Ultrasonic vocalizations near 30 kHz may indicate excitement rather than distress in female Wistar rats

Abstract

Rats emit ultrasonic vocalizations (USVs), sometimes referred to as 50-kHz vocalizations, during activities such as play and lower-frequency USVs, sometimes referred to as 22-kHz vocalizations, when experiencing distress. Definitions of 22-kHz vocalizations vary in terms of which frequencies should be included, with differing upper limits (e.g., 25 kHz, 30 kHz, or 33 kHz). In the current experiments, we examined the extent to which USVs near the upper frequency limit may represent milder anxiety than lower-frequency vocalizations. In Experiment 1, rats received both gentle tickling and simulated rough-and-tumble play with a human hand, while low-frequency (i.e.,18–33 kHz) and high-frequency (i.e., 34–90 kHz) USVs were recorded; rough-and-tumble play elicited significantly more low-frequency USVs (median 29.5 kHz), but also significantly more high-frequency USVs (median 50.7 kHz). In Experiment 2, rats were presented with 23-kHz, 32-kHz, and 52-kHz USVs while foraging in a T-maze. The 23-kHz USV elicited the most freezing, while the 32-kHz USV elicited significantly less freezing than either the 23- or the 52-kHz USV. Taken together, these results suggest that within the range of frequencies referred to as 22-kHz vocalizations, lower-frequency USVs may indicate a more negative affective state than higher-frequency ones. As USV production can serve as an indicator of rat welfare, an understanding of what frequencies represent positive versus negative affective states is crucial.