Birth spacing in the mouse communal nest shapes adult emotional and social behavior

The interactions with the mother and with peers are among the most relevant early environmental factors shaping adult brain function and behavior. In order to investigate the role of these factors, we exploited a novel early manipulation, the Communal Nest (CN), consisting in a single nest where three mothers give birth, keep their pups and share care-giving behavior from birth to weaning. In particular, we reared CD-1 swiss mice in three different CN conditions, each one characterized by a different interval between the three deliveries (Birth Spacing) of 3, 5 or 7 days (respectively, CN ± 3, CN ± 5, CN ± 7). Length of birth spacing affected maternal behavior, CN ± 7 pups receiving the highest levels. At adulthood, mice reared in the different conditions showed differences in emotional response and social skills. In the plus maze test, short birth spacing was found to be associated with enhanced emotionality, CN ± 3 mice showing highest levels of anxiety-like responses in the plus maze compared to the other two CN groups. In the social interaction test, the strategies to achieve dominance differed among the three groups. While CN ± 3 mice appeared to have a more aggressive strategy, displaying high levels of attack behavior in the first encounter, CN ± 5 and CN ± 7 mice displayed a more affiliative strategy based on social investigation. Overall, these findings show that birth spacing shapes the early mouse social environment and, in turn, affects the development of social skills and emotional responses.