Date of this Version
Selection for litter size had been practiced for 21 generations and relaxed selection for 13 generations in mice. Three replicates were used with four selection criteria: index of components (ovulation rate and ova success), uterine capacity, litter size, and an unselected control. Especially with selection for litter size and the index relative to the control, number of pups born had increased, and differences also occurred in mating weight. Dams of the three replicates and their litters were used to evaluate the effects of accumulated selection on pup birth weight, variability in weight of littermates, and dam's weight at mating and after littering. Total number born, number born alive, number of males, and number of females were also recorded and studied. Mean pup birth weight did not differ among the criteria; however, variability among littermates in pup weight tended to differ among criteria of selection. Regressions for pup weight and within-litter standard deviation of pup weight on number born were small and negative but significant ( P < .001). The distribution of pup weight within litter was normal for 77.2% of the litters, with no differences among the criteria. The difference between weight of male and weight of female pups was significant ( P < .001); overall males were 2.5% heavier than females. There was a difference ( P < .02) among criteria in mating weight and littering weight; however, the maternal weight gain between mating and littering was not different among criteria. Number born differed ( P < .003) among the criteria, but there was no significant difference among criteria in numbers of males and females. Selection for larger litters did not have a large effect on the mean or variability within litter for pup birth weight.