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Re Warm-blooded debate


   From Nov 96 to June 97 various discussions have come up about
temperature control and cooling in dinosaurs, especially large sauropods
and whether dinosaurs were warm-blooded.
   Forget the arguments and think of dinosaurs as a combination of
an ostrich, an alligator, a camel, and an elephant.  An ostrich because
of its long neck, bipedal gait, toe claws, small head to body  ratio,
ability to pant, circular flow through bird lungs, bird heart, rapid
growth rate, lack of chewing teeth, warm blooded metabolism,
egg laying, and it relationship to dinosaurs.  An alligator because of its
tail, replaceable teeth, hand and toe claws, flow through lungs, almost
4 chamber heart, stomach stones, continuous growth, and its ancestral
relationship to dinosaurs.  A camel because of its long neck, small head
to body ratio, ability to take hot body temperatures and warm-blooded
physiology.  An elephant because of its replaceable teeth,  ability to
get to water in draughts, 6 sets of teeth, and warm-blooded metabolism.
   Dinosaurs lungs probably would have had  ?flow through tubes?
style lungs (birds and alligators) vs. ?sack-at-the end of a tube? lungs
(mammals) because of their relationship to birds and crocodillians.
The relation ship to birds,  pleurocoelous neck vertibrae, and in some
cases hollow bones suggest that the ?circular one way - oxygen exchange -
while breathing in and out with air sac? system of  birds was possible.
Such a system would provide the oxygen to the blood that a endothermic
dinosaur would have needed. For description, of how the lung might have
worked, see below.
   Dinosaur hearts probably would have had 4 chambers like (birds and
alligators (almost) because of their relationship.  Like in mammals and
mentioned in the mailing list debate, this type of heart could  pump this
oxygenated blood to its body without blowing out the lungs.
   With oxygenated blood available to the cells, the development of
aerobic conversion of energy could take place.  In addition, the oxygen
could have been used to start endothermy is smaller animals to allow
them to funtion regardless of external temperature.
   I presume that the early dinosaur ancestors went through a small phase
where the birdlike ankle (versus the heeled types)and long neck developed.
The development of endothermy here could have developed here and allowed:
1.  flight in pterosaurs,
2.  later flight in birds,
3.  rapid growth rates in dinosaurs,
4.  a advantageous adaptation to other upright gait reptiles that didn?t
     get there, such as crocodilians, relegating them to extinction or
     blooded  environments.
5.  an equivalent system to mammals for head to head competition.
    It seems probable that endothermic, 4 chamber hearts, and circular
through hearts, would be needed for birds or pterosaurs to truly develop
powered flight.  If they evolved from dinosaurs or a common ancestor, it
easy to believe some or all of the dinosaurs may have been endothermic.
    Following this line, early dinosaurs with replaceable socketed teeth
long necks would have allowed them to:
1.   continue to grow throughout their lives and become large.
2.   replace  teeth damaged, worn, or lost.
3.   develop grinding teeth such as in hadrosaurs.
4.   dominate day time land predatory niches.
    Of interest to note is that elephants with their 6 sets of teeth
closest approach
the large dinosaur sizes.
     Dinosaur locomotion suggests movement.  Horner?s large herd
of plant eating dinosaurs would need to move while it foraged to
prevent total consumption of its food (with a short break at nesting
Sauropods would need to move from tree to tree.   Theropods and their
thin legs are clearly meant for some type of running.  Aerobic energy
conversion would definitely be an advantage here, especially if the
predators were aerobic and endothermic.
     The ability of camels and other large animals to live in hot
show that large endothermic animals do not need to keep their bodies at
a constant temperature.  Some people seem to think that endothermic
means constant body temperature.  This might be true for humans to
prevent overheating but not all other animals. In THERMOREGULATION
IN LAND GIANTS (LONG), Tue, 19 Nov 1996, Greg Paul shows that
not only is large size not a problem for endothermic animals in hot
environments, but possibly an advantage.
    The control of  excessive temperature to the brains is more important
animals than body temperature, even in reptiles.   Many mammals keep
their brains cool through evaporative cooling through the initial portions
of the respiratory system where relative humidity is at its greatest
Panting and gullar flutter are examples.  Here air rapidly moves past a
moist surface to increase evaporation.
    Most dinosaurs had small brain cavities with nasal passageways
that would take air around or near the brain.  Nasal curls and hollow
scull features could also increase initial respiratory surface area.
Other features such as neck airsacs (especially if attached like a nasal
opening) or flexible membranes may have also been used to rapidly
move air past evaporative surfaces.  Small brains with air flow around
would not be hard to keep cool.
     The biggest dinosaurs, sauropods for example, have not lost
their toe claws.  Sauropod rear toe claws were probably used to dig
for water or they might have evolved elephant like toe nails.  The
ability to get to water during draughts would be very important for
a long lived large dinosaurs, especially if it needed some moisture to
cool its brain.   (Greg Paul, I am still waiting for an illustration
showing a sauropod digging for moisture in a draught.)  Even if not,
the amount of moisture evaporated from the lungs would need to
be replaced.
    The head on the long neck and a countercurrent blood flow heat
exchanger would help isolate the temperature of the head from the
body during the day.   As mentioned elsewhere, cooling the body occurs
most for large animals at night when the surrounding temperature is
    Some dinosaurs probably stored moisture.  The raised spines on some
dinosaurs may have supported these features.   We should not think that
the only purpose of these ribs was to act as a radiator.
    A high body temperature might be an advantage for a sauropod.
With stomach stones, they may have broken down their food more rapidly.
In addition, a hot body might be able to grow faster.
    Some considerations commonly ignored when talking about large
sauropods are that the lung volume and surface area remains proportional
to the body volume.  As such internal surface area increases in cooling
importance for larger animals.  Likewise, the reduced surface area to
volume for larger animals means less solar radiation absorbing surface
area to volume which should slow down the increase in temperature for
large sauropods when in the sun as compared to smaller animals.  However,
a constantly eating sauropod probably would have spent much of its time
in the cooler and shadier locations of forests.
   Some of the recent arguments or comments are provided below for
those interested in looking deeper.  While this is not proof that
were warm blooded, it sure makes it hard to consider them otherwise.
                                Mark Shelly

