The incentive value of males' 50-kHz ultrasonic vocalizations for female rats (Rattus norvegicus).

Rats emit ultrasonic vocalizations (USVs) of 22 kHz and 50 kHz before, during, and after copulation. The 50-kHz vocalizations can be subdivided into flat and frequency-modulated (FM) trill calls. In this study, the role of 50-kHz USVs in sexual incentive motivation in female rats was examined. USVs were recorded from sexually active males during the precopulatory phase. In the first two experiments, "full 10-min song," flat-call, and FM-trill-call stimuli were used. In Experiment 1, a combination of complex and trill calls was used as the FM-trill call, whereas a mixture of multistep and upward ramp calls was used in Experiment 2. The auditory stimuli were played back to sexually receptive female rats in a sexual incentive-motivation test. For comparison, the odor of an intact male rat was also used as incentive stimulus. The flat-call stimulus did not induce approach behavior in any experiment, whereas the FM trill showed a short-lived effect in Experiment 2. The females approached the "full song" during the first minute of stimulus exposure in Experiment 1, but not in Experiment 2. When the entire 10-min test period was analyzed, the females in Experiment 1 did not approach the full song more than background noise, though they did so in Experiment 2. The olfactory stimulus, to the contrary, had a clear incentive value in both experiments. In a third experiment, a devocalized male, an intact vocalizing male and a female rat were used as incentive stimuli. The females did not approach a vocalizing male more than they approached a silent male, showing that USVs do not contribute to the male rat's incentive value. Overall, the results of the present series of experiments show that the male rat's USVs do not have any consistent incentive value for the sexually receptive female rat. This sharply contrasts the strong and reliable effect of male odor.