Close relationship between the frequency of 22-kHz calls and vocal tract length in male rats

It is known that the size of the components of the sound production apparatus in mammals may affect the acoustic structure of vocalizations. Therefore, some acoustic variables such as voice frequency may change with age in association with body size and body weight increases. However, whether this relationship also applies to ultrasonic vocalizations emitted by laboratory rats has not been investigated. Thus, in the present study, we first recorded changes in three acoustic variables (mean frequency, duration, and bandwidth) of air puff-induced 22-kHz calls in male rats during their growth period and assessed the relationship between these changes and body weight gain (Experiment 1). Then we directly recorded several body size measures including components of the sound production apparatus in 6- and 12-week-old male rats and examined the correlation between these values and the acoustic variables of 22-kHz calls (Experiment 2). In Experiment 1, the mean frequency of 22-kHz calls in male rats during the growth period showed negative correlations with body weight gain, while the duration of 22-kHz calls showed positive correlations. In Experiment 2, only a close negative correlation between the mean frequency of 22-kHz calls and vocal tract length in male rats was found. These results suggest that the age-related decrease in the mean frequency of 22-kHz calls may be ascribed to anatomical elongation of the vocal tract length in association with the growth of male rats. These acoustic differences could inform the receivers about the age of the signaler.