The present study was intended to evaluate the hypothesis that the nursing-suckling relationship of rats proceeds from the mother-leading-type in early postpartum days to the infants-leading-type in later postpartum. Four groups were organized to make tests on the effects of different kinds of mother-infant separation (8-hr) upon nursing-suckling behaviors. Groups were Groups MIS (a separated mother and infants group), MS (mother-separated and infants-fostered group), IS (a foster-mother and separated-infants group), and C (non-separated control group). The separation of each group, lasting for 8-hr, was made at 1-2, 6-7, 11-12, 16-17, 21-22, 26-27, and 31-32 days after parturition. In Exp. I, nursing behavior of mother and suckling and/or eating behavior of infants in mother-infant interactions were observed by time-sampling method for 30 min following 8-hr of separation. In Exp. II, the suckling test of infants (half of the litter) was made on the anesthetized mother at the presence of food pellets. The results of Exp. I showed that nursing-suckling episodes were more frequent in Groups MIS, MS, and IS than in Group C (Fig. 1). Further, the differences among the first three groups were not significant before 11-12 days of age. However, the infant-separated group (Group IS) showed higher level of response at 16-17, 21-22 days of age, suggesting the active role of infants in starting nursing-suckling episodes at these ages. These results suggest that the hypothesis previously mentioned regarding the active role of mother and infants in nursing-suckling relationship is not valid at early stage. Namely, high frequency of nursing-suckling episode in Group IS implies that the separated infants may have activated maternal behavior by changing their stimulus characteristics at these ages. According to the results of suckling shown to the anesthetized mother (Exp. II), separation of infants was the main determinant of the suckling behavior under these conditions of controlled maternal behavior (Figs. 3, 4). Finally, separated infants preferred eating to suckling after 26-27 days of age both in the mother-infant interaction and in the suckling test (Figs. 2, 5, 6). This finding indicates that the weaning process of rats follows next four stages: spontaneous eating of solid foods (15-16 days), equal level of eating and suckling (21-22 days), predominance of eating (26-27 days), and finally disappearance of suckling (31-32 days).
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