wondering whether large theropods
might run in a fundamentally different way from BSBSs (Boring Small
Bipeds :-) like chickens.

I would agree, in a way.  It is an interesting question that we need to look at in more detail.


BSBs run by pushing themselves forward with each step; but one can
imagine a larger animal, with more momentum (and proportionally less
drag effects to worry about) running by essentially falling forwards
all the time, and controlling that fall by taking steps that do not
need to propel the body, but merely get the leg forwards quickly
enough to stop the fall becoming terminal.  Then the primary force
that the leg needs to absorb is the slight downward movement of the
body mass at the end of each step, pushing upwards to turn that into
the slight upward movement necessary for the ILB to make it over the
crown of the next step.  Purely intuitively, it seems that this would
require substantially less muscle mass than the BRB running style.
Yes?  No?

A key distinction between walking and running (in fact, the current biomechanical definition) is that a walking animal works much like you describe above.  Picture an inverted pendulum with a mass on top, swinging back and forth like a leg.  Potential energy and kinetic energy are out of phase (PE is high at midstance when the mass is high off the ground; KE increases as that mass falls down).  That's walking, biomechanically defined.  It is a stiff-legged, pendular swinging gait.  Walking is efficient (you recover maybe 70% of your KE from PE) because it is a more passive form of gait, as you suggest above.

Running is currently defined as a spring-mass-like gait.  Like a pogo stick or bouncing ball.  Instead of the leg behaving like a stiff pendulum, it compresses during stance and is shortest (not longest) at midstance.  KE and PE are in phase, so energy exchange between them is not possible.  However, elastic strain energy is stored in the tissues of the limbs (esp. tendons and other collagenous tissues) and returned at the end of stance, like a spring bouncing back.  This mechanism keeps running fairly efficient.

In the biomechanical definition, running does not require an aerial (suspended) phase where the feet are off the ground.  That's the classical definition.  More often than not, the two definitions coincide,  but there are animals such as crabs and other many-legged animals, birds running at medium speeds, and humans running with a more crouched limb orientation (all called "Groucho running") that are similar.  Humans who train in competition racewalking use a fast gait that is biomechanically a run at fast speeds; a true walk at slower speeds.

I've speculated before that large theropods might have used a "groucho run" in order to move quickly while keeping their feet on the ground.  Groucho running involves lower peak forces on average than running, but higher muscle forces (and ~50% more metabolic energy) than regular running, and it is more limited in top speed (~5m/s in humans).  I don't have any evidence to support that large theropods groucho-ran but it's fun to think about.


Then it occurred to me that a while back when I saw film of an ostrich
running, it seemed to be doing something very similar to that: at
least, it _appeared_ (though all those fluffy feathers might by hiding
the truth) that its centre of mass was _well_ forward of its feet as
it ran.  So that seems an extra reason to be interested in how well
ostriches fit your body-mass/leg-muscle-mass graph.

Ostrich biomechanics have been studied and like other animals their center of mass is above the foot at mid-stance.  They are not terribly unusual in their whole-body mechanics.  At slower speeds, I assume they groucho run like other birds.


A final though.  If large theropods did run in a falling-forward
style, that has implications for manoeuverability.  There wouldn't
have been any :-) Seriously, an animal running in that way would find
it much harder to turn quickly, I imagine -- and might have had
trouble stopping, except perhaps by crashing into a prey animal.  Not
sure what the ecological consequences of this would be, but it does
suggest that a quadruped running from a _rex_ might do well to turn
suddenly (well, more suddenly than the _rex_ can) and charge it.

Perhaps.  Segue to Jim Farlow's work, or David Carrier's "dinosuit".


OK, my rambling ends.  If this is all a load of uninformed nonsense
then sorry for wasting your time; but if some of it is uninformed
non-nonsense, then great!

Thanks again not only for writing the paper, but for discussing it
out here where we laymen lurk, along with the Real Dino Guys.


Truly a pleasure.
Sincerely,
John


 _/|_   _______________________________________________________________
/o ) \/  Mike Taylor   <mike@miketaylor.org.uk>   www.miketaylor.org.uk
)_v__/\  "I have always prided myself on being in a field that has
        no practical application" -- Palaeontologist James Farlow.

===========================================
John R Hutchinson
NSF Postdoctoral Research Fellow
Stanford University
Durand 209, BME
Stanford, CA 94305-4038
(650) 736-0804 lab
(415) 871-6437 cell
(650) 725-1587 fax
===========================================