Sensory structures of reptiles exhibit variations in size and complexity that are roughly correlated with ecological varia tion and phylogeny. For example, lizards considered to be primitive, such as those of the family Chamaeleonidae, are primarily visually guided in the context of predation as well as in the contexts of social and reproductive behavior. This reliance on vision is reflected in the wonderful mobility of the eyes, the size of the optic lobes, and in the brilliant color patterns in the family. The phenomenon of "excited coloration" (color changes reflecting emotional or motivational states) involves socially important signals that can only be appreciated with vision. More advanced lizards, such as those in the family Varanidae, place greater emphasis on their nasal and vomeronasal chemosensory systems. Associated with this characteristic is a shift in the morphology and deployment of the tongue, which in varanids is used mainly to pick up nonvolatile molecules and to convey them to the vomeronasal organs. There is an associated shift from insectivory to carnivory. In snakes, which may be derived from a varanid-like ancestor, these shifts have been carried to an even greater extreme.
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