Forked tongue

A forked tongue is a tongue split into two distinct tines at the tip; this is a feature common to many species of reptiles. Reptiles smell using the tip of their tongue, and a forked tongue allows them to sense from which direction a smell is coming.^{[citation needed]} Sensing from both sides of the head and following trails based on chemical cues is called tropotaxis.^[1] It is unclear whether forked-tongued reptiles can actually follow trails or if this is just a hypothesis.^[2][3][4]

Forked tongues have evolved in these squamate reptiles (lizards and snakes) for various purposes. The advantage to having a forked tongue is that more surface area is available for the chemicals to contact and the potential for tropotaxis.^[5] The tongue is flicked out of the mouth regularly to sample the chemical environment. This form of chemical sampling allows these animals to sense non-volatile chemicals, which cannot be detected by simply using the olfactory system.^[6][7][8] This increased ability to sense chemicals has allowed for heightened abilities to identify prey, recognize kin, choose mates, locate shelters, follow trails, and more.^[8]

Forked tongues have evolved multiple times in squamates. It is unclear, based on the morphological and genetic evidence, where the exact points of change are from a notched tongue to a forked tongue, but it is believed that the change has happened two to four times.^[1][9] A common behavioral characteristic that has evolved in those with forked tongues is that they tend to be wide foragers.^[1][10]

Hummingbirds also have tongues that split at the tip.^[11] Galagos (bushbabies) have a secondary tongue, or sublingua, used for grooming, hidden under their first.^[12]