Acoustic playback paradigms alter affective states in juvenile male Wistar rats. 

Play has been proposed as a promising indicator of positive emotions and welfare in domesticated animals. During play, rats produce ultrasonic vocalisations often referred to as 50kHz USVs (or ‘rat laughter’) which are thought to indicate a positive emotional experience and have a role in social communication. Previous work has shown playback of USVs to individual rats can alter rat behaviour. In a radial maze setting, 50kHz positive communicative calls induce approach behaviour and increase exploration, whereas, 22kHz alarm calls induce behaviours indicative of the fight/flight system, e.g., reduced locomotor activity and freezing. When given the opportunity to express play behaviour, we hypothesised that playback of calls of contrasting valence would alter affective state and result in a matched change in play; playback of 50kHz USVs would increase 50kHz vocalisations and play, with 22kHz USV playback reducing USV production and play. Juvenile male Wistar rats (N = 58; 37 days old) were housed in pairs and assigned to one of 3 groups balanced by bodyweight. Each pair of rats received acoustic stimuli treatments in the home cage in a pseudo-randomised Latin square design across three days. Experiment I tested 50kHz calls, 22kHz calls and background noise, whilst Experiment II tested 50kHz calls, white noise within the 30 – 100kHz range and background noise. Each day a playback session consisted of a habituation phase, then 3 sets of acoustic stimuli followed by a pause for a total of 7 minutes, with each phase lasting 1 minute. Playback of 50kHz calls increased subject-produced 50kHz vocalisations and approach behaviour towards the sound source when the speaker was above the cage compared with background noise. Contrastingly, playback of 22kHz calls reduced the number of 50kHz USVs. When rats were exposed to white noise the number of USVs produced matched the number produced during playback of 50kHz calls. In contrast to out hypotheses, we found that social play was dampened when played 50khz calls but induced when played the white noise compared with background noise. Further investigation is required for use as play an indicator of positive welfare and the need for further work to investigate relationships between playback of USVs and spontaneous behavioural responses. The work described in this abstract conforms with the ISAE Ethical Guidelines.