The main hypothesis, that vertebrates use 2-8% of their resting metabolism for the CNS, applies with re- markable consistency across all vertebrate taxa and to species of greatly different body size. The simplicity and consistency of this relationship is much greater than the relationship of brain size to body size that has been noted by previous investigators. Unlike the latter, the new finding does not require separate equations for cold-blooded and warm-blooded vertebrates. Also, unlike earlier findings, the present findings may be explained with an equation that is more or less linear rather than with complex logarithmic equations.
Several taxa of warm-blooded vertebrates appear to be exceptions to the rule that 2-8% of the resting metabolism is used for the central nervous system. These include primates, domesticated livestock, the whale, the elephant, the ostrich, and the shrew. The shrew, as indicated in METHODS, may be only an apparent exception due to technical problems of measuring its metabolic rate. Very large animals such as the whale, ostrich, elephant, and alligator may have attained a size at which the basic relationship breaks down. After all, most physical laws break down at the extremes. In fact, if animals with body weight over 100,000 g are excluded from the data, the slope of the resulting regression equation is exactly unity, i.e., the relationship is linear.
The apparent fact that domesticated livestock use a lower percent of their resting metabolism for the central nervous system may reflect the purposes of the selection by humans during the process of domestication. The horse, cow, pig, sheep, and chicken may have been deliberately selected for an increase in body size and for no increase, or if anything, a decrease in CNS size. Corroboration of this hypothesis is found in the fact that the brain-to-body weight ratio of domesticated pigs is 30% lower than that of their feral counterparts (53), and a similar relationship holds between domesticated and wild sheep (15). By contrast, dogs and cats that have presumably not been selected for increased body size or reduced intelligence have metabolic ratios similar to those of nondomesticated vertebrates.
Primates devote a relatively high proportion of their body metabolism to the CNS. It would appear from data on brain-to-body weight ratios that this shift began to occur early in primate evolution (78). The greatest increase occurred in the last l,000,000 yr during the evolution of Homo erectus and early H. sapiens (72). Crile (19) noted this fact and suggested an explanation that is consistent with the present thesis: he postulated that when an animal's thinking capacity could control "energy outside themselves, whether it is the energy of a club, of fire, or of another animal, the brains of such animals will be larger than the brains of animals that execute energy entirely within themselves."
The constancy of the metabolic ratio for the CNS and total body metabolism may reflect a general relationship between control and executor systems in the animal kingdom. We may hypothesize that there is an optimal proportion of metabolism used for control systems (the CNS) and executor systems (the muscular systems) such that control systems use 5-10% of the energy used by the executor systems.
To test the hypothesis that there is an optimal control-to- executor energy ratio, we need to obtain data that are not yet available in the scientific literature. First, we need direct data on CNS metabolism from more species to confirm the relationships hypothesized in the present paper. Second, we need data on metabolism for normally active organisms over long time periods, rather than the available data that represent resting animals for short time periods. If such data are obtained, we may find that the relationship is similar to the one hypothesized for resting animals; thus species of fish that are continually active have total body metabolic rates that are twice those of species that are inactive (34), and, similarly, active species of fish have brain metabolic rates (as measured in vitro) that are double the comparable rates in sedentary species (91). In warm-blooded animals the data are less complete, but we know that very active humans may have metabolic rates that are double those of inactive people of comparable size [see Table 48 in Fulton (28)], and brain metabolism also increases as a function of short-term activity (87). Third, we need data on long-term metabolism of the muscular system rather than total body metabolism. Presumably, the muscular system uses most of the total body metabolism as opposed to digestion, nonmuscular heat production, tissue repair, etc., except perhaps during the high growth rates of the young animal and the pregnancy and lactation of the mammalian mother. However, total muscular metabolism must be less than total body metabolism, and for that reason we have suggested that the control/executor metabolic ratio is not 2-8% but more on the order of 5- 10%.
This theory can explain why there was such a great
increase in relative brain size during the evolution of
warm-blooded vertebrates, a fact that previous theories
have not been able to explain very well. The evolution of
warm-blooded vertebrates was marked by a great increase
in energy metabolism that had to be matched by
a proportional increase in food intake and, therefore, an
increase in the amount of work done to obtain food. It
has been estimated that at an environmental temperature
of 16°C, a field mouse of 20 g will require nearly 60
times as much food to meet resting metabolic requirements
as a common frog of the same weight (3). The
converse is also true; warm-blooded vertebrates that
reduce their body temperatures to 20°C during hibernation
have values for resting body metabolism during
hibernation on the order of the cold-blooded values (36).
We know, from the present data, that in the course of
evolution the warm-blooded vertebrates increased not
only their overall metabolism in comparison to cold-blooded
vertebrates, but their CNS metabolism as well,
and that each was increased in the same proportion. The
equivalent proportional increases can be explained by
the theory proposed here; since it is proposed that there
is an optimal relationship between metabolism of the
control and executor systems, any increase of muscle
metabolism should be matched by a similar proportional
increase of CNS metabolism.
 |
 |
 |