The ontogeny of exploration in the rat: habituation and effects of handling.

Exploration, measured by head-dipping in a holeboard, was tested in male rats at 16, 21, 28, 56, and 84 days of age. Head-dipping increased with increasing age from Day 16 to Day 84, and so did the number of rears made. At all ages handled rats showed more head-dipping and made more rears than did their unhandled litter-mates. The age differences in exploration cannot be explained by different patterns of habituation: rats showed significant within-session habituation of head-dipping at all ages tested (Days 16, 21, and 28). Day 16 rats also showed significant between-session habituation.