Mice of strain A2G, living and breeding in an environment at -3°C., have a resting metabolic rate about four times that of controls at 210 C. The corresponding figure for mice of strain C57BL is more than three times that of the controls. In the warm environment C57BL mice have a higher metabolic rate than A2G mice, but in the cold the A2G mice have the higher rate.Abdominal adipose tissue per unit body weight is much lower in the cold in strains A2G and A, but not in strain C57BL. Total body fat in strain A2G is much lower in the cold than in the warm, but is not significantly different in strain C57BL. Strain differences in fat content parallel differences in growth: C57BL mice are lighter than A2G and A mice in the warm but not in the cold; A2G and A mice are lighter in the cold than in the warm, while C57BL mice show no significant difference in the two temperatures.In adipose tissue and body fat, mice of strains A2G and A, but not C57BL, are more variable in the warm than in the cold. A hypothesis is advanced to account for this.

Mice of four strains were reared at 21°C. and then transferred to ‐3°C., in individual cages with nesting material. Controls were kept in similar conditions at 21°C. Many of the mice of two A strains died when stressed by cold at the age of 3 weeks, but none at 5 or 12 weeks. Many C57BL and GFF mice died when stressed at 5 weeks. Death usually occurred during the first 3 days, often on the first day. All stressed mice lost weight, especially in the first 2 days; if they survived, they made up the lost weight but they grew more slowly than the controls at all stages. Mice reared at ‐3°C. and stressed in that temperature when 3 weeks old had a lower mortality than those transferred from 21°C., and survivors did not lose weight. Mice reared at ‐3°C. and transferred to 21°C. at 3 weeks, although initially lighter, grew quickly and after 1 week were as heavy as the controls. Three‐week‐old mice stressed at ‐3°C. lost liver glycogen quickly whether they had been reared at 21°C. or ‐3°C. This was not due to a failure to eat, and there was no evidence that death resulted from inability to mobilize carbohydrate reserves.

Mice of strains A , A2G and C57BL have been bred to the age of 28 weeks at three temperatures: 21 °C (controls), 10 °C and ─ 3 °C. Breeding at 21 °C. In the control temperature the C57BL mice bore more young per pair than either of the A strains, but a higher mortality in the nest gave them a yield of young weaned at 3 weeks similar to that of the A strains. Body weights of C57BL young at 3 weeks were lower than those of the A strains. Breeding at 10 °C. In the intermediate temperature mice of strain A had fewer litters than those of the same strain at 21 °C, but otherwise their reproductive performance was unaffected. The reproduction of mice of strain A2G was unchanged. The C57BL mice bore more young per pair, and weaned more young per pair, than the C57BL controls. The weights of the 3-week young, of all strains, were hardly different from those of the controls. Breeding at ─ 3 °C. In all three strains breeding began later at the low temperature and there was a higher mortality between birth and weaning; consequently, the number of young weaned per pair was one-third to one-fifth of the control figure. Other parameters of breeding were influenced in different ways in the different strains: in particular, ( a ) the number of young born per pair was little changed in strain A , but substantially reduced in the other two strains; and ( b ), complete failure to breed owing to the cold occurred only in strain C57BL . The effect of cold on the weights of the weaned young also varied: in the two A strains, although there were fewer young per litter, the young were lighter than the controls; of the C57BL young, those of the first generation were heavier than the controls, but not significantly so, while those of the second generation were lighter. Litter size. At ─ 3 °C, the number of young born per litter, and weaned per weaned litter, was unaffected in strain A ; but both these parameters were reduced in strains A2G and C57BL . There was no evidence that litter size influenced the body weight of the young at 3 weeks in strain A2G either at 21 °C or at ─3 °C, or in strain A mice at 21 °C. But there was evidence of decline in body weight with increase in litter size in strain A mice at ─3 °C and in C57BL mice at both temperatures. The number of young per litter tended to decline after the third litters in all three strains at both ─3 and 21 °C. Effects of transfer between temperatures . After 5 generations at ─3 °C, mice of strains A and A2G were transferred to 21 °C at 5 weeks and mated; these ‘reversed’ pairs showed no difference in breeding performance from the controls. The young of the A strain reversed pairs did not differ in weight at 3 weeks from those of the controls, but those of the A2G reversed pairs were lighter. Some of the offspring of the reversed pairs were transferred back to ─3 °C at 5 weeks and mated; the number of litters born to these ‘doubly reversed’ pairs was lower than that of mice indigenous to the cold, but the numbers born and weaned per litter were higher. In strain A , the young of the doubly reversed pairs were heavier than those of pairs indigenous to the cold. Variation . Variation was usually higher in the cold environment than in the warm, especially in those parameters of reproduction which were most affected by the cold. This did not apply to weights at 3 weeks, probably owing to the death of the extreme variants before weaning. General . The mice at ─3 °C were breeding in conditions of continued mild ‘stress’. The main effects on reproduction were the delay in the onset of breeding, the high infant mortality and the greater variation of performance.

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