Why Do Snakes Flick Their Tongues? Smelling With the Tongue

How the tongue and Jacobson’s organ work

A snake’s tongue is long, slender and deeply forked. It slips in and out through a small notch in the front of the upper lip, so the snake can flick it even with its mouth closed. Out in the air, the tongue tips sweep up and down several times, touching the ground or simply waving through the air, and molecules stick to the moist surface.

When the tongue is drawn back in, the chemicals are delivered to two small openings in the roof of the mouth. These lead to the paired vomeronasal organs, which are lined with sensory cells that respond to scent molecules and send signals to the brain. Older books often said the tongue tips slide straight into the openings; more recent studies suggest that pads on the floor of the mouth help transfer the chemicals instead. Either way, the tongue is the delivery system and the vomeronasal organ is the sensor.

Snakes also smell normally through their nostrils. The two systems work together, but for following trails and identifying other animals, the tongue and vomeronasal organ are especially important.

Why are snake tongues forked?

A forked tongue lets a snake sample two points at the same time. In a well-known 1994 paper in the journal Science, biologist Kurt Schwenk argued that this works much like having two ears for sound or two eyes for depth. The snake can compare the amount of scent on the left tip with the amount on the right tip.

During a flick, the tips spread widely apart, often to a distance greater than the width of the snake’s head. If the scent is stronger on one side, the snake knows which way to turn. This ability to steer by comparing two signals is called tropotaxis. By moving along a trail and constantly checking left against right, a snake can stay on the edge of a scent trail laid by a mouse or another snake.

Forked tongues are typical of reptiles that actively search for food, such as snakes and monitor lizards. Lizards that wait in ambush or rely more on sight tend to have thicker, less deeply divided tongues.

Following prey

Tongue flicking is central to how snakes hunt:

• Finding prey. Many snakes, including the common garter snake and the corn snake, search actively through grass, burrows and leaf litter, flicking constantly to detect the scent of worms, frogs or rodents.
• Recognizing food. Young snakes respond to the scent of their natural prey with bursts of rapid tongue flicks, even before they have ever eaten.
• Trailing after a strike. Rattlesnakes and other vipers often bite prey and let it go, then follow its scent trail until the venom takes effect. Studies show that tongue-flick rates rise sharply right after a strike, a behavior known as strike-induced chemosensory searching.

Pit vipers add a second sense on top of smell: heat-sensitive pits on the face. That works in a different way, explained in how pit vipers sense heat.

Finding mates and recognizing other snakes

Snake skin carries chemical signals called pheromones, and the tongue reads them. In the breeding season, male snakes follow the scent trails of females, sometimes for hundreds of meters, and can tell from a single trail whether it was left by a female of their species.

Garter snakes show how precise this is. Males at large winter dens distinguish females from males by skin chemicals alone, and some males that briefly carry female-like scent attract courtship from other males; read more in Why do garter snakes form mating balls?. Young snakes of some species also follow the scent trails of adults to find winter dens.

Sensing danger

Snakes also use tongue flicks to check for predators and new surroundings. A snake placed in an unfamiliar place usually flicks rapidly as it explores, and many snakes react with alarm to the scent of predators such as kingsnakes. Faster flicking generally means “I am gathering information,” not “I am about to bite.”

Myths and facts about snake tongues

• Myth: the tongue is a stinger. A snake’s tongue is soft, has no venom and cannot hurt anyone. Venom is delivered through fangs.
• Myth: snakes taste food with their tongues. The tongue has few or no taste buds; it is a scent collector, not a taste organ like ours.
• Myth: snakes hear with their tongues. Snakes sense sound and ground vibrations through their inner ear and jawbones, not their tongues; see Can snakes hear?.
• Fact: a snake flicks its tongue all the time. Even a resting snake flicks now and then to monitor its surroundings, and the rate rises whenever something new or interesting appears.

The flicking tongue is one of the most recognizable things about snakes, and it is also one of their most important tools. With it, an animal that lacks external ears and often has limited eyesight can map a world of scents in remarkable detail.