In Exp I, in standardized tests of huddling behavior, 64 5-, 10-, 15-, and 20-day-old Sprague-Dawley rat pups spent substantial and equivalent amounts of time with an immobile rat or a heated, fur-covered tube, which suggests that the conspecific and inanimate stimuli were equally attractive to the pups. In Exp II, with 32 Ss, 2-choice preference tests showed developmental differences in attraction. Younger Ss preferred to huddle with the warmer, inanimate target, whereas older Ss preferred the conspecific. The emergent conspecific preference appears mediated by attraction to species odors. In Exp III (64 Ss), the 5- and 10-day-olds huddled equally with an immobile rat and an immobile gerbil (stimuli with similar thermal and tactile properties), but older Ss preferred the conspecific. In Exp IV, with 16 15-day-old and 16 20-day-old Ss, intranasal zinc sulfate treatment eliminated preference for the conspecific but did not disrupt huddling per se. Results from all experiments show that thermal cues were sufficient to elicit huddling at all ages, but olfactory cues became a more salient influence before weaning. An ontogenetic transition from "physiological" to "filial" huddling is discussed in terms of changes in sensory control of early behavior. (22 ref) (PsycINFO Database Record (c) 2006 APA, all rights reserved)