ADAPTATION OF MICE TO COLD
Authors: BARNETT S. A. 
Published: January 2008

Since 1953 laboratory mice have been bred continuously in an environment kept at – 3° C. Control stocks are kept at 21° C. All mice have cotton wool bedding. The effects of the cold environment are reviewed, and related to observations on other species exposed to cold.

Physiological adaptation

The principal physiological adaptation, of a small mammal exposed to cold, is increased heat production. Inbred mice, fully adapted to – 3° C., expend up to 4–5 kcal./100 g. body weight/hr., or about four times that of controls at 21°C. This is probably a maximum, maintained only outside the nest and for short periods. Colon temperature is unchanged; skin temperature, though high (31 °C. at the surface), is lower than at 21° C. (34° C.).

Virgin female inbred mice at – 3° C. eat about twice as much food, per unit body weight, as controls at 21° C. The difference during late pregnancy is, however, much less; and, relative to body weight, consumption during the first 10 days of lactation is lower at – 3° C. than at 21° C. There are fewer young at ‐ 3° C. The lactating females may eat the maximum they can digest. Food is probably utilized more efficiently at ‐ 3° C. than at 21° C.

Lowered external temperature reduces activity. Mice in a cold environment economize in general exploration of their environment; they also do no ‘functionless’ gnawing of friable materials.

After increased heat production, the most important adaptive response is nest‐building. When accustomed to a cold environment, mice build better nests than in a warm one. But, given unfamiliar nest material, in a novel situation, they build a nest less quickly at – 3° C. than at 21° C.‐another example of economizing in energy.

On exposure to cold, an increase in body weight would be adaptive. Some wild mammals increase their weight and body fat in winter. But adult laboratory mice, after transfer from 21 to ‐ 3° C., lose weight, largely owing to loss of fat. In this they resemble laboratory rats transferred to a cold environment. Similarly, inbred mice reared at ‐ 3° C. are usually lighter, at all ages, than controls at 21° C.

Appendages, especially tails, are usually shorter in a cold thanin a warm environment; in mice the caudal vertebrae are shorter. The extent to which this reduces heat loss is not known. A genetically mixed stock of mice, selected for fertility at ‐ 3° C., developed longer tails during eighteen generations in the cold.

Laboratory rats, after a few weeks at about +4° C., alone, without nests, have heavier liver and kidneys, intestine, heart and adrenal glands, than controls. Exposure in groups, or to seasonal cold, evokes different responses. In cold‐adapted inbred mice of the first or second generations reared at ‐3′ C., stomach, intestine, liver, and heart are heavier than those of controls. Insulation by hair is slightly increased at ‐ 3° C., though the heat‐conserving effect of this is small; the weight of the cropped skin is reduced in inbred mice, but it was unchanged in a genetically mixed stock after twelve generations in a cold environment. There is a higher proportion of body water, and lower collagen, at ‐3° C. Males, but not females, have a lower relative calcium content.