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Re: Sloping terrain Re: Woman against Abelisaur

On Jul 25, 2011, at 1:20 PM, Don Ohmes wrote:
> I reiterate -- the case wherein sinking does not occur is irrelevant to 
> this thread, or my thesis.

Ah, okay, that's a critical point I missed originally.

> Is it as weak as, "they obviously mired because they were just so big"?

Ha.  No, of course not.  Funny how often animals are labeled as clearly "too 
big" to do things.  As (primarily) a pterosaur worker, I get that all the time 
on flight topics... "but it's really big!"  Me: "And?...."

>> However, the accommodation of compliant substrates is not merely a matter of 
>> stability.
> Actually, that is what the thread is about. Stability, and now morphing 
> into miring. Not relative overall locomotive efficiency.

Duly noted.  Those are different issues.

>> and some of them would have been prone to miring on account of having very 
>> heavily loaded foot sections.
> If you assume that a heavily loaded foot section leads to miring. I 
> assert that it does not, when accompanied by power and limb length, 
> which tend to increase along w/ loading.

A heavily loaded foot leads to sinking, which can lead to miring.  The easiest 
way not to mire is to sink less, which is presumably why most waders and/or 
habitual soft substrate walkers have low foot area loadings.  However, if we 
assume the substrate is really soft such that everything sinks until (if) it 
hits bottom, the scenario changes.

>> Elephants actually have relatively broad feet, while sauropods did not.
> How does it work out size to size -- any sauropods whose lower limbs are 
> known that approximate elephant mass?

Good question - I'll see if I can get the numbers for a smaller sauropod and 
get back to you on it.  Mine were all for relatively big guys.

>> As the sediment becomes even more compliant, it effectively becomes a full 
>> fluid, at which point propulsion is swimming, and theropods again become 
>> more efficient, though I doubt they hunted while swimming very often.
> Here we diverge strongly. As the hypothetical 'swimming fluid' becomes 
> denser, and we are not talking about water, but mud, then the absolute 
> power available to shear the fluid, and the power to 'drag profile' 
> ratio determine the density at which the legs stop moving.

We are referencing two different circumstances.  If the animal can move by 
imparting momentum to the substrate via simple drag-based propulsion, then we 
are talking about swimming, and the substrate is a classic fluid.  Even if it 
is a dense fluid, the theropod limb structure is much more suited to effective 
swimming, in part because they can change the drag profile on the limb by a 
much greater margin on the propulsive and recovery strokes.

However, since you indicated a consideration of resistance to shear, then you 
are not actually talking about swimming (since a fluid doesn't resist shear, 
but only rate of shear) - you're talking about moving in a semi-fluid 
environment with viscosity of sufficient magnitude that significant solid 
properties still exist (this would be a non-Newtonian fluid, of course).  
Swimming in that circumstance is very difficult; this is how quick sand works.  
Under that scenario, we're still at the need to touch the bottom. In fact, in 
that case, anything that can't touch bottom, but doesn't float, probably dies.  
I'm talking about going even more compliant than you are - in other words, at 
some point we transition to a roughly Newtonian fluid, at which point most 
theropods will move more easily than any sauropods I can think of, simply 
because the theropod bauplan is more effective for swimming (not that it's 
great for that).

> Higher buoyancy and what I perceive intuitively to be a superior 
> hydrodynamic profile are likely advantages, too.

These won't matter much until we get to the fully fluid case.

>> I can describe that all in mathematical terms, of course, but I'm not sure 
>> anyone here wants to read that...
> Mike, w/ all due respect, you have assured me that your calculations 
> indicate that "assisted running" is not possible -- empirical evidence 
> contradicts that.

Not sure if it's worth getting back into this again, but just to clarify, my 
rough calculations (over a year ago) indicated that steady state maximum 
running speeds were unlikely to be effectively assisted with wings.  They also 
indicated that assisted running in the sense of augmented burst accelerations, 
leaping distance, leaping height, and turning radius should all be possible.  
There have as yet been no published data to contradict either of these 
suppositions, and several published datasets supporting the "agility" factor 
augmentation.  If you know of force-measurement studies showing otherwise, 
please let me know - it's not as if I don't make mistakes.  I may have made an 
error in my calculations, but I haven't found one so far.  With all due 
respect, as well, observations of baby birds flapping while running is not 
empirical evidence of assisted running in the sense I think you mean it.

Regardless, there is a difference between distrust in the mathematician and 
distrust of math.

--Mike H.

Michael Habib
Assistant Professor of Biology
Chatham University
Woodland Road, Pittsburgh PA  15232
Buhl Hall, Room 226A
(443) 280-0181