RESUMEN
A series of experiments was performed to examine the determinants of the rat pups' 1st excursions from the maternal nest into the novel environment outside the burrow. A standardized, nidic environment consisting of a maternal nest and adjoining open field was studied under a variety of structured test conditions. When the mother was outside the nest, pups readily ventured into the field. The early excursions were not stimulated by the mother's absence from the nest because when the mother was removed from the nest but not placed in the field the pups remained nestbound. Anesthetized dams elicited few excursions, suggesting that olfactory cues are less potent in this situation than behaviorally transmitted signals. Nonlactating adults were not attractive to pups but after induction of "maternal responsiveness" concaveated males facilitated egression. The data from a final experiment suggest that when in the nest, the dam stimulates egression by providing additional "permissive" stimuli that render the weanling more responsive to approach cues outside the nest.