Sexual dimorphism in laryngeal muscle fibers and ultrasonic vocalizations in the adult rat

Abstract

Objective: The human voice is sexually dimorphic in obvious ways, such as differences in fundamental frequency and gross laryngeal anatomy, but also in less apparent ways, such as in the prevalence and types of voice disorders and the manifestation of voice changes in advanced age. Differences between males and females are rarely explored, however, in mechanistic animal studies. The goal of this study was to explore sexual dimorphism in laryngeal function and structure in adult rats by examining ultrasonic vocalization acoustics and muscle fiber size and type in the thyroarytenoid muscle.

Study design: Animal group comparison.

Methods: Spontaneous ultrasonic vocalizations from 10 male adult rats and 10 female adult rats were recorded, classified, and acoustically analyzed. Cross-sections of the thyroarytenoid muscle were stained and imaged for analysis of muscle fiber size and type. Acoustic and muscle parameters were statistically compared between sexes.

Results: Male rats had a lower mean frequency of short ultrasonic vocalizations. Male rats also had a larger mean fiber size in the external division of the thyroarytenoid and larger overall muscle area in both the vocalis and external divisions of the thyroarytenoid. However, muscle fiber type compositions were similar between sexes in both the vocalis and external division of the thyroarytenoid muscles.

Conclusion: Functional and structural laryngeal differences exist between adult male and female rats; therefore, the rat model can be used to further study sexual dimorphism of the voice.

Level of evidence: NA. Laryngoscope, 127:E270-E276, 2017.

Keywords: Larynx; female; rat; sexual dimorphism; thyroarytenoid; ultrasonic vocalizations.

Fig. 1

Muscles fibers are stained accordingly: type I green (a), type IIB red (a), type IIA green (b), type IIX red (b), and type IIL green (c). Laminin is outlined in blue (a) (b). Both images (a) and (c) illustrates that the TA-X co-expresses type IIL and IIB, and both fiber types are also present in the TA-V. Image (b) illustrates that type IIX stained most of the TA-V, with type IIA fibers stained in the rostral portion of the TA-V. TA-V = thyroarytenoid vocalis division; TA-X = thyroarytenoid external division.

Fig. 2

Mean ± standard error and Mann-Whitney U-test results for percentages of muscle fiber types in thyroarytenoid vocalis division and thyroarytenoid external division. Note that fibers can coexpress isoforms; thus, the total percentages exceed 100% within each muscle. F = female; M = male.

Fig. 3

Box-and-whisker plots of the individual muscle fiber sizes of the TA-V (a) and TA-X (b) and the overall muscle sizes of the TA-V (c) and the TA-X (d). A pattern can be noted that male rats typically have larger fiber sizes and muscle area than female rats. TA-V = thyroarytenoid vocalis division; TA-X = thyroarytenoid external division.

Fig. 4

Density plot of the mean frequency of all flat USVs produced by male (solid line) and female (dashed line) rats. Male and female rats have a similar binary distribution of flat USVs in the 40- and 55-kHz range, whereas female rats have another small peak in the distribution of flat USVs centered around 75-kHz range. USV = ultrasonic vocalizations.