Responses to hypothermia in several species of infant mammals.

N EWBORN mammals survive body temperatures that cannot be tolerated by adults of the same species. Edwards (I) showed that infant cats and dogs could be cooled until their rectal temperatures were below 13’ C.; adults could not survive. The exact conditions of temperature and of oxygen required for survival have been worked out for infant rats (2). The infant rats could cease breathing movements, heart beats, and all other detectable signs of life for I to 2 hours and still recover when warmed (3). Our aim was to investigate the progressive limitation of tolerance to low body temperatures during ontogeny. What changes of physiological properties are involved in the loss of the tolerance of infancy? Hypothermic animals are now found to exhibit reproducible relations of tissue temperatures to frequencies of heart beats and of breaths, which differ between infant and adult; hence it is evident that those relations shift in transition from infancy to adulthood. The present investigation was planned to define those relations as they became modified during ontogeny. Golden hamsters, rats, and cats were chiefly used for these quantitative studies; rabbits, mice, and guinea pigs in fewer numbers were included. In rats the adults were found to die at 14’ to do, while the newborns can endure I' for at least one-half hour. During the first three weeks of life (2) the transition to the adult tolerance occurred, In cats the transition both in tolerance and in heartbeat frequency is here found to be slower. In hamsters, the adults themselves withstand body temperatures less than 5’ C.; then, do the young of this species differ from their parents? Studies were planned so that development of the individual infant animal progressed under standard conditions, all individuals being cared for by the mothers. Then each infant +vas removed from the mother for cooling through a wide gamut of temperatures. Most individuals survived the tests and were returned to the mother. Some were re-used at two or three different ages. The limits of low temperature that infants of each species can survive seem to be related to the patterns of heart-beat frequencies and breath frequencies within the range of body temperatures that they tolerate. Each animal was bound, with adhesive tape about each leg, to a suitable frame.