Re: Scaling problems in Hutchinson 2004

[MICHAEL HABIB <habib@jhmi.edu>]

Couple of questions/comments:

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?

2)  Are there any good references regarding scaling in aerial locomotion?  
Presumably
 size has an effect on acceleration (both linear accel and turning), top speed, 
and relative speed in flying animals as well.  I'm particular interested in 
refs on verts for this, anything on birds would be fabulous (which, presumably, 
would be the bulk of that literature anyway).

I find this (aerial scaling) to be less 'intuitive', in a way, because the 
examples are not as easy to sort out.  Fast flying birds come from both 
large-bodied and small-bodied lineages.  This could mean that small birds that 
fly fast (like swifts) simply devote massive proportions of their bodies to 
flight (which seems to be the case), but it might also imply that there are 
tradeoffs that affect speed in both large and small taxa.  For example, large 
birds presumably get more thrust per stroke, but they also have to generate 
more lift (and may have to expend proportionally more energy to recover strokes 
quickly).  Of course, this all pivots to a degree on having accurate speed 
records for flying v
ertebrates, and I am guessing they are not that much better than for 
terrestrial vertebrates (pigeons, ducks, etc should have good records, however).

3) Another concern for the bipedal and quadrapedal comparisons (and this comes 
from Carrano once again) is that birds and mammals do not run the same way (in 
fact, non-avian dinosaurs do not run as much like advanced birds as one might 
think from phylogenetic relatedness).  Birds have highly redistributed muscle 
mass, a nearly horizontal femur position, and a power stroke in the legs that 
pivots mostly at the knee, not the hip.  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.

Answers/comments would be much appreciated.

Cheers,
--Mike Habib

Johns Hopkins School of Medicine
Center for Functional Anatomy and Evolution
1830 E. Monument Street
Baltimore MD, 21205

habib@jhmi.edu