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Re: Sauropod browsing energetics



On Wed, Mar 23, 2011 at 2:44 PM, David Marjanovic
<david.marjanovic@gmx.at> wrote:

> Yeah, if.
>
> Without studies of neck mobility beyond the current controversy, and studies
> of locomotion cost beyond Paul's back-of-the-envelope calculation which
> suggests this cost was very low indeed, and studies of the cost of holding
> the neck horizontally and of raising it (assuming the bones allowed both,
> see above), this conclusion sort of hangs in the air.

My objection is the paper simply looks at the cost of moving vs the
cost of standing still as viewed against their estimation of browsing
envelope expansion with neck elongation.  Which is great, but they
aren't figuring in the extra cost of having the long neck (e.g. M is
held constant)!  All that extra tissue in the neck has to be grown and
maintained like any other part of the body; plus mechanically speaking
simply having the longer neck necessitates greater muscle exertion
_all the time_ (less so if held upright, but still extra work
regardless), and of course more work breathing in through a longer
trachea (although this is partially mitigated if you assume
flow-through lung ventilation...which we should).

If having a longer neck didn't cost you anything, then of course it
would make low-browsing more energy efficient.  Walking may be cheap
in large animals, but nothing is cheaper than free!  The real question
should be "what is the extra cost to maintain that longer neck 24/7
(and also how it impacts survival to reproductive age) and whether
THAT cost is lower than the energy saved while browsing with a longer
neck?"

That actual comparison between models of browsing simply doesn't
happen in the paper, so it's failing to test what it claims to be
testing.  To be sure, their model should be useful to future attempts
at modeling the energetics of sauropod food acquisition, but their
claim to have demonstrated a selective advantage for long necks in
browsing is completely false.

-Scott

P.S. As a smaller pet peeve; as far as I know it's universally
acknowledged amongst locomotion researchers that the cost of
locomotion goes down with size as a percent of your daily energy
budget (hence the "smaller than 1" scaling factor in the equation they
publish).  So why do they keep referring to moving the "enormous mass"
of sauropods, and keep citing size as a factor behind neck elongation
and browsing?  If "neck elongation and not walking" is going to be a
browsing advantage for anyone, it should be especially so for smaller
vertebrates who have to spend a larger amount of their daily energy
budget per unit distance moved.

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