CONCEPTION AND PARTURITION OF MICE AT TWO TEMPERATURES

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Abstract
Mice of strains A/Tb, A2G/Tb, C57BL/Tb, and F1 generation from the last 2, and a genetically mixed stock have been bred in permanently mated pairs. One colony of each type was in a room kept at 21[degree]C, another at -3[degree]C. The dates of birth of their litters, and the number of young born and weaned (at 3 weeks), were recorded. Information on 2708 intervals between parturitions has been analyzed. The distribution of intervals was always bimodal. The 1st mode, which represented conceptions at post-partum estrus, was at 20 or 21 days, at both temperatures, with the following exceptions at 21[degree]C, the C57BL mice had a mode at 24/25 days after lost litters, and at 26 days after surviving litters; at -3[degree]C, the A2G mice had a mode of 31 days, and the C57BL of 28 days, both after surviving litters. Evidently, the C57BL mice were particularly subject to lactation delay. After postpartum conception, delayed birth, due to lactation, was proportional to the number of young weaned in the previous litter, except in strain A at 21[degree]C. The influence of lactation was greater at -3[degree]C than at 21[degree]C in all 3 inbred strains and also in the mixed stock, when the effect was measured by the percentage of intervals of less than 25 days. But in terms of modal intervals between parturitions only strains A and A2G displayed an effect of temperature. There was no evidence that the cold environment lengthened gestation in the absence of sucklings. Second modes, representing post-lactation conceptions, were at 45 to 50 days. The proportion of post-lactation conceptions (about 25% in inbred strains at 21[degree]C) was higher at -3[degree]C, but only when the previous litter survived. At 21[degree]C percentages of post-lactation conceptions were higher among the F1 and mixed stocks than among inbreds. Dispersions about both modes were greater at -3[degree]C than at 21[degree]C. It is suggested that the supply of foodstuffs for anabolism was a crucial influence in determining the interval between parturitions at -3[degree]C.

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