Research reportEmotional contagion in mice: The role of familiarity

- Familiar and unfamiliar mice observed conspecifics receiving electric foot shocks.
- Familiar observers freeze more than strangers when witnessing another in distress.
- Demonstrators receiving shocks freeze the most aftershocks have ceased.
- Observers stop freezing when shocks have ceased in the adjacent compartment.
- Demonstrators show less freezing behavior when observer is familiar.

Empathy is a complex emotional process that involves sharing an emotional state with another organism. The extent to which nonhuman animals are capable of empathizing with others is still far from clear, partly due to a lack of empirical work in this domain, but also due to definitional confusion of empathy with emotional contagion and other related terms. In this study, an observer mouse witnessed a familiar cagemate or an unfamiliar non-cagemate receiving electric foot shocks in an experiment that consisted of three periods: baseline (no shocks), test (shocks) and recovery (no shocks). Freezing behavior in the observer was significantly increased in the cagemate, as opposed to the non-cagemate condition during the test period, but not during baseline or recovery, emphasizing the role of familiarity in empathy-like processes. In agreement with this, we also found a correlation that approached significance between the total number of fecal droppings of the observers, as an indication of distress, and those of the demonstrator in the cagemate, but not in the non-cagemate, condition. While the freezing behavior of the demonstrators increased with time, reaching a maximum at the recovery period, the observers froze the most during the test period while the demonstrators were receiving the electric foot shocks. The observation that the freezing response of the observers ceased when the shocks in the adjacent compartment stopped could be due to a decrease in saliency of the demonstrators' behavioral response. Finally, the presence of a cagemate, as compared to a stranger, possibly reduced the demonstrator's pain-induced behavior, suggesting an ameliorating effect of familiarity on stress responses.