Mice of strains A/Tb, A2G/Tb, C57BL/Tb, the F1 generation from the last two, and a genetically mixed stock have been bred in permanently mated pairs. One colony of each type was in a room kept at 21\s=deg\ C, another at \ p=m-\ 3\ s=deg\ 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 analysed.The distribution of intervals was always bimodal. The first mode, which represented conceptions at post-partum oestrus, was at 20 or 21 days, at both temperatures, with the following exceptions : at 21\s=deg\ C, the C57BL mice had a mode at 24/25 days after lost litters, and at 26 days after surviving litters; at \ p=m-\ 3\ s=deg\ 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\s=deg\C. The influence of lactation was greater at \ p=m-\ 3\ s=deg\ C than at 21\s=deg\C in all three 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\ s=deg\ C) was higher at \ p=m-\ 3\ s=deg\ C, but only when the previous litter survived. At 21\s=deg\C percentages of post-lactation conceptions were higher among the F1 and mixed stocks than among inbreds. Dispersions about both modes were greater at \ p=m-\ 3\ s=deg\ C than at 21\s=deg\C. It is suggested that the supply of foodstuffs for anabolism was a crucial influence in determining the interval between parturitions at \ p=m-\ 3\ s=deg\ C.
Reproduction and food consumption have been studied in three classes of female A 2 G/Tb mice: (i) controls , kept at 21 °C; (ii) mice, reared in an environment at –3 °C, whose parents or grandparents had been transferred, as young adults, to the cold environment ( new stock ); (iii) old stock , of the seventeenth to nineteenth generations reared at –3°C. As virgins (at 8 weeks), and also during pregnancy and lactation, both the new and the old stock at –3 °C were lighter than the controls. As virgins, the old-stock females were about the same weight as the new; but they put on more weight during their first pregnancy, and maintained their weight better during lactation. During second lactation the old stock were much the heavier. The mice of both groups at –3 °C had fewer young than the controls in their first litters, and a higher mortality among their nestlings; but the old stock had a lower nestling mortality than the new. The difference between the second litters of the old and new stocks was greater: more of the former produced second litters; the number born in old-stock litters was higher; and mortality was again lower. Relative to body-weight, the virgin females of both stocks at –3 °C ate about 70% more food than controls. The increase in food consumption during pregnancy and lactation was, however, lower at –3 °C than at 21 °C, both absolutely and relative to body-weight. Old-stock females ate about the same amount of food, relative to body-weight, as those of the new stock, even during their second pregnancy and lactation, when they were more successful mothers than the new stock. The unexpectedly low consumption of food at –3 °C may be related to ( a ) reduced activity in the cold, ( b ) superior metabolic efficiency. In addition, the old-stock mice, females and young, probably utilize food more efficiently than the new stock.
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