A contractile cochlear frame is a common feature of the hearing organs in Gekkota (sauria, Squamata): a comparative study.

Geckos are the most vocalizing animals among Squamata. Previously we discovered a contractile segment (the NAL, noncartilaginous abneural limbus), within the rigid periotic cochlear frame of the gecko Teratoscincus scincus [Ganeshina and Vorobyev, 2003]. Because this unusual cochlear specialization has not previously been described in the vertebrate hearing organs, we have hypothesized that the NAL has evolved within Gekkota as a specialization associated with vocalization and sound communication. Here we show that the NAL is present in ten other species belonging to four major Gekkota clades: Gekkoninae, Diplodactylinae, Eublepharinae and Pygopodidae. The NAL exhibits similar structural organization among the Gekkota species. It is composed of large, tightly packed cells enriched with a filamentous cytoskeleton and extensively interconnected via putative gap junctions. No relationship is found between the extent of development of the NAL and degree of vocalization. However, the species with relatively large body dimensions show larger absolute NAL area and structural peculiarities of the NAL that might affect its mechanical properties. A representative of the non-gekkonoid, non-vocalizing lizard, Pogona barbata (Iguania, Agamidae), possesses a similar cochlear specialization. This provides evidence that the NAL is not the exclusive feature of the Gekkota hearing organs. Our data are compatible with the hypothesis that the NAL appeared before the Gekkota separated from other Squamata groups as a mechanism involved in maintenance of the cochlear mechanical or ionic homeostasis.