Two Japanese Corriedale lambs, 6 months old, were used for investigation under field conditions. Rectal temperature and cardiorespiratory behavior were recorded daily throughout the summer of 1955. In another experiment two 11-month-old and three 3-to 4-month-old lambs were exposed to varying environmental temperatures at low relative humidity in a psychrometric room, and their physiological responses were measured in the same manner.
1) Rectal temperature rose and respiration rate increased following the elevation of environmental temperature from 10° to 30°. The highest rise in rectal temperature was observed when environmental temperature was about 25°. A remarkable change in respiration rate generally occurred when enviromental temperature was changed from 20° to 25°. No change in pulse rate could be ascribed to environmental temperature.
2) These results suggest that the critical point of environmental temperature may be about 25°, at which the body-temperature regulating mechanism of the lamb may be affected by heat stress.
3) Changes of rectal temperature were less in 11-month-old lambs than in younger ones. The bodytemperature regulating capacity of the lamb was also recognized at 3 months of age.