Morphological and kinematic study of the tongue and buccal cavity in the lizard Anguis fragilis (Reptilia: Anguidae)

Abstract

Background. The ability to detect chemical cues is highly developed in Scleroglossa, and particularly in anguid lizards. This ability was predicted because anguids possess a well-developed vomeronasal organ (VNO) (or Jacobson's organ) and rely largely on chemical cues in various behaviours as other active foragers. In this work, we have investigated the possible functional association between tongue flicking and the VNO in the lizard Anguis fragilis. Methods. The morphology of the tongue and the buccal cavity was investigated by light and scanning electron microscopy. The kinematics of tongue and jaw movements was studied by high speed cinematography. Results. The epithelial cells of the ventral aspect of the tongue tips show microstructures (microridges, microfacets, micropores) which are not present on other areas of the mouth. Beneath the tongue, the floor of the buccal cavity shows two concave-like elevations suggesting a structural analogy with the anterior processes described in snakes. The apex and the internal margin of these processes bear parallel oblique ridges. Taste buds occur anteriorly on the buccal floor and on the palate and are abundant on the internal side and on the edge on the anterior processes. The tongue showed three modes of tongue flicking: simple downward extension, single oscillation, and multiple oscillations. At each tongue flick, the ventral surface of the tips was observed contacting the substratum. Immediately after the tongue retraction, the buccal floor moved slightly upward. The observation of tongue flicking with the mouth open showed that the anterior processes moved upward when the tongue was retracted. Conclusions. These observations suggest the following: (1) during tongue flicking the ventral surface of the tongue tips invariably makes contact with the substratum; (2) the microstructures of the tongue tips and the ridges of the anterior processes might be helpful for collecting and receiving, respectively, chemicals during tongue flicking; (3) the anterior processes may be apposed on the roof of the mouth next to the ducts of VNOs when the buccal floor is fully elevated; (4) due to their localization, the taste buds could be equally stimulated by the molecules transferred during tongue flicking.