Some Effects of Cold and of Hybridity on the Growth of Mice

Mice of strains A and A2G, born and reared at an environmental temperature of − 3°C, were lighter at the ages from 3 to 16 weeks than controls at 21°C. The body-weight of mice of strain C57BL was unaffected by the low temperature. F1 mice from the cross A × C57BL were heavier than those of the parent strains at both temperatures. Mice from the cross C57BL × A2G, and from the reciprocal cross, were intermediate in weight between the parent strains in the warm, but superior in the cold. The growth of F1 mice was not depressed by cold. Outbred mice, derived from crossing the above inbred strains and G FF, were selected for superior breeding performance in the cold environment. After twelve generations these mice were heavier than controls kept at 21 °C. A2G mice transferred from −3°C to 21 °C at 3 weeks were lighter at that age than controls kept at 21 °C and failed to make up the growth deficit. Their offspring, too, kept at 21 °C, were lighter than the controls. Outbred mice of the twelfth generation in the cold were transferred from − 3°C to 21 °C at 5 weeks. Their offspring, reared at 21 °C, were heavier than the controls which had been bred permanently in the warm; they were also heavier than the outbred mice kept in the cold. This was evidently a result of selection. In all strains the males were heavier than the females. Weights of kidneys, relative to body-weight, were observed in A2G, C57BL and outbred mice. In all three strains they were higher at -3°C. Of the same strains liver weights were raised in the cold in C57BL and outbred mice but not A2G. There was evidence that these two inbred strains had lighter gonads in the cold; but the outbred mice of the twelfth generation had heavier testes in the cold than in the warm. There was no evidence of an effect of heterozygosis on variance in bodyweight. There was some tendency for growth to be more uniform in the cold than in the warm environment.