Preweaning growth of inbred, F 1 hybrid, and random-bred mice as a measure of mother's lactation

A cross-fostering experiment was performed in which inbred and random-bred mothers suckled three groups of young: inbred (the SWR strain), F1hybrid (SWR × C3H), and random-bred (originally synthesized from SWR, C3H, CBA and C57BL). Two inbred mothers and one random-bred mother were placed in each of 43 cross-fostering sets, and each mother suckled two of her own young and two from each of the other two mothers. Body weights of young were recorded individually at birth and at 12 days.In 12-day weight and gain from birth to 12 days, differences among the three groups of young and differences between inbred and random-bred mothers were significant (P < 0.01). Although the ratios of the between-dam to within-dam mean squares in log scale varied among nine mother-young subgroups, the differences between any pair of these ratios were statistically insignificant. It was therefore concluded that in detecting lactation differences among mothers the value of a given type of young was not different between inbred and random-bred mothers and that as a means of detecting lactation differences among random-bred mothers, inbred and F1hybrid young did not appear to offer any substantial advantage over random-bred young.