Postnatal development of open field behaviour of male and female rats

In a longitudinal study of postnatal behavioural development each 8 male and female Long-Evans hooded rats were exposed daily to an 60 X 75 X 22 cm open field beginning from the 4th postnatal day (pd) for 30 min. The number of infrared beam crossings, of rearings, climbings, sniffing and grooming periods, of grooming duration, of entering and stay in central fields and of defecation were counted and measured in 5 min periods. All parameters increased continuously or in phases with significant peaks until the 27th pd. Females significantly predominated in the second and the third week in ambulatory activity, in entering central fields and in the frequency of grooming periods and in the third and fourth week also in grooming duration. Males clearly predominated in the development of exploratory activity. Every significant peak in the curves represents the appearance of a new behavioural element which is exercised several times and then integrated in a behavioural stereotype, which becomes more and more enriched. Each peak is followed by a significant minimum. Thus, peaks and minima are highly reproducible in their postoperative timing and express the physiological development of functional systems and stereotypes. The data are a useful and important basis for the study of experimental models of perinatal or early postnatal injury.