Encephalization in vertebrates. A new mode of calculation for allometry coefficients and isoponderal indices.

The conventional allometric power function, with its slope near 2/3, works well for interspecific scaling of brain vs. body weight in all groups of vertebrates. It fails, however, in extrapolation to vertebrates of the largest size within their groups: these have smaller brains than the equation would predict. We propose a correction, the hyperbolic tangent, to linearize the data over all sizes, and we discuss evolutionary reasons for the relatively small brain size of the largest vertebrates.