Effects of postnatal litter size on reproduction of female mice.

A group of 98 dams weaned 588 female mice to be mated and allowed to reproduce. These females were assigned at birth to be reared in a litter of eight or 14 mice. Such litters were intended to represent postnatal environments equivalent to small and large litters. However, average litter size was approximately 15 mice in this line and comparisons were more correctly those between small and average litter sizes. A slight bias, genetically in favor of females raised in large litters, was noted due to allotment. A sample of 123 females was slaughtered at 10 days of pregnancy and 325 were allowed to litter. Body weights at 12, 21 and 42 days were positively correlated with each other and with reproductive traits. Weights at 12, 21 and 42 days were consistently larger (P<.01) for females raised in small litters. Sexual maturity (days to vaginal opening) was negatively associated with weights to 42 days. Mice raised in litters of eight were earlier maturing (2.11 days) and reached maturity at heavier weights (1.61 g). These differences (P<.01) indicate that age at maturity could not be completely explained by weight differences. An advantage (P<.05)in corpora lutea numbers (.93) was found for females from small litters. The advantage for number of embryos at 10 days of pregnancy was only .48 and was not significant. Females from small litters had .31 and .23 more mice born and mice born alive, respectively, but these differences were not significant. Heritability estimates for litter size born, numbers born alive and litter weight were .34, .48 and .64, respectively, when calculated for females from small litters. The corresponding