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Re: Testing the "why" of Giraffe necks...
Combined answer to DM and MH: Comment by intersperal below.
----- Original Message ----
From: David Marjanovic <firstname.lastname@example.org>
To: DML <email@example.com>
Sent: Sunday, December 24, 2006 4:44:52 PM
Subject: Re: Testing the "why" of Giraffe necks...
> If the limit of 'head-over-heart' height is the strength of the artery
> rather than the pressure the heart can generate,
That's an interesting idea. Are there any data on this...?
------------ Not that I know of. My reasoning (FWIW) is that the walls of the
heart are unlikely to be weaker than the walls of the arteries, which makes
sense from an engineering perspective, and aneurysms occur at the pressures the
heart can generate.
> a denser atmosphere would therefore allow a longer neck (ie, a greater
> head/heart height differential).
Of course, but so would anything else that would keep the arteries from
bursting. I can think of lots of options: increasing the amount of smooth
muscle around the arteries...
------------ To the first point, one of the postulates I use is: due to
limitations inherent to the materials used to 'build' biosystems, there are
absolute limits to size. So, sooner or later, you run out of "options", per
environment. As to the available options, you are quite correct of course, and
there is data on giraffes in particular. As to whether giraffes are close to
the limit of morphological improvement... very good question. I gather that
some think "yes", while others wonder. Given my assumptions relative to
optimization, I would probably tend to fall in the 'yes' camp, if pushed off my
perch. BTW-- I vaguely remember an ongoing controversy (year 2000 or so?) about
the possibility of a 'siphon effect' in giraffes, where return flow boosted
arterial flow; does anyone know how that turned out? Due to flexwall tube as
opposed to rigid, a siphon seems unlikely to me. Don--------------
> a higher atmospheric pressure would "reinforce" the artery
> wall by increasing the ambient pressure acting on the external wall.
thought. Are you sure this is the case, though? I'm not sure how much
of the pressure on the walls of internal vessels really depends on
atmospheric pressure. High pressures can certainly compress
superficial vessels (and low pressures can cause them to dilate or
burst), but I'm not certain if the carotids, for example, would react
much to atmospheric changes. Any thoughts?
------------ Absolutely positive that I am not sure. I suspect carotids would
respond measurably to short term changes in pressure, but... Anyhow, I believe
two assumptions justify this rank speculation; 1) there is a check valve(s) in
the aortic system to prevent backflow, and 2) the pressure the heart supplies
is intermittent. This implies that in a maximally long (more or less vertical)
neck, the primary internal pressure of the carotids is supplied by gravity when
_between_ heartbeats (due to upper tube collapse and lower tube bulge), and
that this 'resting' pressure on the tube wall would therefore be counter-acted
in part by the ambient (atmospheric) pressure. In an extremely large and
extremely longnecked animal that had a low heart beat frequency, this could be
important relative to functional limits. Or so it seems to me.
Heh. Imagine an extremely large and extremely longnecked animal that cannot
pump blood all the way to it's brain when the neck is in the 'vertical'
position; it therefore must hold it's neck more or less parallel to the ground
"most" of the time, and raise it's head intermittently to feed, As I imagine
others have speculated, the nooks and crannies of the sauropod physique could
serve to store O2 in order to increase the time of feeding 'expeditions' ( }:
D ) by the head, as 'constant' bloodflow cannot be maintained. Subsequent loss
of consciousness would cause the neck to droop, restoring blood flow and
consciousness, thereby forming a natural browsing cycle. If you can visualize
w/out smiling, you are a better primate than I.
I paste this from archival file:
1st female sauro: "If you're having trouble getting a date, try losing some
2nd female sauro: "How do I do that? You know I can't stop eating when I'm
1st female sauro: "I heard that, honey. Hey, that Butch is cute. I hope he asks
me to the prom."
2nd female sauro: "'Cute, huh? That is just random female preference, girl, no
guarantee of fitness in that. I like my date, Stanley. He can raise his head 3m
above the mean, and retain consciousness 5 minutes longer than Butch. Now that
is what I call a selective advantage."
1st female sauro: "I suppose. But who cares? We'll all be extinct soon anyway.
If it feels good, do it, I say."
2nd female sauro: "Whatever blows your skirts up, honey. Damn, I'm hungry
again. See you when I wake up."