Among the vertebrates, mammals' high-frequency hearing is the best, followed by birds, with lizards having the least sensitive hearing at high frequencies; and this decline in sensitivity is matched by the size of their basilar papillae, the organs that detect sound. Geoffrey Manley, from the Technische Universität München and University of Western Australia, has long been fascinated by the evolution of hearing. According to Manley, geckos have some of the most complex basilar papillae of all lizards, equipped with two sets of high-frequency sensitive hair cells that could respond to different frequency ranges. Knowing that pygopod geckos (Delma species), which look more like snakes than geckos, share this relatively complex basilar papillae with their legged cousins, Manley and his colleague Johanna Kraus decided to find out just how sensitive pygopod hearing is (p. ).
Travelling through the remote Pilbara region of Western Australia with a mobile laboratory, the duo measured the hearing of four pygopod species and found that the reptiles could hear frequencies as high as 12–14 kHz at 75 dB SPL. ‘Such a hearing limit has never been observed in any reptile,’ explains Manley, and suggests that the lizards' sensitivity could be attributed to ‘a unique division of labour between groups of sensory cells within the hearing organ’.
Having found that the snake-like lizards have exceptional hearing, the duo recorded the geckos' squeaks as they released them back into the environment and found that the reptiles could hit notes as high as 20 kHz. ‘Our estimates of vocalisation sound pressures and of hearing audiograms suggest that at close range, Delma species can hear their own call components up to a frequency of at least 12 kHz,’ says Manley.
