Emergence of the exploratory motive in rats

As infant rats approach weaning, they must overcome their infantile attraction to the home nest and prepare to leave its safety to forage for food. Although nutritive needs may help to motivate pups to make this bold move, a newly emerging exploratory motive also may play a role. Three experiments are reported here which examine age-related changes in the exploratory motive. In the first experiment, preferences for a novel side over a familiar side of a two-sided testing chamber were examined in postnatal Days 15 (P15), 17 (P17), 19 (P19), and 21 (P21) rat pups. Subjects did not prefer to explore the novel side until P19. In the same test, no preference for a novel object was observed at any of the ages tested, suggesting that this preference, which is expressed in adulthood, does not emerge until a later age; however, additional studies are needed to verify this. In the second experiment, P15 subjects demonstrated their ability to discriminate the two sides of the testing chamber in a novelty-induced locomotion test. In the third experiment, the presence of a home-nest-associated object was shown to inhibit exploration of novel context in P15, but not P21, subjects. These data suggest that home nest egression may be more the consequence of the declining attractiveness of the home nest than of a growing exploratory motive.