A unique sound production mechanism in the pipid anuran Xenopus borealis

Abstract

The totally aquatic pipid frog Xenopus borealis produces long trains of click‐like sound at high sound pressure levels (> 105 dB SPL) underwater at night. While X. borealis retains an essentially terrestrial respiratory tract, the larynx is highly modified in two ways. First, the cricoid cartilage is greatly expanded posteriorly to form a large ‘box’. Portions of this cricoid box are composed of an unusual elastic cartilage. Second, portions of the arytenoid cartilages are elaborated into calcified rods with disc‐like enlargements at their posterior ends. These discs are the only freely moveable components within the larynx–there are no vocal cords. Artificial stimulation of a pair of muscles controlling the discs and discrete lesions that impair their movement demonstrate that motion of the discs is both necessary and sufficient for click production. Unlike all other anurans, X borealis does not use a moving air column in sound production. A possible mechanism of click production involves two steps: (1) at first, the discs are held tighdy apposed in the midline by fluid adhesive forces, and contraction of bipennate muscles is isometric; (2) when the muscle tension exceeds the adhesive force, the discs separate with very high acceleration leaving a vacuum between them. Air rushing into the space at high speed (an implosion) produces the click. The cricoid box shapes the frequency spectrum of the clicks, and opening the box broadens the power spectrum. The power spectrum of clicks produced by males after breathing helium is unchanged.

Central pattern generators for social vocalization: Androgen-dependent neurophysiological mechanisms

“…Extensive neuroanatomical studies by Kelley and colleagues (Wetzel et al, 1985;Zornik and Kelley, 2007) identified a central calling circuit for the fully aquatic frog Xenopus laevis that does not depend upon respiration for calling (Yager, 1992). The Xenopus map closely matches the neurophysiological one for Lithobates depicted by Schmidt (1992).…”

Sexually differentiated, androgen-regulated, larynx-specific myosin heavy-chain isoforms in Xenopus tropicalis; comparison to Xenopus laevis

“…The phylogenetic relationships of extant species have been described using both mitochondrial and nuclear deoxyribonucleic acid (DNA; Evans et al 2004). All species produce calls via contraction of the laryngeal muscles to pull apart a pair of cartilaginous disks, thus producing a "click" (Yager 1992); the temporal patterns of trains of clicks (trills) and click rates are unique to each species. While the mechanism of sound production is highly conserved, the rates at which these clicks are produced and thus the rate at which laryngeal muscles contract vary considerably among species, from two clicks per second in X. borealis (Yager 1992) to 70 in X. laevis (Kelley 2002).…”

“…All species produce calls via contraction of the laryngeal muscles to pull apart a pair of cartilaginous disks, thus producing a "click" (Yager 1992); the temporal patterns of trains of clicks (trills) and click rates are unique to each species. While the mechanism of sound production is highly conserved, the rates at which these clicks are produced and thus the rate at which laryngeal muscles contract vary considerably among species, from two clicks per second in X. borealis (Yager 1992) to 70 in X. laevis (Kelley 2002). This variation in muscle contraction rates provides an opportunity to examine how MyHC isoform expression changes under selection for the trill rates of species-specific calls.…”

Regulation of Respiratory and Vocal Motor Pools in the Isolated Brain ofXenopus laevis

“…Sound production does not require airflow; the contraction of a single laryngeal muscle group causes the snapping apart of paired discs and a resulting click (Yager, 1992), the basic unit of all calls. The simplified mechanism of Xenopus call production makes it an attractive model for addressing how vocalizations are produced and also offers the opportunity to investigate how breathing and calling can be regulated in a temporally distinct manner.…”