# Sex Differences in Reaction to Chronic Unpredictable Stress in the House Mouse (Mus musculus musculus) of Wild Origin

## TL;DR

Male and female wild mice respond differently to chronic stress, with females gaining less weight and showing distinct behaviors and hormone levels.

## Contribution

The study reveals sex-specific stress coping strategies in wild-derived mice using chronic unpredictable stress.

## Key findings

- Stressed females gained significantly less body weight compared to unstressed females.
- Stressed males had higher hair corticosterone levels and shorter immobility time in the tail suspension test.
- Females showed prolonged grooming and fewer immobility episodes, indicating different behavioral coping strategies.

## Abstract

Did you know that male and female mice deal with long-term stress in different ways? It is a crucial question that has often been overlooked in science. To find out, we studied wild house mice, and their stress responses are closer to animals in nature. We exposed them to unpredictable stress for five weeks, then compared them to an undisturbed control group. The differences were pronounced. The stressed females gained much less weight than their unstressed counterparts did. Both sexes tended to have heavier stress-related organs, but their behavioral coping strategies diverged. Stressed females showed a conflict between exploring and avoiding risk. Stressed males, however, became more active in resisting inescapable stress. Male mice also produced higher levels of stress hormones. Our work reveals a key insight: There is no single stress response; instead, male and female mice have unique biological and behavioral ways for coping with pressure. These findings argue that to truly understand stress—in mice and, potentially, in humans—we must always consider sex-specific strategies. This means we may need to develop different ways to measure and treat stress for males and females.

Sex differences in stress response continue to be understudied in basic physiological and behavioral research. The current study aimed to investigate the sex-specific effects of chronic stress in wild-derived house mice subjected to chronic unpredictable stress (CUS). The use of wild-derived mice enhanced the ecological validity of our stress model. We applied CUS for 5 weeks based on protocols previously established in laboratory mice, with regular weighting and welfare checks. Control mice were not subjected to stress. After the 5-week exposure, behavioral tests were performed, blood and hair samples were collected for corticosterone measurement, and stress-sensitive organ weights were assessed. Stressed females, but not stressed males, gained significantly less body weight over the entire CUS period. After CUS, mice tended to have higher adrenal and thymus weights. In stressed females, we observed significantly prolonged grooming time in the open field test and fewer immobility episodes in the tail suspension test (TST). Stressed males displayed significantly shorter immobility time in TST. Stressed males, but not stressed females, had significantly higher levels of hair corticosterone, with a similar tendency in plasma. Our results indicate different CUS coping strategies in males and females and raise a question about the development of different protocols for the assessment of stress responses in males and females.