The Jacobson's organ is located on the roof of the mouth in reptiles and mammals. It is also called the vomeronasal organ. This organ works by sensing the chemicals such as pheromones.
Pheromones are the chemicals that a living organism emits and that organisms of the same species can detect. Scientific research has revealed that plants, vertebrates and insects communicate in this chemosensory way. For example, the female silkworm signals potential mates by releasing the pheromone bombykol, first discovered in 1959 by Adolf Butenandt. When bees swarm, it is in response to other bees that emit pheromones as an alarm.
Reptiles and mammals use their Jacobson's organ to sense pheromones. Elephants touch the tips of their trunks to this organ to enact their chemosensory perception of things. A lion uses it for sensing sex hormones, and will often open its mouth to sniff the pheromones it senses.
The Jacobson's organ also helps some animals perceive other chemical compounds besides just pheromones emitted between species. For instance, snakes find their prey by using it. A snake places its tongue on the two pits in the roof of its mouth after having its tongue in the air to allow it to properly sense the direction of its prey. The reason snakes have a forked tongue is so that the tongue can touch these pits. The deeper the fork in a snake's tongue, the more the snake uses its Jacobson's organ.
Snakes have a fully functioning organ, but humans and some species of bats do not. The vomeronasal organ develops in the fetus, but then does not continue to develop fully. Researchers have found that some people may have at least a partially functioning one, but other researchers consider only a fully functioning Jacobson's organ as counting as having one, so these results are controversial.