On Nov 8, 96 Brian Franczak wrote regarding PLEUROCOELOUS:
>Nick Longrich wrote:
>> Sauropods do something similar, I think, with their pleurocoelous
>>vertebrae- eliminating the material in the middle, where it doesn't do
>>much good.
>Wouldn't airsacs in these pleurocoelous vertebrae add to the surface area
>and aid in keeping the animal from overheating?
Nick Replied:
>Bakker has suggested just this. I think there may be debate as to
>whether there actually were air-sacs - some pleurocoels may not have
>openings through which air passages could have passed. One wonders
>this could have made a big enough difference to make dinosaurs much more
>able to deal with being big than other endotherms ( i.e. mammals).

You can look at a mounted sauropod skeleton from the chest area and see
that 2 airsacs could have extended from the base of the neck almost to the
These air sacs may have been connected to the lungs or the nasal cavity.
Wayne Anderson added:
>Seems to me the airsacs would only help with cooling if they had a way to
>either radiate heat outward (not inside vertebrae!) or a means of
>air, i.e., being an extension of the respiratory system WHERE THE AIR IS

   With the closest relatives to dinosaurs being birds and crocodilians,
It seems
highly possible that dinosaurs had flow through lungs with air sacs on the
far side of the lungs (croc) or air sacs on both sides of the lungs with
a one way flow pattern (birds).  If the dinosaurs were like birds, 2 neck
sacs would be connected to the respiratory system after the lungs.   The
breathing cycle would be similar to birds with the following being one
of many possibilities:  (see attachment if possible)
Step 1    Relax special neck muscles allowing air sacs in neck to expand
              fill up with used air from throat and to draw fresh air into
the throat.
Step 2    Close valve to prevent airflow into neck air sacs trapping used
Step 3    Open valves from the throat to prelung air sacs and lungs.
Step 4    Expand chest bringing fresh air into prelung airsacs and through
              into post lung air sacs to required volume for oxygen needs.
Step 5    Close valves from throat to prelung airsacs and lungs.
Step 6    Open valves from postlung and neck airsacs to throat.
Step 7    Contract chest and force fresh air from prelung airsacs through
              This allows oxygen transfer even while breathing out.   At
same time
              contract special neck muscles to force used air from the
neck air sacs.
        Air from the lungs, neck, and post lung air sacs is forced out the
Step 8    Close valve from the throat to prelung airsacs and lungs.
Step 9    Repeat
   The birdlike respiratory system allows the animal to exchange oxygen
breathing in and out.  Much more efficient than the ?balloon at the end of
tube system? mammals have.  While rarely mentioned, the ability to
and maintain an active lifestyle (commonly called endothermy) probably
requires the ability to rapidly replace the oxygen content in the blood.
one of the key evolutionary developments of endothermic birds (and
dinosaurs) was the one way flow through lung/ airsac system.
Note, it might be possible that neck airsacs were connected to the lungs
the nasal opening.

