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RE: Scaling problems in Hutchinson 2004



Hi, not much time to reply but here's a somewhat brief one (for me):

--I didn't say that small (unless by small one means medium-large, maybe
over 100 kg-ish) animals are absolutely faster than big ones; I suppose this
is a misinterpretation of what I said/meant. I did say (p.433) "There is no
evidence that larger birds such as ratites have reduced locomotor
performance," meaning that there is no evidence that they run absolutely
slower (yes, duh). This led into a partial explanation of how they may do
this: by having larger leg extensor (i.e., antigravity) muscles than smaller
birds. Domestic chickens have 8.8% body mass/leg as extensors (certainly
from artificial selection; jungle fowl tend to be skinnier); turkeys
5.5%/leg (much more typical of galliforms), the tinamou Eudromia 4.2%/leg;
these latter two are typical values for smaller ground birds. I didn't
bother publishing other supportive data on that as I didn't bother modeling
other running birds, knowing well that I'd obtain similar results for them.
Emus and ostriches have 2-3x as much at 14-15%/leg.  That's more than the
other running birds, and Greg notes some of the likely reasons; my goal was
to add real data on what ratites are like and use that to start to
understand why they are so fast.
        I read Hartman's (1961) work but don't fully trust it and hence used
it little, as I think he included tendon, aponeurosis, and other
non-muscular tissue masses in his "muscle" masses, which would explain why
his values tend to run a little high. Unfortunately his methods aren't too
clear on that.  Indeed, what I said about ratites is that (based on the data
so far) they seem to have higher joint "safety factors" than smaller birds,
although I suspect some of this apparent difference is artefactual (e.g.,
there are limited mid-stance data on limb joint angles for ratites and
tinamous, we need better center of mass data for ratites), althought this is
a minor issue. We are testing this now with new experimental data, as we
have 15 ostriches living here for a grad student's great project on their
locomotor mechanics.

--I didn't discuss power vs. mass^2/3 scaling, rather emphasizing muscle
force vs. mass^2/3 scaling, as experimental evidence to date suggests that
power does not necessarily limit speed -- e.g., when animals have mass added
to them, they do not always always slow down proportionately as would be
expected if power limited speed, and similar inconsistent findings have been
obtained by running animals up/down hill; speed often doesn't change as
power limitations would lead one to expect, but does change to fit a muscle
(or limb) force limitation.  This is all in the literature; my 2004 work
cites some of it.  The Fedak and Seeherman paper is about metabolic power,
not mechanical power; the two factors are closely related but not in a 1-1
fashion typically. Hence I did not cite that as it is only tangentially
relevant.  What mechanical factor limits speed (the most; many factors
certainly influence speed) remains unclear; limb or muscle force or
stiffness may be the answer, or who knows maybe power if past experiments
are wrong.  Certainly more experiments (and modeling) are needed.  As I've
stated before in past exchanges on the list, at present, biomechanists
(myself included) who work on this topic experimentally and/or theoretically
tend to agree that mechanical power is probably not the chief limitation on
max speed, that metabolic power almost certainly is not (although it limits
mean speed in the long-term, for endurance, naturally), and the money is
mainly on muscle force output (or total limb force).

--I trust Robert's work on biped vs quadruped leg muscles despite some minor
flaws. The basic point is that bird (esp. ankle) muscle fascicles are long
and hence the muscles are big; read the papers for details, folks.  I don't
think that quadrupeds necessarily should have smaller leg muscles than
bipeds, but so far some seem to.

--The main points of the 2004 studies were to broaden the data
set/validation for bipeds and to refocus the question of fast running on the
"safety factor" of individual joints, as no leg is stronger than its weakest
joint. This emphasis seems to be generally missed.

--Naturally, we've cut up quite a few thoroughbred horses (I work at a
facilty emphasizing horse biomechanics; with Alan Wilson et al.) and weighed
all the muscles , and ditto for some elephants (and now I am doing rhinos).
All of it will be published in due time, as it takes time to do this real,
hands-on work. Whether some of my ideas are right or wrong, at least I'm
contributing real data from living animals to the debate. I do not see Greg
dissecting animals and hence contributing real data, but I welcome it from
anyone, because at least we can agree that more data are needed.

============================================
John R. Hutchinson
Structure and Motion Laboratory
The Royal Veterinary College
University of London
Hawkshead Lane, North Mymms
Herts AL9 7TA, United Kingdom
phone  (+44) (0)1707-666-313
fax    (+44) (0)1707-666-371 or 652-090
mobile (+44) (0)7843-629-162
web    http://rvc.ac.uk/sml
============================================