Mechanics and kinematics of fluid uptake and intraoral transport in the leopard gecko

Abstract

Drinking permits amniote vertebrates to compensate for water loss. Lizards (non-ophidian squamates) use their tongue to imbibe water, but for most lizards the tongue is also employed in other activities. To determine how these various demands can be accommodated alongside the tongue’s role in drinking, it is necessary to firstly determine the tongue’s function in water uptake and intraoral transport in a lizard that does not use this organ for chemoreception or food prehension. We selected the leopard gecko (Eublepharis macularius) for this purpose. We build upon previous morphological observations of the anatomy of its tongue and oral cavity and explore its drinking behaviour through high-speed cinematography and cineradiography. This allows us to follow the movement of radio-opaquely labelled water in association with tongue, jaw and throat movements. The tongue is modified to collect water and to release it into the anterior region of the buccal cavity. Repeated tongue cycles are associated with the shifting of the imbibed water into paired ventrolateral chambers and ultimately to its passage to a single, midline posterior chamber prior to swallowing. Tongue movements, capillarity and pressure changes due to hyolingual movements cause water to be moved posteriorly along specific pathways prior to swallowing, bypassing the airways and dorsal surface of the mid- and hind tongue. In this licking-based lingually driven approach to drinking, only small volumes of water can be gathered at a time, with a drinking bout consisting of 20–30 tongue cycles before emersion and swallowing occur. This mechanism differs from drinking in turtles and snakes, which do not employ licking and may take in larger quantities of water. We advance the hypothesis that all lizards have conserved this pattern of tongue-based water uptake and precisely directed intraoral fluid transport, with specializations for tongue-based food capture and/or lingually based chemoreception being superimposed thereupon.