Environmental enrichment enhances inter-individual variability in nest-building behavior in male mice.

Environmental enrichment (EE) affects brain function and behavior in laboratory animals. In this study, we investigated the behavioral and physiological effects of short-term EE exposure on male C57BL/6J mice. Juvenile mice were housed for 2 weeks under either EE or standard environment (SE) conditions, followed by a series of behavioral tests. Compared with SE mice, EE mice displayed greater variability in nest-building behavior, with some individuals exhibiting delayed initiation. In the open field test, EE mice spent significantly more time in the center zone, suggesting reduced anxiety-like behavior. In contrast, in the elevated plus maze test, EE mice spent less time in the open arms than SE mice. No significant group differences were observed in sucrose preference or tail suspension tests. Although body weight remained unchanged between the groups, EE mice exhibited significantly increased spleen weight and decreased epididymal fat weight, indicating modest physiological changes. These findings demonstrate that EE can enhance inter-individual behavioral diversity, particularly in goal-directed behaviors such as nest building, which emerged as a sensitive and ethologically relevant behavioral outcome for detecting environmental modulation and individual variability. This diversity may serve as a useful indicator for identifying depression-like phenotypes and understanding the vulnerabilities of EE-reared individuals.