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THERMOREGULATION IN LAND GIANTS (LONG)



Having found the current discussion on the supposed vulnerability of land
giants, especially those with high metabolic rates (MR), so poorly informed,
full of mistaken notions and absurd speculations, and exasperating, I have
decided to copy and add to the relevant sections from my paper in Modern
Geology (1991, 16:69-99).

BIG, HIGH MR DINOSAURS WOULD HAVE NOT COOKED IN THE HEAT

Because metabolic rates scale to weight0.7-75 power, while surface area
scales to the 0.67 power, it is almost universally believed that big high MR
animals suffer serious heat stress in tropical climes. If so then big
dinosaurs, sauropods especially, should have had low MR to avoid this dire
fate. Alternately, high MR sauropods needed well developed cooling systems.

This is a major misconception. The difference between the scaling of surface
area and metabolism is not great, and if it were a problem then it could be
solved simply by scaling metabolism to whatever exponent solves the
overheating problem. Yet elephants have the metabolic rate expected in
mammals of their size, so clearly it is not a problem. In fact many tropical
mammals have reached 1-20 tonnes, but no reptile has done so, opposite the
expected pattern if high MR, tropical climes and big size are a bad mix.
Elephants lack well developed evaporative cooling systems, so they cannot
sweat (they can psray themselves with water when it is available). Elephants
living in treeless habitats do not drop dead from heat stroke, even in the
most dangerous conditions of an extremely hot drought when it is not possible
to dump excess heat by radiation or evaporitive cooling, and shade is not
available. Instead large bull elephants have the highest survival rates under
such circumstances, again opposite the presicted pattern (Owen-Smith, 1988).
The giant desert elephants of the Skeleton Coast have been observed walking
across treeless tracts at noon with air temperatures at or above 38C(100F).
Field biologists have long known that big birds and mammals from 100 kg on up
use a classic thermal strategy in which their great mass is used to store a
low mass specific MR for most or all of the day. Heat inflow from the hot
environment is minimized by restricting blood flow to and from the skin, and
because the surface area/body mass ratio is low in giants, heat influx from
the environment is minimized in large forms (so insulation, and light or
reflective skin tones, are not necessary). Body temperatures are allowed to
rise 3-10C(5-18F) up to 46.5C(114F) in some ungulates (brain temperature is
kept normal via vascular counter current heat exchangers), and water loss is
kept to a minimum (see below refs). The built up heat is then unloaded into
the cool night sky by flushing the skin with hot blood from the body core
(insulation would therefore be a bad idea in tropical giants). The bigger an
animal is, the better the effect works. This makes large high MR animals
practically invulnerable to overheating under the harshest natural conditions
(I have yet to come across an documented example of a wild elephant dying
from heat stroke, and I have looked hard [mahuts have driven their domestic
elephant mounts to overheating, but you can do the same with a dog or
horse]). Big giraffes continue to browse in the sun at midday. This same
should have been true of dinosaur giants. A normally active 100 tonne high MR
dinosaur would take 12 hours to heat to maximum tolerable body temperature.
In contrast, a dog exposed to the sun and without water will overheat in
about one hour. (In the MG paper there is a graph illustrating this effect).
Low heat storage capacity, high mass specific MR and high surface area/mass
ratios force small forms to either use large amounts of water cooling if is
available, and/or quickly seek thermal refuge when it is hot.

In this view, large body size is not a problem for tropical animals with high
MR, it is a positive adaptation for preventing overheating. If so, high MR
dinosaurs may have evolved large size in part to cope with a warm Mesozoic
world.

Because it becomes increasingly difficult and then impossible to radiate heat
from the body as environmental heat loads approach and then exceed body
temperature (elephants stop ear flapping when air temperature reaches body
temperature, Hiley, 1975), heat radiators are not effective when it is really
hot. They are therefore not critical adaptations, and the great attention
applied to them regarding various parts of dinosaurs is exaggerated.

In the same issue of Modern Geology Spotila et al (203-227) have a paper in
which computer simulation show that giant high MR dinosaurs would have
overheated. How reliable this data is becomes apparent when it is considered
that their model of a 3.6 tonne high MR animal has a body temperature
18C(32F) higher than the average tropical daily temperature. If so, then
elephants should be frying all the time! Classic case of bad data into the
computer and absurd results out.

Regarding using air-sacs for cooling, this requires panting, which is a
specialized respiratory function in which the chest works at its resonant
frequency to rapidly ventilate the lungs with minimal energy cost. Ostriches
can pant, but as size increases the resonant frequency of the chest decreases
below the rapid level needed to breath fast enough to cool the body with
internal air flow. Breathing faster than the resonant frequency requires so
much work that more heat is generated than is unloaded. Therefore giant
theropods and sauropods could not use their air-sacs for cooling.

I suggest all those interested in the overheating problem read the below
references before contributing to the discussion on overheating and
radiators. It cannot be overemphasized that the notion of overheating land
giants is considered obsolete among modern biologists, and how obsolete
paleontologists are when they dredge up this tired old myth. It's getting
embarrasing. As for myself, I am tired of how this issue keeps coming up
every few months as though there is no learning curve in action; it is like
stamping out cockroachs. I shall therefore save this piece and spring it upon
the unsuspecting when the issue inevitably rears its ugly head yet again.

Gordon M 1972 Animal Function MacMillan Co
Finch V & Robertshaw D 1979 American J Physiology
          237:R192-R196
Hiley P 1975 Natural History 84:23-31
Langman V 1982 Smithsonian Jan:95-103
Owen-Smith R Megaherbivores Cambridge Univ Press
Paul G 1988 Predatory Dinosaurs of the World
Schmidt-Nielsen et al 1957 American J Physiology 188:
           103-112
Taylor C 1969 Scientific American 220:88-95
              1970 American J Physiology 219:1136-1139
              1972 131-142 In Maloiy G ed Comparative Physiology
                        of Desert Animals Academic Press
Whittow G 1978 Natural History LXXXVII:44-48

GSPaul