Large northern mammals like the eskimo dog or the arctic fox are provided with such good insulation that their thermoneutral zone may extend as far down as the temperature of the coldest winter night. They enjoy a complete insulative adaptation to cold (Scholander, Walters, Hock, and Irving 1950a; Scholander, Hock, Walters, Johnson, and Irving 1950b; Scholander, Hock, Walters, and Irving 1950c; Scholander, Lange Andersen, Krog, Vogt Lorentzen, and Steen 1957). If we consider the smaller animals living in the same climate, however, we find that the insulation dwindles with decreasing size, in some cases down to values no greater than those we find in tropical animals (Palmgren 1944, Scholander et al. 1950a, Irving, Krog, and Monson 1955). The northern species do not have an elevated basal heat production compared to their tropical relatives and as a consequence their critical temperature is well above zero, as for example +20'C for red squirrel, +15'C for lemming, +20'C for wild Norway rat and + 100C for snow bunting. All of these were measured in arctic Alaska in the wintertime. Whenever the environment goes below the critical temperature, the animals must increase their inetabolic rate to maintain their body temperature. This situation prevails most of the wintertime. The extra heat may readily be supplied in the daytime by locomotor and other activities of the animal. The question is, however, what happens during the night time when the animal is at rest and asleep and the temperature is even lower than in the day ? To meet this situation most small mammals and some birds avoid the cold night by seeking shelter. Ptarmigans and snow buntings dig down under the snow on cold nights. Chickadees spend the night in hollow trees, bird-boxes and similar places, but many small birds evidently spend the night in the open. Huddling is a common occurrence in small mammals such as mice (Sealander 1952). L6hrl (1955) describes treecreepers (Certhia brachydactyla) gathering into tight bunches of 10-20 individuals during cold nights. When rats are exposed to cold stress for a long time they develop a significant metabolic acclimation to cold (Hart 1957). This increased