The relationship between mature weights of domestic species and the times they take to mature in live weight is extended to breeds and strains within a species and to sex differences within a breed or strain. Results based on the Gompertz growth curve are found to be essentially the same as those based on Brody's growth curve. Both confirm the relationship of proportionality between the time a species takes to mature in live weight and its mature weight raised to the power 0·27 or some power close to this. The overall weighted average coefficient was 0·273 ± 0·029 for Brody's (1945) growth curve data and 0·276 ± 0·030 for the Gompertz growth curve data of Laird (1966). The weighted average coefficients of 0·358 ± 0·098 for strains within a species and 0·343 ± 0·049 for sexes within a strain, although not significantly greater than 0·27, suggest that a coefficient of ⅓ might be a better value to adopt when the relationship is confined within species.

Birds as a class matured significantly more rapidly than expected from their mature weight and mammals significantly more slowly. Body temperature could account for this class difference in maturing time but not for deviations of individual species or strains. Relative to their mature weight, some species appeared to take much less time to mature than others, with possibly a 2- to 3-fold genetic difference between extremes of the range.

The main use of the relationship obtained lies in providing a ‘metabolic’ age scale on which animals or species can be examined for differences in immature physiological characteristics such as maturing time, body composition or efficiency of food utilization independently of differences in adult size.