Re: Mamenchisaurus Posture Paper

[Roger Seymour <roger.seymour@adelaide.edu.au>]

You are quite correct.  The blood pressure at the head would be higher
than at the heart under these conditions.  So how can one pump blood
from a low pressure to a high pressure? Gravitational potential energy
almost counterbalances the hydrostatic pressure of the surrounding
water.  I say 'almost' because the density of blood is about 5% higher
than that of water.  If one ignores this trivial difference, an animal
submerged in water is under essentially zero gravity with respect to the
cardiovascular system.  That's why they put pilots in water-filled
"g-suits"--to eliminate the effects of "g".
A sauropod in water would not be affected by gravity on its circulatory
system.  It could hold its head vertically in either direction and it
would have almost no effect on the pressure at the heart.  Its neck
could be as long as you like.
Pressures in the lungs are another matter.  An animal walking on the
bottom and using the neck as a snorkel is probably not an option for two
reasons.  First, as recognised long ago, it would be difficult for the
animal to inflate its lungs against the hydrostatic pressure of water.
(Whales are almost horizontal when they breathe, so the lungs are near
the surface.)  Second, the gas in the alveoli would have to be less than
atmospheric pressure during inhalation.  This would create terrific
pressure gradients across the thin alveolar walls because the blood
pressure would have to be close to external hydrostatic pressure.  The
walls might burst or at least leak fluid into the airspace.  This is why
I like floating sauropods with their lungs on the surface.
The problem for this idea comes from the Morrison Formation, which is
usually described as upland, floodplain overbank.  Where is their deep
water?


don ohmes wrote:
> 
> If a sauropod were immersed in water and lowered the
> head below the heart it would again be pumping blood
> "up" a pressure gradient, although with the assistance
> of gravity. Is this correct?
> 
> How deep (vertical head/heart distance) could the
> heart pump blood, after accounting for gravity?
> 
> --- Roger Seymour <roger.seymour@adelaide.edu.au>
> wrote:
> 
> >
> >
> > frank bliss wrote:
> >
> > > I honestly don't know the answers to the following
> > since I have not
> > > ever really focused on the big guys as they didn't
> > appear to hang out
> > > up here on the Montana/Wyoming border.  Why do
> > sauropods have to hold
> > > their necks up high?  The advantage of a long neck
> > may have been like a
> > > long arm, reaching out and picking up sweet
> > morsels instead of reaching
> > > up.  The tail counterbalances and a really big
> > lawn mower is born (so
> > > to speak).  Is the mainstream opinion that they
> > were high browsers? (Do
> > > their neck verts allow articulation in that way?)
> > Is it necessary to
> > > look for a mechanism to allow high browsing when
> > it may not have been a
> > > method used for feeding.  (Granted it has to be
> > thought out.)
> >
> > Yes, I believe that the high browsing is favored,
> > and some (including the
> > Barosaurus in the AMNH) have sauropods tripoding to
> > reach higher.   I prefer
> > the vacuum cleaner analogy: a less mobile body with
> > a long flexible feeding
> > head that could reach from ground up to a couple of
> > meters above the
> > shoulders.  This would allow access to a great deal
> > of food.
> >
> > > Another
> > > metaphorical analogue  comes to mind.... How much
> > vertical distance
> > > does a large whale have from its heart to it's
> > brain when it is
> > > swimming straight up?  It has to be several
> > meters. Does water pressure
> > > alter the mechanism somehow?  Did the big
> > sauropods spend most of their
> > > time browsing in lakes below the water level
> > instead of terrestrially
> > > feeding or is it all conjecture at this point with
> > no data to support
> > > either hypothesis?
> > >
> >
> > In water, gravity is not a problem, because the
> > hydrostatic pressures in the
> > blood column are practically matched by external
> > hydrostatic pressures that
> > transmit through the soft body tissues.  We studied
> > blood pressure in land
> > and sea snakes.  The sea snakes have relatively low
> > blood pressures and if
> > you take them out of the water and hold them
> > head-up.  Their hearts cannot
> > support the blood column and circulation stops in
> > the brain.
> > So whales do not worry about blood pressure
> > problems; they seem to have
> > slightly lower BP than expected, but there are no
> > really good data on them
> > under natural conditions.  They are in their own
> > 'gravity suits', like
> > pilots.
> > Aquatic sauropods would also be protected from BP
> > problems, using their necks
> > to reach deep vegetation (or ?) while keeping the
> > lungs near the surface to
> > ease the strain of breathing.
> >
> >
> >

-- 
Roger S. Seymour
Environmental Biology (Darling Building D418)
School of Earth and Environmental Sciences
University of Adelaide
Adelaide, SA 5005, Australia
Phone:  61-8-8303-5596
Home:  61-8-8390-2260
Fax:  61-8-8303-4364
email:  roger.seymour@adelaide.edu.au