Larynx morphology and sound production in three species of Testudinidae

Although the ability to vocalize is widespread
among tortoises, the mechanisms of sound production
in chelonians remain undescribed. In this study, we
analyze the morphology and histology of the larynx of
three species of Testudinidae (Testudo hermanni, T.
graeca, and T. marginata) in order to ascertain the presence
of vibrating acoustic structure, and based on our
findings we propose a general model for phonation in
tortoises. The structure of the larynx of the three tortoises
analyzed is simple: three cartilages (the cricoid and two
arytenoids) form the skeleton of the larynx, while two
pairs of muscles (the dilators and constrictors) control the
widening and closing of the glottis. The larynx is supported
in the oral cavity by the hyoid cartilage, which in
tortoises assumes the same functions of the thyroid cartilage
of mammals. Two bands of elastic fibers are inserted
in the lateral walls of the larynx just upstream of the
glottis, and can be stretched away from the hyoid by the
movements of the arytenoids. Their position and structure
suggest that these bands are capable of vibrating during
exhalation, and therefore may be considered vocal cords.
The cricoid of T. marginata and T. graeca hold two diverticula,
not previously reported, which might function as a
low-frequency resonating chamber, improving the harmonic
structure of tortoise calls. The structure of the
larynx is compared with that of other vertebrates and the
relationships between morphology and phonation are discussed.
This is the first detailed description of anatomical
structures possibly devoted to vocalization in chelonians.