J. theor. Biol. (1979) 80, 145-147
LETTERS TO THE EDITOR
Thermodynamics
and Body Temperature
Calloway (1976) has described an optimal body temperature (7”) for a wide variety of organisms based on the thermodynamic properties of water. Presumably, the optimal 7” is related to the midpoint for the equilibrium of reactions between 0 and 100°C and can be calculated by dividing the temperature difference for the boiling and freezing points of water by the base of natural logarithms (e). At sea level, the predicted optimum Ts, 100/2~718281 = 36.8”C, is reasonably close to the observed TB for man, 36~7°C (Hurtado, 1964). Recently, Gunther (1977) has questioned Calloway’s arguments based on the observation that TB in man and cattle are not influenced by changes in the thermodynamic properties of water which occur at high altitudes. However, Gunther’s case seems weak in that one must assume : (a) T, in man and cattle are fairly labile, (b) these organisms have the metabolic and/or genetic plasticity to adjust to new TB optima. Neither assumption is easily met. Homeotherms generally are extremely sensitive to changes in TB. Moreover, since a change in Ts will affect the entire physiology of the organism, the genetic “cost” required to convert the sum of an organisms’ machinery to a new optimum may far exceed the constraints imposed by functioning at a Ts slightly removed from the proposed thermodynamic optimum. This may be especially true for examples involving intraspecific comparisons since : (a) the period of exposure may be too short for genetic changes to have occurred. (b) gene flow from sea level populations could swamp any tendencies for genetic change. Data on Ts in lizards seem particularly pertinent to the considerations raised by Calloway and Gunther. Although lizards are ectotherms, i.e. they elevate and maintain TB by behavioral means, most lizard species have quite specific functional TB (Dawson, 1967) in a range considered optimal by Calloway (p. 331). More importantly, TB in sceloporine lizards are inversely related to their altitude of capture (Bogert, 1949; Brattstrom, 1965). The differences in T, are for distinct species that are evolutionarily adapted to different altitude conditions. Thus, if Calloway’s assumptions are correct, the differences in thermodynamic properties of water that would be expected at 145
0022-5193/79/170145+03
$02.00/o
ii;, 1979 Academic Press Inc. (London) Ltd.
G. SNYDER
146
I 0
I
, I-0
03 Alhtude
I l-5
I 2.0
(km)
FIG. 1. Effect of altitude on the predicted optimal and observed body temperatures in sceloporine lizards. The observed body temperatures are from Bogert (1949). The data on optimal body temperatures were based on the differences in boiling and freezing point of plasma as described in the text. See text for explanation of black body temperatures.
the different altitudes should have affected the evolutionary development of the observed T, for each of the species. In Fig. 1 I show the relation of T, and altitude for the sceloporine lizards (Bogert, 1949). The coefficient of determination, 081, is taken as evidence for a good agreement between TB and altitude. Also shown in Fig. 1 are regression lines for the predicted optimal TB, based on Calloway’s thermodynamic considerations, and black-body temperatures based on data from Bartholomew (1966) and Pearson (1976). The relation of black-body temperature and altitude is included to indicate the effect of altitude on the temperature of a body which freely exchanges heat with its surrounding thermal environment, but does not produce heat internally; loosely speaking, an ectotherm. At sea level there appears to be a correspondence between the optimal and observed TB. However, changes in the observed Ts cannot be predicted from changes in the thermodynamic properties of water. Rather, changes in the observed T, appear to follow changes in the lizard’s thermal environment (Fig. 1). Thus, any direct influence of the thermodynamic properties of water on TB remains to be demonstrated (Fig. 1 ; Gunther, 1977). Department of Environmental, Population and Organismic Biology, university of Colorado, Boulder, Colorado 80309, U.S.A. (Received 18 December 1978)
G.
SNYDER
LETTERS
TO THE EDITOR
147
REFERENCES BARTHOLOMEW,G. A. (1966). Copeia 1966. BOGERT,C. M. (1949). Ann. Inst. Biol. Me-r. BRATSTROM, B. H. (1965). Am. Mid/. Nat. CALLOWAY, N. 0. (1976). J. theor. Biol. 57,
241. 20, 415. 73, 376.
331.
(W. W. Milstead. ed.) p. 231. Univ. of Missouri Press, Columbia. G~IWTHER.B. ( 1977). J. /heor. Biol. 66, 593. HURTADO, A. (1964). In Handbook of Physiology (D. B. Hill, E. F. Adolph & C. G. Wilbur. eds), Sect. 4, p. 843. Washington: American Physiological Society. DAWSON,
W. R. (1967).
PEARSON, 0. P. (1976).
In Lizard
Copeia
Ecology:
1976. 155.
