Acute and vicarious effects of social defeat stress on social behaviour in California mice,

Agonistic social interactions are a significant source of stress across many species. Stress responses can have adverse effects on the brain and body, so avoiding stressful social situations can be advantageous. There is increasing evidence that, in social species, individuals can detect threatening social situations by observing others in their social group, a phenomenon referred to as social contagion. In this study, we tested the extent to which olfactory cues or physical interactions are used to transmit stressful states in California mice. We found that a single episode of social defeat reduced social approach and increased social vigilance in both male and female mice. This result is unlike previous studies that examined the long-term effects of social stress in California mice, in which the effects of stress on social behaviour were stronger in females. Neither volatile nor nonvolatile olfactory cues from stressed individuals were sufficient to alter the behaviour of familiar cagemates. In a second experiment on male California mice, we observed that mice that witnessed defeat and physically interacted with a defeated cagemate showed reduced approach to a novel empty cage. Effects were weaker in social contexts. These results suggest that, in California mice, more direct interactions with a stressed individual are required to induce social contagion effects. These results have important implications for how social defeat research is conducted and informs future studies to examine differences in activation of neural circuitry between males and females.