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On Nov. 15, Nathan Myhrvold wrote (in response to a posting by

>I am not confused that animals regulate their temperature. I am confused
>about what you mean by "store heat" and "dump heat".  Let me clarify
>questions I asked:
>1. Does the core body temperature of elephants increase during the day
>and then drop at night?
>2. If not, then what did you mean by "store heat during the day"?

        Medium to large sized mammals and birds can store heat in their
body tissues.  This results in an increase in body temperature, which in 
some species may exceed 10C.  The stored heat can be a product of basal
metabolism, exercise, or radiative/conductive heat gain from the external
environment.  In some animals, such as camels, this heat storage is under
direct physiologic control; in others, like giraffes, it appears to be a 
passive process.  Like many other large mammals, the body temperature of
elephants does increase during the day, and falls at night.
        Permitting body temperature to rise in hot environments has two
advantages; it reduces/eliminates environmental heat gain, and results in
significant savings of body water, since the animal is not trying to 
maintain a stable body temperature by sweating and panting.  In a 500kg
eland, a 7.3C rise in body temperature results in the storage of about
3,000 kilocalories of heat.  If the eland were to dissipate this heat by
evaporation, it would require more than five liters of water!  At night,
this heat can by lost by radiation/conduction to the environment.  This
method of heat loss is so effective that some large mammals have to 
_elevate_ their resting heat production at night, in order to prevent
        Large size of endotherms is a distinct advantage here, because
large mammals have a greater available "heat sink".  Their greater thermal
inertia also isolates them more effectively from the environment.  It's
sometimes stated this greater thermal inertia might pose problems with
heat dissipation; however, mass-specific metabolic rates decline with
increasing body mass, so each gram of elephant tissue is much less 
metabolically active (and produces far less heat) than the same gram of 
tissue from a mouse.  In addition, mammals and birds from tropical
habitats typically have lower resting metabolic rates than temperate zone
relatives, further reducing any potential heat stress/body size problems.

        Langman, V.A. & Maloiy, G.M.O. (1989).  Passive obligatory
heterothermy of the giraffe.  Journal of Physiology, 415, 89P.
        Mitchell, D., Laburn, H.P., Nijland, M.J.M., Zurovsky, Y. & Mitchell,
G. (1987).  Selective brain cooling and survival.  South African Journal of
Science, 83, 598-604.
        Porter, R.K. & Brand, M.D. (1995).  Cellular oxygen consumption 
depends on body mass.  American Journal of Physiology, 269, R226-228.
        Taylor,  C. R. (1969).  The eland and the oryx.  Scientific 
American, 220 (1), 88-95.
        Taylor, C. R. (1969).  Metabolism, respiratory changes, and water
balance of an antelope, the eland.  American Journal of Physiology, 217
(1), 317-320.
        Taylor, C.R. & Lyman, C.P. (1972).  Heat storage in running antelopes:
independence of brain and body temperatures.  American Journal of Physiology,
222 (1), 114-117.
        Taylor, C.R. & Rowntree, V.J. (1973).  Temperature regulation and
heat balance in running cheetahs: a strategy for sprinters?  American 
Journal of Physiology, 224 (4), 848-851.

Guy Leahy
Dept of PEHR
Western Washington University
Bellingham, WA 98225