Ontogeny of spontaneous locomotor activity in rabbit, rat, and guinea pig

Postnatal locomotor activity was investigated in rabbit, rat, and guinea pig. A peak was found in this behavior at 5 days of age in the rabbit and, in confirmation of earlier studies, at 15-20 days in the rat. Neonatal guinea pigs showed no variation in activity levels due to age. The guinea pig is born at an advanced stage of neural maturation in comparison with the rat, and the rabbit is born at an intermediate stage. This difference accounts for the observed variations in postnatal activity profiles if it is assumed that the behavioral peak coincides with the transition from a stage of primarily mesencephalic arousal system maturation to a stage of primarily diencephalic and telencephalic inhibitory system maturation.