Again Nick
>I see the airsacs as having no other purpose that structural lightening.

Neck air sacs, while providing the important structural lightening, may
also have helped solve the problem of ?dead air in the throat? allowing
fresh air to flow across the lungs.  The amount of clearing might be
adjusted to
control the amount of carbon dioxide in the lungs.

On 10 Nov Greg Paul added:
>How little most people understand about heat problems in land giants.
>1 - Giant animals are not in particular danger of overheating, they store
>in the daytime and dump it at night.
>2 - Large animals cannot use their deep respiratory tract to shed heat.
 >Doing so requires breathing both rapidly and energy efficiently. Doing
>requires breathing at the resonant frequency of the ribcage, and
>drops off with size. Even ostrich ribcages are too big for them to use
>air-sacs for air cooling. Sauropods could not use their air-sacs for
>This overheating nightmare will never end.

   While large animals can use their respiratory system to shed heat (they
not help it as moisture evaporates from the moist surface of the lungs),
points out that it would not be enough to avoid high body temperatures.
lung surface area to skin surface area increases as animals are scaled up.
This means that internal cooling becomes more important for larger
If the environment is cooler, convective cooling can also take place in
the lungs
(at night in hot climates).  When the environment is as hot or hotter that
Sauropod, evaporative cooling is the only cooling means available.
However, it
would not be practical or possible to evaporate cool the entire body
because of
the water and surface area required.   If you think about temperature
control as
 ?Endothermic does not mean body homothermic? it makes more sense.
Modern animals let their body heat up and do not die.   We can not
this easily as people must keep their body at a constant temperature to
keep our
brains from overheating.  A big brain requires a lot of blood.  Fairly
bodies and sweating to keep a constant temperature goes with a short neck
large brain size for us, not every mammal.
   Regardless, Paul points out large mammals let their body heat up.  A
sauropod body might allow it more rapid growth (birds) and quicker
of food.
    Greg Paul also added:
>Sauropod air-sacs where not for cooling, and probably not for structural
>lightning either since giant mammals lack them. They were probably part
of a
>bird-like respiratory system. The main function was to draw air
>through the lungs, overcoming the dead air space created by the super
>trachea. The thorough lung ventilation may have also increased aerobic
>exercise capacity to elephant-like levels.

    While supporting some of what I discussed above, neck air sacs would
result in lighter necks.  Body airsacs would be used for breathing.  Only
airsac that can also take in air without it going through the lung could
previously breathed air from going through the lungs repetitively.

   Tom Canning wrote:
> Greg, is radiation the sole means of elephant cooling, or is evaporative
>cooling (via sweat) significant? This does make a difference as I know of
> evidence that dinosaurs could sweat.
Van Smith wrote:
>Sauropods, with their extremely elongated necks and pneumatic
>would have a tremendously effective evaporative cooling system as air
>passed over the moist membranes of the very long trachea, and, perhaps,
>over the walls of air sacs as well.

    This is true if you understand that it is not enough to keep the body
at a
constant temperature.   The key part is to understand what a sauropod
need to keep cool and understand how he would keep it cool.

Ray McAllister wrote:
>Evaporative cooling is more related to the fact that the atmosphere is
>usually considerably lower in relative humidity than the 100%RH drop or
>film of water. Thus it tries to evaporate and in doing so removes 540
>small calories, per g of water evaporated, This enormous heat removal
>cools the body holding the water. It would work somewhat in a long neck
>but the RH would certainly be higher in the inside of the body (as it is
>in our sinuses and lungs) and much less effective..

