Rodent diversification is associated with a large diversity of species-specific social vocalizations generated by two distinct laryngeal sound production mechanisms- whistling and airflow-induced vocal fold vibration. Understanding the relative importance of each modality to context-dependent acoustic interactions requires comparative analyses among closely related species. In this study, we used light gas experiments, acoustic analyses, and laryngeal morphometrics to identify the distribution of the two mechanisms among six species of deer mice (Peromyscus). We found that high frequency vocalizations (simple and complex sweeps) produced in close-distance contexts were generated by a whistle mechanism. In contrast, lower frequency sustained vocalizations (SVs) used in longer distance communication were produced by airflow-induced vocal fold vibrations. Pup isolation calls, which resemble adult SVs, were also produced by airflow-induced vocal fold vibrations. Nonlinear phenomena (NLP) were common in adult SVs and pup isolation calls, suggesting irregular vocal fold vibration characteristics. Both vocal production mechanisms were facilitated by a characteristic laryngeal morphology, including a two-layered vocal fold lamina propria, small vocal membrane-like extensions on the free edge of the vocal fold, and a singular ventral laryngeal air pocket known as the ventral pouch. The size and composition of vocal folds (rather than total laryngeal size) appears to contribute to species-specific acoustic properties. Our findings suggest that dual modes of sound production are more widespread among rodents than previously appreciated. Additionally, the common occurrence of NLP highlight the nonlinearity of the vocal apparatus, whereby small changes in anatomy or physiological control trigger large changes in behavioral output. Finally, consistency in mechanisms of sound production used by neonates and adults underscores the importance of considering vocal ontogeny in the diversification of species-specific acoustic signals.

In many mammals, spectral properties of acoustic signals scale with body size within and among species. In rodents, however, despite drastic changes in body size, fundamental frequency (F0) range of ultrasonic whistles produced for social communication remain relatively uniform from birth to adulthood. Such divergent patterns may be due to a novel sound production mechanism unique to rodents involving an intralaryngeal midline pocket termed the ventral pouch. In this study, we analyzed the postnatal shape and size of the laryngeal airway in CD1 mice over ontogeny to better understand the association between ventral pouch geometry and F0 of ultrasonic whistles. Ventral pouch volume (0.06 ± 0.01 mm3) did not differ between pups and 1‐year‐old adults despite extensive shape‐inducing remodeling of the intralaryngeal musculature and connective tissue. In contrast, ventral pouch volume was 50% less in 2‐year‐old compared to 1‐year‐old mice. Thus, allometry of the laryngeal airway appears to explain spectral overlap between ultrasonic whistles of young, small mice and older, larger mice. The causal association between the reduction in vocal behavior and a seemingly shrinking ventral pouch in geriatric mice remains unclear. Together, these data inform our understanding of the postnatal development and remodeling of the intralaryngeal airway in Mus musculus.