Maternal Exposure to Witnessed Social Defeat Stress Induces Sex‐Specific Disruptions in Juvenile Social Play Behavior in Rats

ABSTRACT Stressing rodent dams during lactation can impair offspring neurodevelopment, altering behavioral and physiological outcomes. Witnessed social defeat stress (WSDS) is a paradigm in which a juvenile or adult rodent observes aggressive encounters, affecting stress physiology and behavior. We developed a novel early‐life stress model by exposing lactating rat dams to WSDS and assessed its impact on juvenile social behavior and stress‐related physiology. At postnatal day 24, same‐sex, age‐matched pairs of offspring were tested for social play by scoring pouncing (play initiation), pinning, chasing, and evading. Locomotor activity was assessed in the open field on the same day. After behavioral testing, adrenal cortex weight was measured as an indirect marker of stress‐related physiology. Maternal WSDS exposure was associated with a significant sex × WSDS interaction in pouncing behavior, with reduced pouncing in juvenile males, whereas no comparable effect was observed in females. WSDS exposure also increased adrenal cortex weight overall, although the sex × WSDS interaction for this measure was not significant. The reduction in pouncing was characterized by substantial within‐group variability, and randomization‐based sensitivity analysis indicated that the statistical significance of the sex × WSDS interaction was sensitive to focal‐animal assignment. These findings suggest that maternal WSDS exposure may produce a specific and variable alteration in social play initiation, together with changes in stress‐related physiology, during early juvenile development. This model may provide a useful framework for investigating how maternal psychosocial stress influences offspring behavioral and physiological development.