All reptiles have a sense of smell: odours are detected by sensory cells in the nasal cavity very much in the same way as mammals. Many species depend on smell for detection of prey, predators and mates, although some lizards and crocodiles do rely primarily on sight for finding their food. The sense of smell is generally more acute and important to the lifestyle of those species that are nocturnal or live in subterranean darkness and cannot rely on vision. However many diurnal species also depend largely on their sense of smell. Even tortoise appear to have the ability to locate a food source at a distance by detecting is scent.
Most reptiles (excluding crocodiles) also have a vomeronasal organ known as ‘Jacobson’s organ’. This is a fluid-filled, bi-lobed sensory organ in the roof of the mouth that also supplies the animal with sensory information on odours. This sense is analogous to, but different from, the sense of smell and is better termed chemo-reception or vomerolfaction. It is especially well developed in snakes and some species of lizard and is facilitated by the tongue of the animal. Although the tongue is not sensitive to odours, it serves as a vehicle for the collection of odour particles as it waves up and down. It is then withdrawn into the mouth and particles are transferred to the openings of the Jacobson’s organ where the odours are detected.
The forked tongue of snakes and some species of lizards means the vomerolfaction can be directional and allows them to follow scent trails. Each of the two tongue tips supplies odour particles to the respective lobe of the Jacobson’s organ and the further apart the tongue tips are held while collecting odour particles, the better able the animal is to detect gradients in the strength of the scent. Thus, it is probably the advantage of being able to follow scent trails that has resulted in the evolution of the characteristic forked tongue of snakes and monitor lizards.
Picture duplicated and information with permission of Johan Marais from ‘A Guide to the Reptiles of Southern Africa.