Close relationship between the frequency of 22-kHz calls and vocal tract length in male rats
Highlights
► The frequency of 22-kHz calls in male rats decreases with age. ► The frequency of 22-kHz calls shows negative correlation with body weight gain. ► There is obvious negative correlation between the frequency and vocal tract length. ► The decrease in the frequency with age may be ascribed to growth of the vocal tract.
Introduction
The sound production systems of all mammals exhibit a number of fundamental anatomical and acoustic similarities; mammal vocalizations result from a source signal, generated by vibrations of the vocal folds in the larynx using expiratory airflow from the lungs; this is then filtered by vocal tract resonance [1]. The sizes of components of the sound production apparatus have important effects on acoustic output, for example, voice frequency and call duration. Voice frequency is characterized by two components, fundamental frequency and formant frequencies; the former correlates with the length of the vocal folds [2] and the latter is closely influenced by the length of the vocal tract [3]. Because large animals have longer vocal folds and longer vocal tracts than smaller ones they generally produce acoustic signals with lower fundamental frequencies, with less dispersed formant frequencies, and with sound energy clustered in the lower frequencies [4]. In addition, larger animals should emit longer calls than smaller ones, because they possess larger lungs and have a greater air volume available for calling [4]. Supporting these ideas, the voice frequency of animals generally decreases with age while the call duration increases in association with body size increase and body weight gain [[5], [6], [7], [8], [9], [10]].
Although rodents produce vocalizations through the larynx, their larynxes have two modes of function. The first is, as described above, common to most other mammalian species. In the case of rats, sound of 2–4 kHz is produced [11] and emitted under situations related to pain [12] or direct warning against predators [13]. The second mode of larynx function is used to make ultrasounds, which are pure sounds of a constant or variable frequency with few or no overtones caused by stabilization of the larynx such that it is used as a whistle with a very small orifice created by the vocal folds, that are tightly constricted and cannot vibrate [14], [15]. It is commonly reported that laboratory rats emit long (0.3–3.0 s) ultrasonic bouts of 20–30 kHz with a narrow bandwidth of 1–4 kHz, referred to as “22-kHz calls.” These ultrasonic calls are observed when rats are faced with potentially harmful or life-threatening danger or expect a known unpleasant stimulus without exact information about when it will happen, for example, the presence of a predator [16], confrontation with a dominant and aggressive rat [17], and even a light but unpredictable and sudden air puff [18] or tactile stimulus [19].
In contrast to audible vocalization in mammals, little is known about the relationship between this type of ultrasonic vocalization and body size in rats, although such information could be very useful in rodent models for disorders to disentangle in some models the effect of body size and the effect of abnormal vocal production due to a genetic mutation. However, as it is postulated that 22-kHz calls should be produced by a whistle sound generated in the larynx using expiratory airflow from the lungs and filtration through the vocal tract [20], we hypothesized that the acoustic variables of 22-kHz calls would be closely correlated with body size including the sizes of components of the sound production apparatus, as observed in the relationship between sonic vocalization and body size in other mammals [[4], [5], [6], [7], [8], [9], [10]]. To test this hypothesis, we first recorded changes in three acoustic variables, i.e., mean frequency, duration, and bandwidth, of 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 measured the length of the vocal tract, the lung weight, and other major body size variables (body weight, body length, etc.) 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).
Section snippets
Animals
A total of 32 male Wistar rats (Clea Japan, Tokyo, Japan) were used in this study. All animals were housed in pairs in wire-topped transparent cages (410 × 250 × 180 mm) with wood shavings for bedding, were provided with water and food ad libitum, and were kept on a 12-h light–dark cycle (lights turned off at 20:00). The vivarium was maintained at a constant temperature (24 ± 1 °C) and humidity (40–45%).
Experimental apparatus and procedures
Rats aged 4 weeks (n = 8) participated in Experiment 1, at the beginning of which they were moved to
Results
In Experiment 1, the mean frequency of 22-kHz calls gradually decreased with age (Figs. 2 and 3A) and showed a high negative correlation with body weight gain (r = − 0.75, p < 0.0001, Fig. 3A). The duration of 22-kHz calls gradually increased with age (Figs. 2 and 3B) and showed a moderate positive correlation with body weight gain (r = 0.67, p < 0.0001, Fig. 3B). The bandwidth of 22-kHz calls remained almost constant with age (Figs. 2 and 3C) and showed a non-significant negative correlation with body
Discussion
In Experiment 1, the mean frequency of 22-kHz calls in male rats during their growth period showed negative correlations with body weight gain while the duration of 22-kHz calls showed positive correlations. In Experiment 2, the mean frequency of 22-kHz calls in male rats aged 12 weeks was significantly lower while the duration of 22-kHz calls was significantly longer than those in male rats aged 6 weeks. In Experiment 2, there was significant negative correlation between the mean frequency of
Conclusion
In conclusion, we showed that the mean frequency of 22-kHz calls in male rats decreased with age while the duration of 22-kHz calls increased. The former phenomenon may be correlated with the anatomical elongation of VTL in association with growth. These acoustic differences with age could inform the receivers about the age of the signaler, leading to individual recognition within the social group based on the analysis of perceived vocalizations. It is also speculated that maturing brain
Acknowledgment
This study was supported by a Grant-in-Aid for Scientific Research (C) (22500382) from the Japan Society for the Promotion of Science.
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