Heat production and thermal insulation were measured in three groups of sheep, control, outdoor, and indoor, which were exposed to the effects of season, cold climate, and cold temperature respectively. The experiment was for 24 weeks, from November 1967 to April 1968. Sheep in the control, outdoor, and indoor groups gained 16.2, 10.0, and 14.9 kg respectively. Average feed intake in both control and outdoor groups was 2.65 kg hay/100 kg sheep per day. Food intake was highest in the indoor group and was inversely related to ambient air temperature. Resting heat production was constant throughout in the control group but increased with time in the outdoor group. Resting heat production in indoor sheep was related to intensity of prior cold exposure. Wool growth and thermal insulation did not differ significantly between groups. Critical temperature for the outside group fell from −15 °C in week 4 to −35 °C in week 20. After these trials, all sheep were shorn. The heat production of the shorn indoor group at −30 °C was greater, and that of the outdoor group was less than that of the controls. The results suggest that, in sheep, acclimation to cold temperatures induces rapid, brief increases in resting heat production and summit metabolism. Acclimatization to cold climates slowly induces a persistent elevation in resting heat production, but appears to reduce the initial metabolic response to an intense cold stimulus.

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