When kept at ambient temperatures of 17-5 and 24 degrees C the colonic temperatures of joeys younger than 166 days declined to near ambient temperature. Pouch joeys of 166 days and older were however able to maintain their colonic temperatures at about 35 degrees C. 2. Joeys first developed the ability to sustain high O2 consumption rates in response to cooling, when aged between 144 and 169 days. Only when this latter facility was fully developed in ontogeny could body temperature be maintained in a cool environment. 3. When joeys older than 130 days were kept in a metabolism chamber at pouch temperature (37-5 degrees C) and at high humidity their body temperatures quickly rose to lethal levels, demonstrating the need for cooling mechanisms whilst still contained within the pouch.
