Steady State Nature of Evolution, Learning, etc.
247
of the weight; specifically, n = 1 expresses weight proportionality; n = 2/ 3 ,
surface proportionality; and n ~ 3/ 4 , metabolic body size within homeothermic mammals 1.
Figure 1 illustrates a highly significant linear relation between logarithm
of lifespan CY) and logarithm of metabolic rate (x) in kg
3 / 4 for 63 species
of mammals (replot after SACHER, [31]). The interspecies relationship expressed by the regression line is:
J! = .270 (x) + 1.0649
(2)
The standard error of estimate (Sx.y) is equal to .209, which implies that
the ratio between log (actual lifespan) and log (calculated lifespan) is
generally less than 1.618. This means that most of the predicted life spans
are within 60 % of the actuallifespans (for details see [13]).
The above relation between metabolic rate and lifespan also holds true
on the single organ level, for instance, when plotting brain weight in kg 3 / 4
against lifespan in the same manner as in Figure 1 (see also [13]).
The approximately two-fold superiority of metabolic brain size over
metabolic body size as a predictor of lifespan is shown by the tighter
clustering to the regression lines and specifically man's closer fit within the
relationship. Since man lives three times longer than expected in terms of
his body size [32], the closer fit may indicate that man's brain may have something to do with the prolongation of his lifespan. That metabolic brain size
can be related to performance is also indicated by VANDENBERG's [41]
studies in which it was found that in identical twins, the larger the head,
the higher the intelligence test score. This statistically significant relationship is obscured by other factors in the general population.
It may be of interest to note that indirect calorimetry of the whole brain
of man has yielded high values of energy metabolism [36], an average of
12 m Watts per gram hrain tissue [1]. There is also evidence that the significant correlation between regional cortical blood flow and EEG frequency
content, shown in man and other animals, may reflect the dependency of
both variables upon the oxidative metabolic activity of the nervous tissue
[20].
Mitochondrial oxidative metabolic activi~y in relation to unit of homeothermic mass has been discussed elsewhere earlier [5, 11, 34]. Spectroscopic
observations of intact, respiring mitochondria have shown that the electron
carrier molecules of the respiratory chains participate in an exquisitely
balanced dynamic steady state [26].
1 For practical purposes KLEIBER [24] recommends the kg 3 /4 as a useful
generalization expressing the metabolic unit of body size, although VON SCHELLING [33] favors, on mathematical grounds, the 0.73 power instead of 0.75.
247
of the weight; specifically, n = 1 expresses weight proportionality; n = 2/ 3 ,
surface proportionality; and n ~ 3/ 4 , metabolic body size within homeothermic mammals 1.
Figure 1 illustrates a highly significant linear relation between logarithm
of lifespan CY) and logarithm of metabolic rate (x) in kg
3 / 4 for 63 species
of mammals (replot after SACHER, [31]). The interspecies relationship expressed by the regression line is:
J! = .270 (x) + 1.0649
(2)
The standard error of estimate (Sx.y) is equal to .209, which implies that
the ratio between log (actual lifespan) and log (calculated lifespan) is
generally less than 1.618. This means that most of the predicted life spans
are within 60 % of the actuallifespans (for details see [13]).
The above relation between metabolic rate and lifespan also holds true
on the single organ level, for instance, when plotting brain weight in kg 3 / 4
against lifespan in the same manner as in Figure 1 (see also [13]).
The approximately two-fold superiority of metabolic brain size over
metabolic body size as a predictor of lifespan is shown by the tighter
clustering to the regression lines and specifically man's closer fit within the
relationship. Since man lives three times longer than expected in terms of
his body size [32], the closer fit may indicate that man's brain may have something to do with the prolongation of his lifespan. That metabolic brain size
can be related to performance is also indicated by VANDENBERG's [41]
studies in which it was found that in identical twins, the larger the head,
the higher the intelligence test score. This statistically significant relationship is obscured by other factors in the general population.
It may be of interest to note that indirect calorimetry of the whole brain
of man has yielded high values of energy metabolism [36], an average of
12 m Watts per gram hrain tissue [1]. There is also evidence that the significant correlation between regional cortical blood flow and EEG frequency
content, shown in man and other animals, may reflect the dependency of
both variables upon the oxidative metabolic activity of the nervous tissue
[20].
Mitochondrial oxidative metabolic activi~y in relation to unit of homeothermic mass has been discussed elsewhere earlier [5, 11, 34]. Spectroscopic
observations of intact, respiring mitochondria have shown that the electron
carrier molecules of the respiratory chains participate in an exquisitely
balanced dynamic steady state [26].
1 For practical purposes KLEIBER [24] recommends the kg 3 /4 as a useful
generalization expressing the metabolic unit of body size, although VON SCHELLING [33] favors, on mathematical grounds, the 0.73 power instead of 0.75.