Seasonal Changes in Heat Balance of Dogs Acclimatized to Outdoor Climate

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Heat production (M), dry heat loss (R+C), evaporative heat loss (E) and rectal temperature (T_re) were measured in a direct calorimeter in female mongrel dogs acclimatized to outdoor climate at Kanazawa (latitude ; 36°35'' N), Japan. M and total dry and evaporative heat losses (H_L) were minimum at calorimeter wall temperatures (T_W) of 26-29°C in summer and 22-26°C in winter (thermoneutral temperature; TNT). The seasonal shift of the lower critical temperature was confirmed. At T_W, below TNT, the values of M and H_L were significantly higher in summer. At T_W above TNT, these values increased. At TNT and above, M and H_L, were significantly higher in winter.(R+C) decreased linearly with increasing. 7; in both seasons. At T. below 26°C, (R+ C) were significantly higher in summer. At TW, above 26°C, E increased greatly. The values of E were significantly higher in winter at TT_W, 29- 32°C. T_re remained nearly constant at TNT and below, and increased at T_W above TNT in both seasons. Mean body surface temperature (T_sf) decreased with decreasing T_W. Body thermal conductance (K) was minimum at T_W below 26°C in summer and at T_W below 22°C in winter. At T_W above these temperatures, K increased significantly. Whole body insulation (I) was significantly higher in winter, particularly atT_W18°C. These results suggest that the dogs reared outdoors in winter acclimatized to cold in two ways ; by increasing the insulating effect of the fur coat and by elevating resting heat production.