Relation of Gestation Time to Brain Weight for Placental Mammals: Implications for the Theory of Vertebrate Growth
Duration of gestation in homothermic placental mammals has a much more precise alaxensis relation to neonatal brain size than to neonatal body size or any of three other neonatal or maternal dimensions. This was shown by a multivariate analysis of data on gestation time, neonatal and adult brain and body weight, and litter size for 91 species. These results can be explained by a theory of kinetics of fetal growth based on three hypotheses: (1) the brain is the slowest-growing organ in the fetal mammal; (2) it is also the pacemaker for growth of other tissues, which are held to its rate of growth; and (3) brain growth proceeds at the maximum rate allowed by its intrinsic growth law, so that the growth of all mammalian brains is governed in great degree by a single functional relation of attained size and time. The invariant rate constant for mammalian brain growth could be due to nutritional limitations or to the existence of an upper limit on rate of information flow during development.