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Re: Sauropod browsing energetics



On Thu, Mar 24, 2011 at 7:01 PM, Don Ohmes <d_ohmes@yahoo.com> wrote:

> That is not true -- the problem that evolution is "trying to solve" in
> animals that trend toward maximal size is this -- what shape allows the
> maximum mass to be fed, which equates to what shape allows the most biomass
> to be harvested ?

Technically the question evolution is trying to answer in any lineage
is "how can I produce a dusproportionate number of offspring who
themselves will exhibit disproportionate reporductive success?"  If a
lineage trends towards large size then that size increase is in some
way serving that larger goal.

And to the authors' credit they were trying to test this in their
paper, in that they were trying to establish that long necks
themselves provided an energetic advantage in low browsers, and to do
that they have to test the cost of having those necks against the
energy savings during food gathering.  They failed to do this, so they
failed to test their hypothesis.

That's not to say that testing different shapes in sauropods wouldn't
be interesting, but then you are asking a different (and even more
complex) question.

> Comparing the efficiencies of various designs (i.e., morphologies) at equal
> mass is entirely justified.

Sure, if you are testing a different question.  But as long as the
hypothesis is "longer necks provide an energetic advantage" then you
need to constrain the other variables.

> An environment wherein an animal that can swing it's head in an arc w/ a
> radius of 9m and angle (arbitrary number alert) of 60 degrees, yet find only
> one bite per step, is going to demand a *lot* more than one step per bite on
> average from a minimally necked animal...

Most likely, but you still have to demonstrate that taking all those
steps is more expensive than having a long neck all your life.  Fail
to test that and you fail to demonstrate any benefit to having a long
neck for browsing.  Remember that steps get relatively cheaper with
size, not more expensive.


> Some things we know -- e.g., as size increases w/in a given body-style, the
> total mass of food that must be ingested per day to survive increases. Also
> a long neck unquestionably means you can gather not only (much!) more browse
> per step, but also more browse in a day, all else being equal. Therefore you
> can be bigger than your cousin who has the short neck, and that can be well
> worth any cost.

Hold up.  A longer neck will almost always mean you can browse more
bites per step.  That's not at all the same as saying you can take
more bites (browsing) per unit time.  If resource allocation and neck
speed motion is such that your long neck takes more time to sweep from
one side to the other than it would take to just take a few steps then
it's wrong.  Furthermore, even if it's true in a given environment
it's only energetically favorable if the costs of having the neck
don't outweigh the increase in browsing rate.

I'm not saying these things couldn't be true (or at least true in some
situations) I'm pointing out that this paper completely fails to test
for them.

> There is indeed in theory a size at which the neck is "too costly", but it
> is obviously more than 9 or 10m, because necks evolved to that size.

Ok, that would be a very general look at overall neck length that
would then be searching for a maximum size for sauropod necks
(presumably driven by some physical constraint, like blood pressure or
tracheal length, etc.).  And that may well be (it's certainly an
interesting question in its own right) but the Ruxton and Wilkinson is
making the specific claim that they can demonstrate that all things
being equal a longer neck is energetically more favorable for low
browsing than a short one.  And all I'm saying is the completely
failed to even attempt to calculate the cost of having a longer neck,
therefore they didn't even test their hypothesis.

Perhaps long necks can be demonstrated to be favorable to low
browsing.  Even if they weren't, perhaps sauropods did it anyways and
the long necks evolved for altogether different reasons.  I'm not
trying to defend any of the varying hypotheses here, I'm just saying
you can't claim something is energetically more favorable without
comparing the cost savings to cost increases.

Here's my tortured analogy: I can deliver 47 newspapers on my bike in
the morning before school.  In a car I can deliver 153.  Since
delivering more papers means I make more money, I've proven that it's
economically more viable to have a car as a paper boy.

Think maybe there are a couple of other costs there I should examine
before publishing my work?

Best,

-Scott

-- 
Scott Hartman
Scientific Advisor/Technical Illustrator
(307) 921-9750
www.skeletaldrawing.com