Prenatal and postnatal influences on growth, prolificacy and maternal performance in mice.

I N mammals the growth rate of the nursing young is influenced considerably by the lactational performance of the mother. The magnitude of this influence in mice has been investigated by BATEMAN (1954) and Cox, LEGATES and COCKERHAM ( 1959) through the use of cross-fostericg experiments. These experiments permitted the variance for a trait to be partitioned so that the importance of prenatal (genetic and uterine) and postnatal maternal (lactational influences) could be estimated. Among the traits studied was 12-day litter weight, a trait that FALCONER (1947) suggested as a convenient measure of milk production in mice. Results obtained by BATEMAN (1954) and by Cox et al. (1959) were quite different for this trait. The former found only 32% of the total variance in “performance” for litters of eight mice to be due to postnatal differences, while the latter estimated that postnatal effects were responsible for 71.5% of the variance in the 12-day weight of litters of six mice. BATEMAN’S results lead to questioning the advisability of using weights of the nursing young to measure postnatal maternal performance, since so small a portion of the variance in these weights appeared to be conditioned by postnatal factors. On the other hand, the results of Cox et al. (1959) suggest that weights of the nursing young may be a satisfactory phenotypic measure. This investigation was undertaken to assess the relative importance of prenatal and postnatal influences upon body weight and growth and to examine the genetic and phenotypic relationships between different growth and maternal characteristics. From this information, a further assessment was sought of the usefulness of the 12-day weight of litters standardized to six mice as a selection criterion for postnatal maternal performance. Results from other aspects of the investigations are presented in a companion paper (YOUNG and LEGATES 1965).