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  • Author: S. A. BARNETT x
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Mice of strains A2G and C57BL were kept in male-female pairs throughout their breeding life, (a) in an environmental temperature of -3° C, (b) at 21° C (controls). The mice at -3° C had an ancestry of several generations in the cold and had a much lower nestling mortality than mice recently introduced into the cold. In both strains, breeding began later and intervals between litters were longer in the cold than in the warm. The A2G mice produced and reared about the same number of young in the two temperatures; however, the breeding of the controls ended at about 40 weeks, while that of the mice in the cold was not completed until 80 weeks. The C57BL mice produced just over half as many young in the cold as in the warm: their breeding ended at about 52 weeks in both temperatures. Adrenal weights at 80 weeks were higher in the cold in C57BL mice but not in A2G. Kidney weights were higher in the cold in both strains, but especially C57BL. Spleen weights were lower in the cold in both strains; this may reflect a heavier infection with a pathogen in the control mice and is possibly relevant to the remarkably low nestling mortality in the cold.

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Mice of strain A2G/Tb, of a stock adapted to an environment kept at −3° C, were fostered shortly after birth on to A2G/Tb mice breeding at 21° C. The parents of one group (a), were transferred to 21°C before the birth of a litter; those of a second group (b), were left in the cold. The group (b) young, which were exposed to cold for a few hours after birth, were heavier at 10 and 21 days than those of group (a). Mice of both groups were mated : those of group (b) had a lower death rate among their nestlings than those of group (a). Offspring of group (b) were also mated; the mortality among their nestlings resembled that of group (a) nestlings.

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Four classes of A2G/Tb mice were studied: (i) controls kept in an environmental temperature of 21° C; (ii) mice transferred at mating from 21 to −3° C; (iii) mice of the first generation reared at −3° C; (iv) mice of the twenty-fifth to the twenty-ninth generations of a stock kept permanently at −3° C. Of each class, some were killed, with their litters, at either first or second parturition, and some on the 10th day of either the first or second lactation. Litters kept for 10 days were reduced to four young at birth.

The females at their first parturition were compared with virgins of a previous study. Pregnancy was accompanied by increase in the relative weight of heart, liver and stomach, and in the length of the small intestine; but the small intestine lost weight, and there was no change in kidney weight. The percentage of collagen and calcium in the body declined, but the absolute amount of nitrogen in the body increased.

During lactation, the liver and stomach, but not the heart, increased in relative weight still further; and the weight of the small intestine also rose. The percentage of fat and calcium in the body declined.

The relative weights of the heart, kidneys and small intestine were higher in the cold environment than at 21° C, but liver weight was not affected. The proportion of fat, nitrogen and collagen in the body was lower in the cold.

The young had a much higher proportion of fat in their bodies at 10 days after birth than the neonates. They also contained more collagen and calcium and less water per unit body weight. The main effect of cold on the young was a higher fat content at birth.

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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° C, another at —3° 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° C, the C57BL mice had a mode at 24/25 days after lost litters, and at 26 days after surviving litters; at —3° 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° C. The influence of lactation was greater at —3° C than at 21° 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° C) was higher at —3° C, but only when the previous litter survived. At 21° 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° C than at 21° C.

It is suggested that the supply of foodstuffs for anabolism was a crucial influence in determining the interval between parturitions at —3° C.

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Mice, Mus musculus, were bred in permanently mated pairs, in two environments, at 21° C and −3° C, respectively. Observations on strain A2G/Tb are described in detail; others studied were strain C57BL/Tb, outbred laboratory mice, and wild mice bred in the laboratory.

Second to fourth litters were observed at birth, 10 days and 21 days. Most deaths were due to losses of whole litters. Mortality was higher at −3° C than at 21° C, but not among A2G/Tb mice of a stock bred for many generations in the cold.

Litter weights at birth, for a given litter size, were unaffected by cold, but there were fewer large litters at −3° C than at 21° C; hence mean litter weight at birth was lower in the cold. At 10 and 21 days, litter weights, except those of strain C57BL/Tb, were lower in the cold at most litter sizes.

Individual weight at 3 weeks was unaffected by number of litter mates at litter sizes around the mode, and declined only at high litter sizes; hence the sizes of most litters were within the range which allowed most growth.

The findings illustrate the importance of the uterine environment in determining litter size and survival in the nest.