LONG-TERM REPRODUCTIVE PERFORMANCE OF FEMALE MICE

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Abstract
The relation of litter size to litter order was studied in eighteen normal mice (two-ovary females) and twenty-two mice in which one ovary had been removed before sexual maturity (one-ovary females). Both groups showed an initial increase in litter size, followed by a plateau period, and finally a period of linear decline in which the mean number of young born decreased by one in each successive litter. A survey of other published data on long-term reproductive performance in polytocous mammals shows that this pattern is the usual one among mice and rats, but that in pigs the decline in litter size in later life is much less marked. The mean number of young in the first litter was not significantly different in the two groups. After the first litter, number of young was consistently lower in the one-ovary than in the two-ovary group, and the linear decline of litter size started sooner. The number of corpora lutea in the ovaries at autopsy, 16 weeks after the last litter had been born, was high, and the number in the remaining ovary of the one-ovary animals did not differ significantly from the number in both ovaries together in the two-ovary group. We conclude that the lower mean litter size in the one-ovary group during the plateau period is an effect of prenatal mortality due to the presence of double the normal number of embryos in a single uterine horn. In both groups, the decline in litter size in the second half of reproductive life, which limits the total reproductive output of the female, must be due to a factor causing increased embryonic loss rather than a diminished supply of eggs. Ageing of the uterus itself is probably involved, leading to increased embryonic mortality both at and after implantation. The earlier onset of the decline in one-ovary females would then be due to overloading and premature ageing of the single functional uterine horn. In the pig, the regulation of total reproductive output is probably not primarily uterine.
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