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More than 170 million years ago a huge shift happened in squamate evolution. One group of squamates evolved a sensitive mechanism for detecting chemicals in the air and on the ground.
In living squamates the sensing system has two key parts: a cleft, or split, tongue and a sensory structure called the vomeronasal (VOH-mer-oh-NAY-zal) organ. Living squamates that rely on this sensing system are the so-called "nose hounds": they include skinks, Monitor Lizards, the Gila (HEE-luh) Monster and all snakes.
Some groups of squamates rely on vision to get their dinner. But others find prey or mates, or avoid danger, aided by a sensitive odor detection system. Odors may carry long distances in the air and can linger on the ground. Being able to pick up these tiny traces gives the so-called "nose hounds" an edge: they can track hidden prey and search for mates over a much larger area.
When squamates flick their tongues, they're not trying to sting. Instead, they are "tasting" the surroundings for chemical traces of other animals—that is, for their smells. When the chemical signal of a food item is stronger in one direction, that's the way the animal moves.
A deeply forked tongue samples the intensity of odor molecules at two places, not just one. The space between the tongue forks in some squamates can be nearly as wide as the animal's head, which means it can sample a bigger area and stay on the right track.
