Monitor lizards, like snakes, have a pair of sensory organs located above the roof of the mouth called the Jacobson’s organs (see page 43 of this issue for more). The organs are used to detect scent particles within the air. These lizards continuously flick their forked tongues to collect these particles and to ‘taste’ the air; this extra sense is used mainly for hunting as monitor lizards are very active predators and are almost constantly foraging around for food. During the breeding season it is also used to help these usually solitary reptiles find a mate; the male will use his tongue to follow the scent of a female.
Answered by Adam Bland.