Evolution and function of lingual shape in lizards, with emphasis on elongation, extensibility, and chemical sampling

Major squamate taxa exhibit extreme variation in lingual morphology, presumably due to correlated variation in trophic and chemosensory functions. Data are presented on evolution of lingual shape documenting several trends: (1) Resting lingual elongation is greatest in families specialized for lingual chemosensory sampling. (2) The greatest increase in elongation achievable by intralingual means including elasticity and foretongue retractility occurs in families with intermediate degrees of lingual specialization for chemosensory sampling. Sampling efficiency may be enhanced by the ability to extend the tongue well beyond the mouth, with resting elongation and intralingual extensibility perhaps jointly determining distance extended. In families lacking sufficient resting elongation, augmentation of intralingual extensibility may be a means of approaching optimal protrusion distances. Decreased extensibility evolved in tandem with the greatest resting elongation, suggesting that resting elongation may be more efficient for protrusion and that elasticity declines as optimal resting length is approached. The optimal shape for chemosensory sampling may be predicted to be highly elongate, as in teiids, varanids, and colubrids. The tongue should be broad at the tip for prehension (as in iguanians), fleshy for manipulation and swallowing, and broad at the base for tamping prey into the esophagus. (3) Lingual surface area relative to that of a rectangle of dimensions length × base width varies accordingly. Relative area is high in families that do not tongue-flick much while foraging because tongues are broad and fleshy throughout their length. It is low in families that have wedge-shaped tongues and intermediate specialization for chemosensory sampling. Narrowing of the anterior tongue may improve chemical sampling. Relative lingual area in chemosensory specialists is very high, with progressive narrowing toward the base as optimal sampling shape is approached in taxa lacking lingual function in swallowing, prehension or prey manipulation.