Wild house mice, Mus musculus, were bred in (a) warm (control) and (b) cold laboratory environments. More pairs were barren in the cold than in the warm. Fecund pairs in the cold reared fewer young than controls, but their litters were larger at birth and their young were heavier. Nestling mortality declined over ten generations in the cold. Adult mice in the cold were heavier than controls, but not longer. Mice of the tenth generation in the cold, transferred to the warm environment and bred for three generations, were heavier, fatter and more fertile than the controls, but had lighter heart, stomach and kidneys. The secular changes observed were probably genetically determined, but may have included a maternal component.
Wild house mice, Mus musculus L., were trapped, and their descendants reared, in permanently mated pairs, for a number of generations in two laboratory environments, at about 21°C (controls) and -3°C, respectively. All mice had sawdust and cottonwool for bedding; but the nests of those at -3°C were colder than those in the warm, and fluctuated greatly in temperature.
Reproductive performance was inferior in the cold environment: more pairs were barren, and the fecund pairs reared fewer young than the controls. Yet litters at birth were usually larger in the cold, and the young at three weeks were always heavier. Over ten generations nestling mortality declined at -3°C.
From generation 1 on, adult mice at -3°C were heavier than the controls, but there was no corresponding increase in body length. Tails were much shorter relative to body length in the first generations in the cold, but returned to control proportions by generation 10. Most of the structural changes in the cold accord with the “rules” of Bergmann and Allen.
The incidence of abnormal sixth lumbar vertebrae was low in all generations at both temperatures.
After nine generations, some mice were transferred from the cold to the warm environment, and bred for a further three generations. There they outstripped the controls both in reproductive performance and in growth. They also had more fat, and a heavier and longer small intestine; but the heart, stomach and kidneys were lighter than those of the controls. Adrenal weights at 21°C declined over the generations, but those of the mice at -3°C did not.
The secular changes observed, especially those in the cold environment, are attributed principally to differential selection of genotypes, not to inbreeding; but maternal effects may also have been involved.