Long-term behavioral and physiological consequences of developmental group size history in mice

The social environment during development is a critical determinant of adult mental and physical health. While extreme conditions like social isolation are well-studied, the long-term impact of variations in group size during the post-weaning developmental period remains less understood. This study investigates how developmental social density shapes adult behavioral and physiological reactivity, specifically examining whether behavioral phenotypes induced by housing in a larger group persist after the group size is reduced in adulthood. Male C57BL/6 N mice were housed in groups of either six (6-3 group) or three (3-3 group) from postnatal weeks 3-8. At week 8, the number of mice in the 6-3 group cages was reduced to three to match the 3-3 group. Physiological and behavioral assessments, including tests for anxiety-like behavior, locomotor activity, sociality, and working memory, as well as serum corticosterone measurements, were initiated from week 9. Mice with a history of being housed in a larger group (6-3 group) exhibited persistent physiological impairments, including significantly suppressed weight gain, reduced forelimb grip strength, and increased pain sensitivity. Behaviorally, however, the 6-3 group displayed a unique phenotype characterized by reduced anxiety-like behavior in the light/dark transition test and consistently increased spontaneous activity in the open field, social interaction, and Y-maze tests. Furthermore, the 6-3 group showed improved spatial working memory performance and significantly lower serum corticosterone levels compared with the 3-3 group. These findings demonstrate that being reared in a larger group during the critical post-weaning developmental period induces a persistent behavioral and physiological phenotype that remains after the group size is matched to the 3-3 group in adulthood. This state is characterized by behavioral hyper-reactivity and neuroendocrine adjustment, representing a long-term scar of the early-life social environment. This study highlights the profound influence of developmental social history on adult traits and provides a framework for understanding how the early-life social environment dictates the long-term stability of physiological and behavioral phenotypes.