If you guessed because it makes them look bad apples, you’d only be half right.
The reason snakes have forked tongues is because they use them to "smell."
By flicking its tongue in the air, a snake can collect odor-causing particles that it then delivers to a sensory organ in its mouth. The vomeronasal organ (also called Jacobson's organ) is located just behind the nose and is only accessible via two thin grooves in the roof of the mouth. To “smell” the particles it collects from the air, the reptile runs its tongue along pads at the base of the grooves, allowing the particles to travel up the grooves to the sensory organ.
Diagram of the Vomeronasal or Jacobson's Organ. Photo credit: Fred the Oyster/Wikipedia (GFDL)
The split tongue of a snake is thought to be advantageous because it allows the snake to smell in three dimensions. Since the two tips pick up odors from slightly different locations in space, the snake can detect the direction of the source of the smell. If an odor is slightly stronger on the left tip, for example, the source must be somewhere to the snake’s left. Using a similar method, humans can often detect where a sound is coming from, thanks to the distance between our two ears.
Because snakes also have noses, we know that the vomeronasal sense is an additional sense, not a replacement for smelling, once again busting the myth that there are only five senses. Vomeronasal perception is thought to have evolved specifically for mating because it facilitates reproduction by detecting pheromones, but snakes also use it for detecting food and following their prey.
Mammals also have vomeronasal organs, in fact, you’ve probably seen your cat using its vomeronasal organ. Unlike snakes, cats don’t need to flick their tongues through the air to collect pheromones — instead they taste the particles using a distinctive routine known as the Flehmen response. The animal lifts its head, wrinkles its nose, and lifts its upper lip. In horses the Flehmen behavior is often followed by neighing.
A stallion lifts his lip in response to pheromones. Photo credit: Waugsberg/Wikipedia (CC BY-SA 3.0)
In case you’re wondering, yes, human also have vomeronasal organs, although we no longer seem to use them. The human vomeronasal organ is classified as a vestigial feature — in other words, a trait that our ancestors used but has gradually become defunct from generations of disuse. Other examples of vestigial features include your appendix, the ability to wiggle your ears, and wisdom teeth.
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