Thus primate brains scale with body weight with almost exactly the same exponent value (0.75) as for the nonprimates (0.74), which is also expressed by the nearly parallel regression lines in the log brain~log body plot. Thus primate brains tend to increase at nearly the same rate as a function of body weight as do those of nonprimates. However, primate brains tend to be about 2.3 times larger than the brains of nonprimates of the same body weight, and this is expressed by the difference in the scaling factor.
**
(First graph)
Primates have larger brains for their body weight than do most other mammals, a relation expressed by the regression line for primates, which is nearly parallel to the lower regression line for nonprimates. On a log-log scale, power-law relationships such as the allometric equation relating brain and body weight plot as straight lines. Note, however that the mammals nearest to humans in terms of the brain-body relationship are porpoises and dolphins, which also have large brains for their bodies.
The data used to construct this graph were rovided by Robert Martin.
(Second graph)
The relation between brain and body weight for primate species. The residual variance in brain weight after removing the effect of body weight is calculated by measuring the vertical distance between the data point for each species and the regression line. The graph on the right, based on the brain residuals, shows that pl-i mates that eat mainly fruit have significantly heavier brains than do those eating mainly leaves.
Evolving Brains
by John Morgan Allman
http://authors.library.caltech.edu/25078/1/Evolving_Brains.pdf
**
For primates, the equation becomes
log brain weight = 0.75 log body weight + 2.06
For nonprimates, the equation becomes
log brain weight = 0.74 log body weight + 1.7
Thus primate brains scale with body weight with almost exactly the same exponent value (0.75) as for the nonprimates (0.74), which is also expressed by the nearly parallel regression lines in the log brain~log body plot. Thus primate brains tend to increase at nearly the same rate as a function of body weight as do those of nonprimates. However, primate brains tend to be about 2.3 times larger than the brains of nonprimates of the same body weight, and this is expressed by the difference in the scaling factor.
**
(First graph)
Primates have larger brains for their body weight than do most other mammals, a relation expressed by the regression line for primates, which is nearly parallel to the lower regression line for nonprimates. On a log-log scale, power-law relationships such as the allometric equation relating brain and body weight plot as straight lines. Note, however that the mammals nearest to humans in terms of the brain-body relationship are porpoises and dolphins, which also have large brains for their bodies.
The data used to construct this graph were rovided by Robert Martin.
(Second graph)
The relation between brain and body weight for primate species. The residual variance in brain weight after removing the effect of body weight is calculated by measuring the vertical distance between the data point for each species and the regression line. The graph on the right, based on the brain residuals, shows that pl-i mates that eat mainly fruit have significantly heavier brains than do those eating mainly leaves.
(sk)
霊長類の動物たち、つまり一般に類人猿とかサルとか言われている動物の仲間たちの「脳の重さ」と「体の重さ」を両対数グラフで表すと、「0.75 乗」の線に見事に乗る。
霊長類以外の動物たちの「脳の重さ」と「体の重さ」は 「0.74 乗」の線に乗る。
つまり、霊長類の動物たちの（体の重さに対する）脳の重さは、他の動物たちのより重い。だから霊長類は他の動物たちより優れているという。
人間の脳はこの関係式から少しだけ重い側にずれている。つまり人間は他の霊長類の動物たちに比べて少しだけ重い脳を持っていることになる。だから人間はどの動物より優れているという。
でもそういうことは簡単には言えない。ネズミイルカ（porpoise）やイルカ（dolphin）の（体の重さに対する）脳の重さは、人間のとほぼ同じ。人間だけが特別だとは言えないのだ。