Effect of Long-Term Selection for Litter Size in Mice on Lifetime Reproduction Rate

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Abstract
Litter size at subsequent parities and total number of litters in a 308-day production period of mice of the 25th generation of a line selected for litter size (L line) and a control line (C30) were compared. Within the L line the effect of standardization of litter size at birth to eight young was studied. In both the L and C30 lines, interval and permanent breeding methods were compared. In interval breeding the males were removed from the mating cage just before a litter was born and were returned when the litter was weaned. In permanent breeding the males were left with the females permanently. In permanent breeding the total production of young of the L line was lower than that of the C30 line, because the L line was unable to continue the high initial production level (14 young) under such an intensive method. In interval breeding total production of young in the L line was much higher than in C30 because higher litter size of L line was maintained at subsequent parities. It was speculated that the rapid decrease of litter size of L line dams in permanent breeding was caused by effects of overloading the uterus. As effects of standardization of litter size were not significant, it was concluded that stress at permanent breeding was caused by shorter litter intervals combined with large litter size. This lack of effect of standardization suggests that milk production was not the major limiting factor for dams in the L line. Copyright © 1978. American Society of Animal Science . Copyright 1978 by American Society of Animal Science.