Heightened Play following Play Observation in the Absence of 22-kHz Calls in Juvenile Rats

Emotional contagion is often studied through aversive-state transmission, leaving positive affect sharing less understood. Here, we investigated whether observing social play facilitates a subsequent play opportunity in juvenile male rats. Observer dyads were socially isolated for 30, 120, or 270min before observing either highly playful demonstrator dyads that had been socially isolated for 24h, or less playful demonstrator dyads that were socially housed. During observation, visual, auditory, and olfactory cues were available, but physical contact was prevented. When observers were reunited, their behavior and ultrasonic vocalizations were quantified in sessions without 22-kHz calls -often associated with negative affect, alarm, or social conflict- detected. Play observation increased playful pins in observers most distinctly after 30 or 120min of isolation, whereas total play duration showed a weaker concordant pattern. Play observation was not detectably associated with increased non-social or non-play social exploration, arguing against a purely non-specific arousal effect. As individual callers could not be identified, vocalizations are reported as session-level acoustic measures. Among these, 50-kHz calls - a heterogeneous class associated with appetitive and social contexts - were emitted at higher rates following play observation. These findings suggest that observing social play can facilitate subsequent play in juvenile rats while underscoring the importance of 22-kHz calls and caller identity in positive emotional contagion paradigms.