Dinosaurs
Threads - Heads Down, Tails Up?
Responding to a post by Chris Munson, Paul Williams posted:
> I can't recall
if this has been suggested before, however to me it seems simple.
> I seem to recall Peter Macinnis thinking aloud that dinosaurs may have
had
> valves in their neck arteries which prevented rapid blood loss when
> dinosaurs lifted their heads to eat leaves from the tops of trees. The
> question was then: how did the dinosaurs have big enough hearts to pump
the
> blood up their necks to their head?
Wild Giraffes have massive hearts
(about 12 kilos). One would imagine that sauropods had suitably massive hearts
as well.
There have been some arguments in palaeontology circles about the possibility
of sauropods having multiple hearts.
There is no need to postulate such an unlikely physiology when one thick-walled
perhaps 100 kilo heart would do the job (with suitable 'plumbing')
> Could the answer
be in their tails? I imagine a great big bunch of tail
> raising dinosaurs, lifiting their tails to deliver blood to their head
and
> necks, many metres away. Tails up while heads were down, then
the
> opposite... Seems simple
>
They would still have needed to constantly deliver oxygen rich blood to their
heads when feeding up high.
Of course, it may be that sauropods didn't raise their heads as high as some
current reconstructions envisage? Nevertheless, reading of the special adaptions
of giraffes leads me to think that the sauropods probably had similar systems
to regulate blood pressure and flow.
Giraffes get over the pressure problem by restricting blood flow (and draining/pumping
off some) when the head is moved downwards and storing some blood in blood
vessels when raising the head.
Also, it appears that the strong tight skin on giraffe's legs act along with
muscles to help prevent blood pooling.
Giraffe's blood pressure regulation:
"As in most ruminants, the blood reaches the brain from the heart by way
of the common and external carotid arteries. The two external carotids divide,just
before each reaches the brain, into many small vessels forming a tight network
that is called the rete mirabile. The vessels of the giraffe rete have elastic
walls which can accommodate excess blood when the head is lowered so that
the brain is not flooded. As a further safeguard for the brain while the
giraffe is in this position, a connection between the carotid artery and
the vertebral artery drains off a portion of the blood even before it reaches
this network. The walls of the rete mirabile vessels are also elastic enough
to retain sufficient blood when the head is raised so that the brain's supply
is not depleted momentarily during the system's pressure changes."
http://www.chaffeezoo.org/animals/giraffeHeart.html
This on Sauropod blood pressure from a paleo forum may be of interest:
(best to take these posts with a grain of salt)
http://www.cmnh.org/dinoarch/2001Dec/msg00815.html
"Why Don't Giraffes Have Dropsy?":
"The problem with the blood supply to the brain is even more severe but different.
In the brain arteries, the pressure will be less than in the heart by about
50 percent and in the veins, it would be about 15 percent below atmospheric
pressure; in fact, it is slightly above. This is not achieved by valves,
as in the legs, but by collapsing the jugular vein to increase the resistance
to flow.
Even more astonishing is the fact that the giraffe can lower its head to
the ground to graze thus creating the same problem in the brain as in the
legs. Now the return mechanism completely changes and the rhythmic working
of the jaw muscles pumps the blood back up the vein assisted by valves which
work only in this direction and for this purpose."
http://www.physics.uoguelph.ca/summer/scor/articles/scor179.htm
It seems possible (even likely) that Sauropods would have had similar blood
pressure regulation mechanisms.
Allied with a possible 'fire fighting pump' 100 kilo heart, I imagine that
they would have got along all right.
David
Maddern replied:
It seems
a bit human-centred to suggest that a bigger animal may not have a pump bigger
than ours and just as good as ours
They are
after all a product of fairly direct natural selection, if your heart is
not up to it then you can't (naturally) reproduce (ever tried it at 10,000
ft unpressurized) it puffs a person out (not acclimatised and a sea level
Walla).
Gerald Cairnes responded:
All blood vessels have valves along their length this why you can pump up
the blood in the veins in forearm with a partial tourniquet to the upper
arm. Tis been thus for a very long time and with each local movement the
blood is automatically moved forward through the system giving two mechanisms
of propulsion i.e. central pressure from the heart plus muscular movement
voluntary and involuntary. The arteries pulse of their own accord also. There
is probably some rebound effect also.
This doesn't necessarily negate your hypothesis but I would have thought
that there would be a lot of pooling and damping effects in such a long animal.
Sorry no time to dig out my old haemodynamics text, buried somewhere or even
ratted like some of my files.
Charles
de G added:
Paul Williams
has provided some interesting Sites.However i cant accept some of the suggestions
made,some are based on Bakker's "Dinosur Heresies". A intersting enough
summary of ideas on Dinosaurs But.
To which Ray replied:
I don't see and problem,
especially since we have a hard time being fully competent in understanding
dinosaur physiology. There may be sufficient skeletal remains, but
not much in the way of organs.
As I see it, a giraffe has no problem of passing out every time it lifts
it head, and whilst it has no tail as a hydrolic counterweight, I believe
it has some kind of valving and musculature reticulation (if that is the
right word) in its neck.
It would just be difficult to glean information like that from petrified
bone, and fact is, the long-necked sauropods were so some adaptation must
have defied gravity for blood flow.
Also, it is erroneous to consider dinosaurs as having simple 3-chambered
hearts (I believe one species had its heart fossilised?) when the crocodile,
unlike other reptiles, has a 4-chambered heart a bit like ours.
Paul Williams also responded to Charles:
Which suggestions
don't you accept?
Bob Bakker
in his book "The Dinosaur Heresies" perhaps pushes the 'warm blooded' dinosaur
'barrow' a little too far.
The book
is, of course, designed to sell.
Nevertheless,
Bakker makes some good points.
There is
no doubt that at least some dinosaurs were 'warm blooded'.
The giant
sauropods may not have needed to be. (Size has it's benefits)
The recent
Chinese feathered dinosaurs add credence to both the idea of warm bloodedness
and the thought that many dinosaurs had efficient bird-like heart lung systems.
Birds have more efficient lungs than mammals and similar 4 chambered hearts.
One point
which may be neglected is that for 140 million years the myriad species which
we blanketly call the dinosaurs survived and prospered through every (read
'almost every') change imaginable. They (in their various forms) outcompeted
mammals in almost every niche for at least 50 million years of this time.
Gerald added:
Just a point to which you may have alluded.
The sheer size of the large vegetarian dinos would have presented the benefits
of heat conservation through large volume to surface area and large volume
"rumination" by itself would have produced significant amounts of heat. I
doubt they would have lost much heat overnight either. Even in the absence
of an active homeothermic mechanism this should have provided a degree of
selection pressure for homeothermy. However, I suspect that even the small
dinos such as those with feathers, as you point out, probably developed homeothermy
also.
There are plenty of examples of parallel evolution and I don't have any difficulty
in accepting that at least some of the dinos were homeothermic. I also
suspect the trend to very large sizes was selected by multiple influences
e.g. those above and as a defensive mechanism against increasingly large
predators plus the advantage of being able to reach high to forage. There
is a possibility that in reaching large sizes and numbers with a homeothermic
metabolism they grazed around the clo ck and tended to
degrade local areas forcing them to migrate continuously. Large size would
then tend to make migration more difficult and energy economy may then have
become a problem and sleeping/resting would need to have been done in short
snatches while standing.
