Various factors concerned in the regulation of litter size and total reproductive output in mice have been investigated. Length of daylight did not significantly affect litter size. The decline of litter size as the animals aged was associated with a high level of embryonic loss after implantation. Ligature of the Fallopian tube on one side or unilateral ovariectomy caused total reproductive output to be about halved. In the latter case, initial litter size was about three-quarters of control but decline of litter size started earlier and reproductive life-span was shorter, whereas in the former case, litter size was approximately half control level throughout but reproductive life-span was approximately equal to that of the controls. Reproductive performance of mothers that had been kept virgin or continuously pseudopregnant for the first 9 months and then paired with fertile males did not differ significantly from the performance over an equivalent period of mothers bred continuously from puberty.
The cause of the decline of reproductive capacity with age is discussed. It is suggested that it is due to the declining ability of the uterus to maintain pregnancy, associated primarily with chronological ageing.
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