Behavioral thermoregulation and immobilization: conflicting demands for survival.

In three studies, core temperatures of immature chicks rose during immobilization induced by simulated predation and fell rapidly when immobilization terminated. Immobilization termination was predicted by proximity of the core temperature to the daily resting core temperature of adult fowl. Although immobilization duration increased with age and was shorter in cold and hot environments, immobilization terminated at the same core temperature at all ages and ambient temperatures. The common termination temperature and pattern of temperature change across all Ss suggests that immobilization is terminated in response to the demand that the rising core temperature be behaviorally maintained within a vital range. The generality of this finding was confirmed in a fourth study with preweanling kittens during exhibition of the transport response--an immobilization reaction with a different evolutionary history. These findings reveal that when immobilization and behavioral thermoregulation present conflicting survival demands for avian and mammalian young, the control of core temperature assumes behavioral priority.