Gular pouch diversity in the Chamaeleonidae

Abstract

Numerous chameleon species possess an out‐pocketing of the trachea known as the gular pouch. After surveying more than 250 specimens, representing nine genera and 44 species, we describe two different morphs of the gular pouch. Species of the genera Bradypodion and Chamaeleo, as well as Trioceros goetzei, all possess a single gular pouch (morph one) formed from ventral expansion of soft tissue where the larynx and trachea meet. Furcifer oustaleti and Furcifer verrucosus possess from one to four gular pouches (morph two) formed by the expansion of soft tissue between sequential hyaline cartilage rings of the trachea. In Trioceros melleri, examples of both morphs of the gular pouch were observed. Morphometric data are presented for 100 animals representing eight species previously known to possess a gular pouch and two additional species, Bradypodion thamnobates and Bradypodion transvaalense. In the species with the absolutely and relatively largest gular pouch, Chamaeleo calyptratus, a significant difference was found between sexes in its width and volume, but not its length. In C. calyptratus, we show that an inflated gular pouch is in contact with numerous hyoid muscles and the tongue. Coupled with the knowledge that C. calyptratus generates vibrations from the throat region, we posit that the tongue (M. accelerator linguae and M. hyoglossus) and supporting hyoid muscles (i.e., Mm. sternohyoideus profundus et superficialis and Mm. mandibulohyoideus) are involved in the production of vibrations to produce biotremors that are amplified by the inflated gular pouch and used in substrate‐borne communication.

The role of hyoid muscles in biotremor production inChamaeleo calyptratus

“…Our results confirm that C. calyptratus is also a species capable of producing biotremors as observed by Barnett et al (1999). These results also support the hypothesis that the SS, SP, MH and LS play a role in the production of biotremors by C. calyptratus (Huskey et al, 2020).…”

“…This might occur as the biotremor is passing through other tissues. It is also possible that the inflated gular pouch of C. calyptratus acts as a resonator (Huskey et al, 2020), which could increase the dominant frequency of the call. The size-related shift in frequency, which is also dependent upon the sex, suggests that the resonance of the gular pouch is involved.…”

“…According to Tornier (1905), and further supported by Huskey et al (2020), chameleons lack the functional vocal cords that are thought to be needed to facilitate biotremor production. Barnett et al (1999) recorded vibrations emanating from the throat region, and all authors observed a head click during biotremor production (personal observations), suggesting that muscles situated in the throat and neck are likely the source of the biotremors.…”

“…Barnett et al (1999) recorded vibrations emanating from the throat region, and all authors observed a head click during biotremor production (personal observations), suggesting that muscles situated in the throat and neck are likely the source of the biotremors. Huskey et al (2020) further observed that the M. sternohyoideus superficialis (SS) and M. sternohyoideus profundus (SP) are secured by connective tissue directly to the surface of the gular pouch, an expansive modification of the trachea that is present in at least 21 species of chameleons (Huskey et al, 2020). The Mm.…”

“…mandibulohyoideus (MH) also contact the rostral surface of the inflated gular pouch. It was hypothesized by Huskey et al (2020) that the gular pouch serves as an amplifier of the vibrations produced by the tongue and hyoid muscles.…”