The ratios of central nervous system (CNS) metabolism to resting body metabolism for 42 vertebrate species are presented in Table 1. The data on weights, metabolic rates, and calculated metabolism of the body, brain, and spinal cord are also shown for each species in Table 1 so that the reader can follow the derivation of each ratio. Sources for the empirical data and methods of calculation for estimated data are indicated in notes to Table 1. As explained in METHODS, each set of data represents the values for a single healthy mature individual of that species.
` Most of the vertebrates use from 2.7 to 7.7% of their total resting metabolism for the CNS, with a mean of 5.3%. There are several exceptions, however. The mouse uses 8.5% of its total body metabolism for its CNS and all primates studied use over 10%. Other species use less than 2.7% of their metabolism for the CNS, including domesticated livestock (horse, cow, chicken, and pig) with values from 1.6 to 2.0% and the shrew at 1.5%. Very large animals also have low ratios; in addition to the domesticated horse and cow, the ostrich has a metabolic ratio of 0.7%, and the extraordinarily large elephant and finback whale have ratios of 2.0 and 0.5%, respectively.
There is a close correlation between total body metabolism and calculated CNS metabolism among all of the 42 species in our sample. A linear regression equation expressing CNS metabolism as a function of total body metabolism for these data takes the form
log y = 1.27 + 0.91 log x (11)
where y is CNS metabolism and x is total body resting metabolism as they have been given in Table 1. The correlation coefficient for this equation is 0.97 and the standard error of estimate (corrected) is 0.32, which indicates that the relationship is a close one. The data are shown in Fig. 3, along with the regression line for the equation shown above.
A more conservative conclusion can be drawn from the animals in which there have been direct measurement of brain metabolism. Data from those animals are shown in an insert of Fig. 3. From these data two other regression equations can be drawn, depending on whether or not the human data are included. With human data, the regression equation is
log y = -1.43 + 1.2 log x (12)
Excluding the human data (since by all accounts the human data cannot be taken as representative of the vertebrates as a whole), one obtains a regression equation with a slope that is close to unity
log y = -1.16 + 0.99 log x (13)
There is not much difference between cold-blooded
and warm-blooded vertebrates in the proportion of metabolism
used for the central nervous system. The mean
metabolic ratio of CNS to body metabolism for the eight
cold-blooded vertebrates shown in Table 1 is 4.8 ± 0.6%
(mean ± SE). The mean metabolic ratio for the 34 warm blooded
vertebrates is 5.5 ± 0.7% (mean ± SE).
 |
 |
 |