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SUMMARY1. Measurements Of 02 consumption at 9 or 10 temperatures in the 20-40' C ambient temperature range were made on joeys with ages selected to cover the 180-day period of pouch occupancy.2. The rate of 02 consumption of joeys younger than 100 days increased directly with ambient temperature.3. After 100 days of age the 02 consumption rate at low temperatures rose and at about 140 days of age a constant rate was maintained over the full ambient temperature range.4. Heat transfer from joey to mother commenced after 100 days of age. 5. At 150-180 days of age the rate of 02 consumption at 20°C was approximately 12 times greater than at ages less than 100 days. A thermal neutral zone was established in the range 32-36°C by joeys older than 150 days.6. At the usual pouch temperature of 36.50 C, 02 consumption per unit wet body weight rose from 12 ml./kg.min at birth to 17 ml./kg.min at the end of pouch life. On a unit dry body weight basis it fell from 120 to 56 ml./kg. min. This decline parallels the decrease in growth rate.

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Twelve dingos were divided into three groups each containing four animals. One group was warm‐acclimated at +30° to +45°C, the second cool‐acclimated at −20° to − 40°C and a third control group was kept in animal quarters maintained at about +23°C. Three coyotes were also kept in animal quarters but they had free access to outside runs which they used during all seasons. The four warm‐acclimated animals tolerated temperatures of +45°C at 25% relative humidity with a mean rectal temperature of 39.3°C and a mean O2 consumption rate of 8.2 ml/kg min. Cool‐acclimated dingos became distressed at ambient temperatures between +37.5 and +42.5°C and at 40% relative humidity the mean rectal temperature of this group was 40.6°C. The O2 consumption rate of one animal at an ambient temperature of +42.5°C was 16.7 ml/kg min. The mean minimum O2 consumption rate for the warm‐acclimated group in its thermal neutral range was 4.4 ml/kg min and the comparable rate for the cool‐acclimated group was 7.8 ml/kg min, a difference of 77% in their standard metabolic rates. At an ambient temperature of −70°C the mean O2 consumption rate of the four warm‐acclimated animals was about six times the mean rate recorded in their thermal neutral range. Despite this high energy usage all had low rectal temperatures and were frostbitten when they were removed from the chamber. The cool‐acclimated animals increased their O2 consumption rates by a factor of only 2.2 at −70°C when compared with the standard rate in their thermal neutral zone. At −70°C the mean O2 consumption rate of the cool‐acclimated group was only 69% of the warm group mean. With this lower energy usage they maintained their rectal temperature at normal values and did not suffer any frostbite. The weight of fur insulation per unit area increased in the cool‐acclimated animals and in some body areas the mean value was three times that of the corresponding mean for the warm‐acclimated group. When calculated from the slope of the O2 consumption vs. ambient temperature relation below the lower critical temperature the cool‐acclimated group had 64% more insulation than the warm‐acclimated group. The three coyotes had a mean O2 consumption rate at −70°C (17.7 ml/kg min) which was about equal to that of the cool‐acclimated dingos at the same ambient temperature. This rate was 2.4 times the standard metabolic rate in their thermal neutral zone and is about the same proportional increase as that shown by the cool‐acclimated dingos. However the lower critical temperature of the coyotes (−10°C) was much lower than that of the cold‐acclimated dingos (+ 10°C). When compared with other canid studies these results suggest that, even when acclimated to heat, dingos have a tolerance of high temperatures which is little or no better than that of the domestic dog. When held at −70°C cool‐acclimated dingos maintained their body temperatures with moderate energy expenditure but even so were not as metabolically economical as the wolf or the husky but were comparable with the coyote. It is concluded th...