Note, high humidity tends to reduce temperature swings.  Dry areas get
hotter than moist environments increasing the need to cool.  Evaporative
cooling would take place primarily where air inters the body

Greg Paul wrote:
> It has been known since the 1950s(!) that large tropical mammals and
> regularly store heat on hot days by allowing body temp to rise as high
> to 114 degrees F. (The brain is kept cool by countercurrent heat
exchangers.) F

     These two have identified the sauropod cooling problem.  The body can
hot but the brain might not be able to take as much heat.  With the body
hot as or hotter than the environment, evaporative cooling is the only
to keep the brain from overheating.  Countercurrent flow works if you want
keep one lower temperature part of the body from raising.  However, you
still cool the part you want cooler.  Evaporative cooling would be most
at or near the point of entry.
    Sauropods, as well as many other dinosaurs, have their heads
on long necks.  The air passages can take the air flow around the smallish
area.  Evaporative cooling would reduce the air temperature in head region
countercurrent blood flow would help isolate the head temperature from the
rest of the body.  In addition, some evaporative cooling could take place
in the
neck air sacs helping reduce the temperature of the blood flowing to the
Additional surface area found in nasal passage ways may have also
to evaporative cooling of the head.
   This is not a new idea.  A recent show on desert animals on the
Channel described the same thing for some desert animals.  The only
with evaporative cooling the brain is getting enough water in times of
Sahara desert border elephants migratory path is dependent on the
availability of water.  They use their tusks and feet to dig for water
Indian elephants with smaller tusks).
   Now, imagine a giant sauropod that needs water in the dry season to
keep his head from overheating past its critical point (which might be a
high temp) or just to replace lost fluids.   Its large hind foot toe
claws, curving
slightly outward would be rotated and driven through lake or stream
to reach the water level.  I think this is the most likely reason
sauropods do not
have elephant toes but huge claws.

Bruce in the warm-blooded debate said 0n 17 June
>Some dinosaurs did have sails similar to the pelycosaur Dimetrodon--why
>should those sails have been used for any different purpose? (ectotherm)
The sails may have been used to support a large water carrying feature
to a camels hump.  If an animal needed extra water to help control brain
temperature, then various types of dinosaurs might have developed the same
The warm up a cold blooded animal, a dinosaur in a cold environment would
to need the feature more than those in warm environments.  As mentioned
when the temperature is as hot or hotter than you need to be, evaporative
is the only cooling method that can be used and them primarily to control

JSW replied :
>.The difference between ectothermy and endothermy is not simply a matter
>of "powering up."  Endotherms don't simply have more energy, they have
>more endurance and a very different cellular chemistry.  Ectotherms have
>no endurance because they can only generate energy anaerobically.  (A
>lizard or a crocodile has to kill in its initial rush -- if it doesn't,
>it has no energy left for a second attempt.  That's why all modern
>reptiles are ambush hunters.)  Endotherms like mammals and birds can
>generate energy both anaerobically and aerobically.  No anaerobic
>organism could have developed the energy for hours of steady cruising at
>ten or twenty strides per minute, as hadrosaurs and ceratopsids must
>have done on their migrations.

Again, do not forget the lungs and their importance for aerobic energy.

DinoGeorge: discussed
>...a key paper by Regal and Gans on the role of
>the four-chambered heart in aerobic metabolism. Endothermy is only
>connected with endurance in tetrapods; the four-chambered heart much more
>Since crocs >almost< have four-chambered hearts, and birds have them, we
>use the Extant Phylogenetic Bracket to argue that all dinosaurs, which
>phylogenetically between crocs and birds, had them. This, rather than
>endothermy--which developed much later, perhaps only in the theropod
>that eventually evolved into birds--was necessarily responsible for
>dinosaurs' erect stance and cruising ability.

>All we know is that modern birds are fully endothermic, so complete
>endothermy must have developed >somewhere< in the lineage leading to
>from the common ancestor of birds and crocodiles (which aren't
>Indications are that adult fully endothermic sauropods would have
>particularly since they have no known anatomical structures with which to
>dump excess body heat, so full adult endothermy >likely< developed in the
>aforementioned lineage leading to birds >after< the sauropods diverged.


Bruce wrote:
>I haven't heard, in several years, any discussion of how sauropods
>dealt with their relatively tiny heads.  Has that conundrum been
>Seems to me that an herbivorous endotherm the size of a C-130 Galaxy
>aircraft, but equiped with a horse-sized head, would have to eat at
>shrew-like rates to keep warm.  Yet if the thing were ectothermic,
>that reduce the caloric rate requirements and thus make the tiny head
The tiny head is an advantage if you try to keep it cool.  Many large
such as hippos do not eat all day.  Others spend most of their time
cud.  Therefore, the size of the head does not limit their food intake to
mammal levels and hence keep them from being warm-blooded.
Stomach stones would help the large sauropods break down their food.
In addition, a high body temperature would help break down the food.  The
bulk of the body by itself would help keep it warm.