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A series of arctic and tropical mammals and birds at Point Barrow, Alaska (lat. 71° N.) and in Panama (lat. 9° N.) was subjected to various air temperatures in a respiration chamber where the heat production was determined by oxygen consumption or carbon dioxide production. The larger arctic mammals and birds showed no increase in metabolism at — 30° C. and from observations on sleeping animals it is probable that their zone of thermoneutrality extends to — 40° C. or — 50° C. The smaller arctic species show a high critical temperature and the tropical species even higher. Metabolic heat production increases rapidly with lowering of the temperature in a tropical mammal or bird, and slowly in an arctic animal. It can be shown theoretically that in a thermoregulated system with a fixed basal energy level and variable insulation the critical gradient is proportional to the maximal insulation and the basal energy level. In a large series of experiments including our tropical and arctic animals, and all animals affording enough data in the literature, it is shown that the heat loss below the critical temperature is essentially proportional to the body-to-air gradient. This means that the overall insulation evidently reaches a maximum at the critical temperature and from then on the heat loss follows essentially Newton's law of cooling. It follows from this that an arctic mammal with a critical gradient of 70° C., by doubling its metabolism, theoretically would double the gradient. Only 40 per cent increase of its metabolism (or insulation) would suffice to take it down to — 70° C. which is near the lowest recorded temperature on earth. The very broad zone of thermoneutrality in the larger arctic species, from + 30° C. to — 40° C., shows their ability to balance an 11-fold increase in gradient and hence the animal can change its heat dissipation by a factor of 11 even when lying down. It is believed that vasomotor control of the poorly insulated legs must play an important role in the general thermoregulation of these animals. In the tropical mammals and birds the critical gradient is low, often only 10° C., which makes them sensitive to even small temperature changes. A 10° lowering of the air temperature from the critical temperature doubles the gradient for the tropical mammal; a 9° increase decreases the gradient 10 times, and in order to maintain the body temperature tropical animals must be able to adjust insulation and metabolism in the same proportion as the gradient. They are thus extremely sensitive to temperature changes. The whole range of heat regulation from tropical to arctic mammals and birds is represented on two charts, Figures 10 and 11.