Litter size, the mother-infant interaction and adult behaviour in the mouse.

Physical and behavioural development and the interaction between mother and young were examined in C3H mice reared in litters of different sizes. At weaning animals were put either into isolation or into bisexual groups until ten weeks of age when 'emotionality' was assessed using the conventional open-field test and a modified version of this test. The response to a trained submissive animal was also observed. Females rearing smaller litters were found to pay more attention to their young; they spent more time in the nest, more time nursing and more time licking individual pups than mothers with larger litters. They also spent less time eating and drinking outside the nest. The differences were interpreted in terms of the differential rate of development of the pups and differences in the physiological requirements of the mothers. Pups reared in smaller litters grew faster and were heavier at weaning but feu differences were found in the appearance of hair, opening of the eyes, development of the Preyer reflex, or in the nest temperature and structure. The method of sampling may have accounted for the absence of differences on some of these measures. The standard open-field test failed to discriminate between the groups but a modified version indicated that animals reared in larger litters were higher on 'emotionality'. Hales raised in larger litters also responded to a submissive male with less exploratory and aggressive activity. A number of possible interpretations of these differences, including the level of stimulation received from the mother in infancy were suggested. larger litters also responded to a submissive male with less exploratory and aggressive activity. A number of possible interpretations of these differences, including the level of stimulation received from the mother in infancy were suggested. Isolation at weaning was found to decrease 'emotionality' and increase the level of aggression towards a submissive animal. This unexpected finding was interpreted in terms of the relative novelty of the test situation. No consistent interactions were found between the effects of pre- and post-weaning experience.