Responses to ecologically relevant stimuli in the rat fetus: interactive effects of milk and an artificial nipple.

Stimuli normally encountered during suckling after birth--the nipple and milk--elicit behavioral responses from fat fetuses on E20. A series of experiments demonstrated that milk, unlike other chemosensory stimuli, consistently evoked mouthing activity during the 15 s after presentation. Unlike other perioral tactile stimuli, an artificial nipple evoked mouthing activity and an active oral grasping response during presentation. Prior exposure to milk altered fetal responsiveness to some forms of perioral tactile stimulation, but did not affect grasping of the nipple. In contrast, prior exposure to an artificial nipple increased mouthing activity in response to milk. The nipple and milk exhibit a temporally asymmetric relationship in fetal rats, which lack experience with suckling stimuli. These findings suggest that responsiveness to ecologically relevant stimuli and their temporal presentation develops before birth, and may facilitate functional behavioral responses of the newborn during the first suckling episode.