Functional‐morphological Diversity and Complexity of the Larynx as a Basis for Vocal Complexity in Birds: Analogies to Human Speech

The larynx of birds differs significantly in its structure and function from that of mammals. The avian larynx forms a laryngeal chamber that is devoid of vocal folds, but instead affects the resonance of sounds that are generated in the syrinx at the base of the long trachea. It is movably attached by ligaments and extrinsic muscles to the basihyal‐urohyal bone of the complex avian hyoid skeleton which is suspended cranially by extrinsic lingual muscles and fasciae to the mandible and caudally by hyoid horns within vaginal hyoid sheaths to the cervical fascia. In general, the avian larynx comprises a ring‐ or trough‐shaped cricoid whose lateral limbs converge caudo‐dorso‐medially to form the crico‐procricoid joint with the unpaired procricoid bone, which articulates rostrally with the bony bases of the paired arytenoids. The cartilaginous rostral parts of the arytenoids support the glottal lips, which are opened and closed by the intrinsic dilatator and constrictor muscles. Whereas the procricoid and arytenoids and the intrinsic laryngeal muscles vary little among birds, the cricoid and the extrinsic laryngeal protractor and retractor muscles vary considerably, so that three types of avian larynges can be distinguished. (1) The avian akinetic larynx, most common among birds (e.g., chickens, pigeons, herons), comprises a trough‐shaped cricoid whose floor and sides are formed through a fusion of tracheal half‐rings. The extrinsic protractor and retractor muscles move the larynx along the basihyal‐urohyal, but do not affect the shape or size of the laryngeal chamber. (2) The psittaciform kinetic larynx is characterized by a flat ring‐shaped cricoid that is movably suspended above a laryngeal chamber whose sides and floor are formed by tracheal half‐rings. The complex extrinsic muscles protract and retract the larynx along the basihyal‐urohyal support, but also pivot the caudal end of the cricoid up‐and‐down, which modifies the shape and size of the laryngeal chamber. (3) The passeriform kinetic larynx is characterized by a trough‐shaped ventral cricoid forming the floor and sides of the laryngeal chamber through a fusion of tracheal half‐rings. A pair of dorsal cricoids articulate with the caudal end of the ventral cricoid and converge caudo‐medially to form the crico‐procricoid joint. Special extrinsic muscles raise and lower the caudal end of the larynx by rotating the dorsal cricoids up‐and‐down and, thereby, modify the shape and size of the laryngeal chamber. The larynx itself is moved along the basihyal‐urohyal by the simple protractor and retractor laryngeal muscles. The passeriform and psittaciform types of larynges are correlated with a capacity for producing complex vocalizations with vowel‐like and consonant‐like sounds presumably by being able to modify the shape and size and, thus, the resonating properties, of the laryngeal chamber. In addition, the raising and lowering of the caudal end of the larynx also modifies the size and shape of the oral cavity as a secondary resonating chamber. Hence, the modifiability of dual resonating chambers in birds (the oral cavity and laryngeal chamber) and in humans (the supralaryngeal and oral cavities) are analogous mechanisms for the production of articulated vocalizations or speech. Support or Funding Information LSU Foundation to D.G. Homberger