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Effects of Prenatal and Postnatal Fraternity Size on Long-Term Reproduction in Mice

University of Wisconsin, Madison 53706

Abstract

Effects of prenatal and postnatal fraternity size (size of litter in which an animal develops prior to birth or is reared following birth) on long-term reproduction were studied by rearing 178 female ICR mice in standardized prenatal and postnatal fraternities. Three levels of prenatal and postnatal fraternity sizes were used in a 3 x 3 factorial experiment. Prenatal fraternity size was standardized by selectively terminating fetal development in pregnant females carrying at least 14 conceptuses. Prenatal fraternities were standardized to either 6, 10 or 14 fetuses, and postnatal fraternities were standardized by randomly assigning individuals to nurse litters of 5, 10 or 15 pups. Prenatal fraternity size negatively affected average pup weight at birth (P<.05) but had little subsequent effect on growth or reproduction. Postnatal fraternity size negatively affected weight at weaning (P<.01), with mice reared in smaller postnatal fraternities being heavier than those reared in larger fraternities. Following weaning, mice reared in smaller fraternities gained weight less rapidly (P<.01) but still tended to be heavier at maturity (P=.11). Vaginal opening occurred at older ages in females reared in larger postnatal litters (P<.01). An interval mating system was used to examine fraternity size effects on long-term reproduction. Females were exposed to males six times at 8-wk intervals with initial mating at 7 wk of age. Postnatal fraternity size and age at mating jointly affected litter size (P<.05). The effect of postnatal fraternity size on subsequent litter size was negative for the first two matings; mice reared in small fraternities produced larger litters than mice reared in large fraternities. However, the effect was reversed at the sixth and final mating, with mice reared in small fraternities producing smaller litters than mice reared in large fraternities. Such effects, if observed in litter-bearing livestock species, could be beneficially exploited because breeding animals in production systems are seldom retained for their natural lifetime.