Visual and olfactory signals of conspecifics induce emotional contagion in mice.

Emotional contagion occurs in many animals, including rodents. To determine the social signals of emotional state-matching between individuals in mice, we conducted an empirical laboratory experiment using visual, olfactory and auditory stimuli. The Japanese wild-derived mouse strain MSM/Ms (MSM) was tested as observers, since our initial experiments indicated that MSM mice showed higher sensitivity to others' pain compared with the laboratory strain C57BL/6J (B6). MSM observers were shown footage of an unfamiliar B6 mouse receiving painful foot shocks via a screen. For olfactory stimuli, one of the following was presented during observation: (i) urine collected from a shocked B6 mouse, (ii) urine collected from an unshocked B6 mouse, or (iii) reverse osmosis water. Consequently, MSM mice observing the footage with urine from shocked mice demonstrated significantly higher fear-induced freezing behaviour than in the other two conditions. Regarding visual and auditory stimuli, observing the pixelated video clip was significantly associated with reduced freeze responses, whereas blocking auditory cues did not affect the duration of freezing. These results provide clear-cut evidence that multiple cues, including olfactory and visual information, are sufficient social signals for emotional contagion in mice.