Body heat loss was attenuated and oxygen consumption was reduced by huddling in litters of developing rats. Rat pups derive physiological benefits from huddling similar to those enjoyed by adult mammals; these findings contrast with previous characterization of the altricial rat as poikilothermic. Huddling insulates by lessening the explosed body surface area of the participants, thus retarding heat loss and enhancing the efficiency of thermogenesis. These physical mechanisms of the clump are actively regulated by the pups. A novel quantitative measure of huddle size revealed a form of group regulatory behavior in rat pups whereby the surface expanded and contracted with increases and decreases in ambient temperature. The individual basis of this group regulatory activity was investigated by marking individual pups and observing them in huddles by means of time-lapse videography. It was found that individual animals circulate throught the huddle, frequently exchanging locations in the group. By studying the huddle positions of an anesthetized pup and a marked control sibling, dynamics of the pup flow were clarified. Ordinarily, the direction of movement was actively downward, into the pile; immobile pups "floated" on the surface. When the nest temperature was raised to thermoneutral, the direction of pup flow reversed and an immobile animal sank to the depths of the huddle. Through individual competitive adjustments the huddle behaves as a self-regulating unit which provides warmth and insulation to all its active members.