Developmental Changes in Nest Egression Behavior and Its Relation to Novel Food Availability.

Nest egression constitutes a major developmental milestone for rats toward independence and is thought to be motivated in part by the need to acquire novel sources of nutrition when milk supply and nursing frequency decline. Here, we tested the hypothesis that nest egression is facilitated by the presence of food outside of the home cage. Spatial location of rat pups was tracked throughout a large environment where they were free to explore away from the home cage through tunnels that gave access to satellite cages. The availability of novel foods in satellite cages was systematically varied on a daily basis. Pup movement was continuously tracked via permanently implanted radio-frequency ID tags. The results show that nest egression first occurred at P12 and increased in spatial extent until P16; at P16, locomotion patterns underwent an abrupt shift toward a mature pattern. The emergence of mature locomotion patterns preceded the onset of food consumption, which was first evident on P17. Preference for locations containing foods emerged only after the onset of consumption; prior to P18, rats avoided locations containing foods. These findings suggest that the emergence of nest egression is not motivated by sensory cues associated with foods and provide insight into the processes that enable independent feeding.