The ear of the chameleon: Chamaeleo höhnelii and Chamaeleo jacksoni

The study of the chameleon ear that began with Chamaeleo senegalensis and Chamaeleo quilensis has now been extended to two further species, Chamaeleo höhnelii and Chameleo jacksoni, with results that are different in important respects. As measured in terms of cochlear potentials, the two species of the present study show definitely poorer sensitivity to aerial sounds. Also their responsiveness to these sounds is not made much worse by severing the columella, an observation that stands in contrast with the serious impairment that this operation produces in C. senegalensis and C. quilensis. The explanation lies in a difference between the sound receptive mechanisms in the two groups. In the species examined earlier a substitute tympanic membrane was found: beneath the surface at the side of the head is a diaphragm formed by a thin plate of the pterygoid bone and a membrane extending its surface to the border of the quadrate bone. In addition there is a branch process (anterior process) of the extracolumella that attaches to the middle of this pterygoid plate, and transmits the movements made by the plate in response to sounds to the stapedial footplate in the oval window. In C. höhnelii and C. jacksoni this anterior process is lacking, and the pterygoid plate cannot function as a sound receptive surface.