Pregnant mice were placed in rooms at three different environmental temperatures: hot, temperate and cold. The hot and cold environments were less favourable than the temperate as judged by incidence of both prenatal and postnatal mortality of the young. The growth and development of the young was studied during the first 4 weeks of life.
Body weight was expressed on the logarithmic scale throughout, and since males were found to be significantly heavier than females the two sexes were considered separately. The inverse relation between body weight and number of mice in a litter, reflecting competition between litter mates, was particularly marked in the cold and was still increasing 4 weeks after birth. In the hot and temperate environments the effect reached a maximum at 2-3 weeks of age.
When allowance had been made for the effect of litter size on body weight, no significant differences in rate of growth or development were found between the mice in the hot and temperate environments. Both growth and development were markedly retarded in the cold.