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Horwitz, Sara



Schwenk, Kurt

Department of Ecology and Evolutionary Biology

University of Connecticut

Storrs, Connecticut USA

 

The aim of our research is to gain a better understanding of the evolution of vomeronasal chemoreception in lizards and snakes by means of tongue-flicking (TF). Snakes have evolved a unique, oscillatory TF mechanism that differs significantly from the simple, single flicks of most lizards. We examined the kinematics of TF in the Gila monster (Heloderma suspectum), a possible relative of snakes, and some other anguimorphan lizards, to explore possible ancestral forms of TF. TF in eight, wild-caught Gila monsters was analyzed using a high-speed videography. The vast majority of Gila monster TFs end with substrate contact using a characteristic ‘touch and drag’ motion with the tines of the forked tongue tip progressively spread apart as they are moved posteriorly across the surface. This pattern presumably concentrates non-volatile chemicals on the tongue tips’ ventral surfaces. The tongue tips are protruded together, but they at retraction they are fully separated so that independent delivery of chemicals to ipsilateral vomeronasal organs is possible, suggesting that, like snakes, Gila monsters can evaluate chemical signal strength separately on each side. We identified three patterns of TF. These differed in duration, vertical height of the tongue tip during protrusion, and drag distance on the substrate. Individuals varied significantly in the frequency of each flick type they used and in the kinematics of TF, generally. Neither protrusion distance nor maximum tongue velocity is correlated with body size, but TF duration is negatively correlated. Protrusion distance and velocity are positively correlated. However, head size (and therefore, tongue size) is not correlated with body size, and might be a better predictor of TF kinematics. Varanids and most other anguimorphans use single flicks kinematically similar to Heloderma, however some anguids, such as Pseudopis and Ophisaurus, often use multiple, rapidly sequential TFs (with the tongue withdrawn between flicks) that could represent a kinematic intermediate between ancestral single-flicks and snake oscillatory TF.

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