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.