Laboratory mice (Mus musculus L.) have been bred at an environmental temperature of - 3° C, with controls at 21° C. The mice were given cotton-wool for nesting.

Body temperatures, taken in the colon, of adult control mice were about 38° C.; those of mice reared in the cold were slightly lower. Skin temperatures, taken below the hair, and temperatures on the surface of the hair, were lower in the cold than in the warm. Surface temperatures were, nevertheless, high (30° C. for females in the cold), indicating a high rate of heat loss.

Mice less than 3 weeks old have lower and more variable body temperatures than adults.

Better nests are built in the cold than in the warm. Nest temperatures, taken among nestlings less than 7 days old, are high (up to 34.5° C.) in the cold as well as in the warm; but nests and nestlings in the cold are subject to sharp declines in temperature when both parents leave the nest to feed.

Adults reared in the warm were transferred to cages in the cold without cotton-wool; most of these mice died within 48 hr. Similar mice, reared in the cold, and transferred to cages without cotton-wool, survived for some weeks, though not indefinitely. The difference reflects the state of partial metabolic (endothermic) adaptation achieved by mice reared in the cold.

The results in general show that both endothermic and ectothermic adjustments play a part in the maintenance of mice at - 3° C. The ectothermic or behavioural adaptations are those involved in the efficient building and utilization of nests.

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