Ontogeny of behavioral arousal in rats: effect of maternal and sibling presence.

Spontaneous locomotor activity of 5- to 30-day-old rats housed either alone or in the presence of various components of the normal nest and litter stimuli was recorded by means of time-lapse videography. Developing animals observed alone showed a sharp increase in total daily locomotor activity from Day 5 to Day 15 followed by a rapid decline from Day 15 to Day 30. Individual animals observed in the context of the normal litter environment showed an entirely different pattern of development. They were relatively inactive during the first 15 days of life and then began a gradual increase of activity which continued for the next 15 days. The heightened activity characteristic of the isolated 15-day old inhibited by the presence of four siblings, an anesthetized lactating female, or an anesthetized adult male rat. Thermal conditioning, including heating of the floor to approximately nest temperature or the presence of a heated tube in one corner of the test apparatus, did not inhibit the heightened activity. These results question the generality of the ontogenetic sequence of excitation and inhibition proposed by Campbell and his associates - at least a portion of the heightened activity seem around 15 days of age was the result of isolation distress, not merely maturational changes in the brain.