Albino mouse pups develop homoiothermy during the first 19 days of their life and show three distinct stages during this period. From day 1 to day 6 they are almost completely poikilothermic at low ambient temperatures; from day 7 to day 14 or 15 is a transitional period during which they develop homoiothermy at medium temperatures (e.g. 22°C); from day 15 or 16 to day 19 or 20 is the final stage when homoiothermy at very low ambient temperatures is developed. These changes are very closely correlated with changes in the ultrasounds produced by isolated pups within the 19 day period. Thus their acoustic response to cold is weak from day 1 to day 5 or 6 and very strong from day 6 or 7 to day 12 or 13, on exposure even to medium temperatures. From day 14 onwards the pups progressively stop producing ultrasounds depending on the ambient temperatures, and there is complete cessation of calling after day 19 or 20 even on exposure to temperatures as low as 2°C. In general, decreasing the ambient temperatures increases the rate and intensity of ultrasounds produced but very low ambient temperatures have an inhibiting effect and at 2–3°C breathing and calling cease completely after a time which varies with age of the pups.

With 8 figures in the text)Environmental temperature changes are capable of evoking ultrasonic responses from the young of Wistar rats and Golden hamsters. The changes with age in the intensity of these sounds and the rate of calling are found to be related to the pattern of development of homiothermy in the two species. In the Golden hamster, the same temperature changes also evoke audible calls which show a different pattern of variation with age. These findings are discussed in comparison with those previously reported for albino mice.