Olfactory preference for mother over home nest shavings by newborn rats.

The developmental course and behavioral processes by which infant rats come to prefer one of two prominent natural odors within their rearing environment, those of their mother and home nest shavings, was studied. Pups as young as 4-5 days and as old as 9-10 days prefer their mother's odor that of their home nest shavings in a two-choice test chamber; pups deprived overnight prior to testing express that preference more strongly than nondeprived littermates when the mother is awake, but not when anesthetized. Encountering maternal and shavings odors in a two-choice paradigm engages a behavioral sequence that progresses from arousal and active sampling of both odors during the establishment of preference to increased mouthing after the choice has been made. A .5 degrees C thermal gradient augments the maternal preference response from 4-5 days of age on, and is necessary to it in 2-day-olds. One-day-old pups do not express a preference for their mothers in our test apparatus on the basis of odor even with the addition of a .5 degrees C thermal gradient, suggesting that preference for the odor of mother over home shavings may be acquired between 1 and 2 days of age.