Ontogeny of thermal and olfactory determinants of huddling in the rat

In standardized tests of huddling behavior, 5-, 10-, 15-, and 20-day-old 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. Two-choice preferences tests, however, revealed dramatic developmental differences in attraction. Younger pups preferred to huddle with the warmer, inanimate target, whereas older pups preferred the conspecific. The emergent conspecific preference appears mediated by attraction to species odors. 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 pups preferred the conspecific. Intranasal zinc sulfate treatment eliminated preference for the conspecific in 15- and 20-day-olds but did not disrupt huddling per se. 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.