Seasonal changes in heat balance of dogs acclimatized to outdoor climate.

Heat production (M), dry heat loss (R+C), evaporative heat loss (E) and rectal temperature (Tre) were measured in a direct calorimeter in female mongrel dogs acclimatized to outdoor climate at Kanazawa (latitude; 36 degrees 35" N), Japan. M and total dry and evaporative heat losses (HL) were minimum at calorimeter wall temperatures (TW) of 26-29 degrees C in summer and 22-26 degrees C in winter (thermoneutral temperature; TNT). The seasonal shift of the lower critical temperature was confirmed. At TW below TNT, the values of M and HL were significantly higher in summer. At TW above TNT, these values increased. A TNT and above, M and HL were significantly higher in winter. (R+C) decreased linearly with increasing TW in both seasons. AT TW below 26 degrees C, (R+C) were significantly higher in summer. At TW above 26 degrees C, E increased greatly. The values of E were significantly higher in winter at TW 29-32 degrees C. Tre remained nearly constant at TNT and below, and increased at TW above TNT in both seasons. Mean body surface temperature (Tsf) decreased with decreasing TW. Body thermal conductance (K) was minimum at TW below 26 degrees C in summer and at TW below 22 degrees C in winter. At TW above these temperatures, K increased significantly. Whole body insulation (I) was significantly higher in winter, particularly at TW 18 degrees 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.