Evolution of brain size in neonatal and adult placental mammals: a theoretical approach.

An index of encephalization has been proposed for neonatal placental mammals, indicating the progress of relative brain size of a species at birth. It appears that the neonatal index is directly proportional to neonatal brain weight and an inverse linear function of the metabolic rate of the newborn. An identical relationship has recently been found for adult mammals. A further inquiry has been made into the allometry of gestation time, litter size, neonatal brain weight and maternal metabolic rate. A hypothetical model in which these variables were incorporated reveals that the maternal energy supply to the fetal mass is the principal determining factor in setting limits to the length of gestation.