Behaviour 2019
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Nervous Laughter: Rougher Tickling Elicits More Low and High Ultrasonic Vocalizations in Rats  
Grace M Eldridge1, Gwen J Lupfer2. 1Columbia University, New York, NY, United States; 2University of Alaska Anchorage, Anchorage, AK, United States

Rats emit 50-kHz ultrasonic vocalizations (USVs) during activities such as playing with another rat or human hand, eating, and sex. They also emit 20-kHz USVs when experiencing distress. However, the boundary distinguishing 50-kHz from 20-kHz USVs is in contention, with some researchers defining distress calls as 20-25kHz and others including calls up to 33kHz. In the current experiment, 8 female 5-month-old Wistar rats experienced two types of heterospecific play: rough-and-tumble (e.g., being pinned on back during belly tickles) and gentle tickling (e.g., tickling belly while rat stands) while USVs were recorded. With low-frequency calls defined as 18-33kHz, and high-frequency calls defined as 34-90kHz, we found that the rough session elicited significantly more low- and high-frequency USVs. Additionally, within each session, low USVs were highly positively correlated with high USVs. Because stressors such as predator odor and bright light have been reported to suppress high-frequency USVs, rougher tickling in the current experiment does not appear to have caused true distress, and USVs up to 33 kHz may be better described as nervous laughter than as distress calls.