The effect of 22 kHz calls and artificial 38 kHz signals on activity in rats

Ultrasonic calls at 22 kHz are emitted by adult rats Rattus norvegicus when submissive, after and sometimes before copulation and in aversive situations, but the function of these calls, if any, is not known. This study examined the effect of 22 kHz calls on the behaviour of individuals. Rats were exposed singly for 5 min to a tape-recorded test signal; 22 kHz calls or artificial 38 kHz signals or tape noise. All rats were exposed to tape noise before and after the test signal. Both of the ultrasonic signals, and particularly the 22 kHz calls, decreased locomotor activity in rats during replay and for 5 min afterwards. Rats sniffed the loudspeaker less during the replay of 22 kHz calls than during the other test signals. These results complement others showing that constant frequency ultrasound from visual display units and oscilloscopes also depresses activity. In natural aggressive situations 22 kHz calls could allow a subordinate to escape and prevent the dominant wasting time in pursuit. In the laboratory exposure of rats to natural or artificial ultrasound could affect ongoing or subsequent behaviour and so be a confounding experimental variable.