Rats (Rattus norvegicus) emit a variety of ultrasonic vocalizations throughout their lifespan that reflect different forms of emotional arousal and accompanying affective states. In this study, high frequency recordings of ultrasonic vocalizations were made during mating, aggression, and both conspecific and heterospecific (dubbed "tickling") rough-and-tumble play behavior. We found that frequency modulated 50-kHz calls (trills and step calls) were positively correlated with positively valenced appetitive behavior during mating, play, and aggression. These calls were also positively correlated with the reward value of these social encounters. However, constant frequency (i.e., flat) 50-kHz calls were not related to appetitive behaviors or reward. In contrast, 22-kHz calls were positively related to aversive/withdrawal behaviors during mating, play, and aggression. Finally, we found that rats self-administered playback of frequency modulated 50-kHz trill calls and avoided playback of 22-kHz calls. Playback of flat 50-kHz calls or tape hiss was neutral. These results suggest that frequency modulated 50-kHz calls index a positively valenced, appetitive, social-emotional state in rats.
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