A S~mposiwn
LETTERS TO THE EDITOR
Thermodynamics
and Body Temperature
Calloway (1976) has described an optimal body temperature (7”) for a wide variety of organisms based on the thermodynamic properties of water. Presumably, the optimal 7” is related to the midpoint for the equilibrium of reactions between 0 and 100°C and can be calculated by dividing the temperature difference for the boiling and freezing points of water by the base of natural logarithms (e). At sea level, the predicted optimum Ts, 100/2~718281 = 36.8”C, is reasonably close to the observed TB for man, 36~7°C (Hurtado, 1964). Recently, Gunther (1977) has questioned Calloway’s arguments based on the observation that TB in man and cattle are not influenced by changes in the thermodynamic properties of water which occur at high altitudes. However, Gunther’s case seems weak in that one must assume : (a) T, in man and cattle are fairly labile, (b) these organisms have the metabolic and/or genetic plasticity to adjust to new TB optima. Neither assumption is easily met. Homeotherms generally are extremely sensitive to changes in TB. Moreover, since a change in Ts will affect the entire physiology of the organism, the genetic “cost” required to convert the sum of an organisms’ machinery to a new optimum may far exceed the constraints imposed by functioning at a Ts slightly removed from the proposed thermodynamic optimum. This may be especially true for examples involving intraspecific comparisons since : (a) the period of exposure may be too short for genetic changes to have occurred. (b) gene flow from sea level populations could swamp any tendencies for genetic change. Data on Ts in lizards seem particularly pertinent to the considerations raised by Calloway and Gunther. Although lizards are ectotherms, i.e. they elevate and maintain TB by behavioral means, most lizard species have quite specific functional TB (Dawson, 1967) in a range considered optimal by Calloway (p. 331). More importantly, TB in sceloporine lizards are inversely related to their altitude of capture (Bogert, 1949; Brattstrom, 1965). The differences in T, are for distinct species that are evolutionarily adapted to different altitude conditions. Thus, if Calloway’s assumptions are correct, the differences in thermodynamic properties of water that would be expected at 145
0022-5193/79/170145+03
$02.00/o
ii;, 1979 Academic Press Inc. (London) Ltd.
G. SNYDER
146
I 0
I
, I-0
03 Alhtude
I l-5
I 2.0
(km)
FIG. 1. Effect of altitude on the predicted optimal and observed body temperatures in sceloporine lizards. The observed body temperatures are from Bogert (1949). The data on optimal body temperatures were based on the differences in boiling and freezing point of plasma as described in the text. See text for explanation of black body temperatures.
the different altitudes should have affected the evolutionary development of the observed T, for each of the species. In Fig. 1 I show the relation of T, and altitude for the sceloporine lizards (Bogert, 1949). The coefficient of determination, 081, is taken as evidence for a good agreement between TB and altitude. Also shown in Fig. 1 are regression lines for the predicted optimal TB, based on Calloway’s thermodynamic considerations, and black-body temperatures based on data from Bartholomew (1966) and Pearson (1976). The relation of black-body temperature and altitude is included to indicate the effect of altitude on the temperature of a body which freely exchanges heat with its surrounding thermal environment, but does not produce heat internally; loosely speaking, an ectotherm. At sea level there appears to be a correspondence between the optimal and observed TB. However, changes in the observed Ts cannot be predicted from changes in the thermodynamic properties of water. Rather, changes in the observed T, appear to follow changes in the lizard’s thermal environment (Fig. 1). Thus, any direct influence of the thermodynamic properties of water on TB remains to be demonstrated (Fig. 1 ; Gunther, 1977). Department of Environmental, Population and Organismic Biology, university of Colorado, Boulder, Colorado 80309, U.S.A. (Received 18 December 1978)
G.
SNYDER
LETTERS
TO THE EDITOR
147
REFERENCES BARTHOLOMEW,G. A. (1966). Copeia 1966. BOGERT,C. M. (1949). Ann. Inst. Biol. Me-r. BRATSTROM, B. H. (1965). Am. Mid/. Nat. CALLOWAY, N. 0. (1976). J. theor. Biol. 57,
241. 20, 415. 73, 376.
331.
(W. W. Milstead. ed.) p. 231. Univ. of Missouri Press, Columbia. G~IWTHER.B. ( 1977). J. /heor. Biol. 66, 593. HURTADO, A. (1964). In Handbook of Physiology (D. B. Hill, E. F. Adolph & C. G. Wilbur. eds), Sect. 4, p. 843. Washington: American Physiological Society. DAWSON,
W. R. (1967).
PEARSON, 0. P. (1976).
In Lizard
Copeia
Ecology:
1976. 155.
A S~mposiwn
