Cycles of presence and absence of mother mouse entrain the circadian clock of pups.

Recent findings on neural and endocrine rhythms in infant mice and rats show that maternal coordination has an important role in setting the phase of the developing circadian clock both in the fetus and soon after birth. However, less information is available about the influence of the mother on activity/rest cycles of infants. Separation of the mother from infants in guinea pigs, monkeys and rats results in an increase in sleep disturbance (enhanced activity?). In this context it may be a common feature that during the postnatal period there is enhanced activity of pups during the hours when the mother is not nearby. Conversely, the social influences exerted by the mother while present with her young possibly leads to a relative rest stage. We have now tested this assumption in the night-active mouse Mus booduga. Our study addressed the postulate that the circadian activity/rest cycles of the pups are controlled by cyclic(?) presence and absence of the mother. The results reported here clearly indicate that the circadian locomotor activity of pups kept under continuous illumination or continuous darkness do entrain them to a regime of imposed 12:12-h cyclic presence and absence of the mother. The characteristics of this entrainment confer on the mother mouse the role of zeitgeber.