The vomeronasal organs, also called Jacobson's organs, are specialized scent detectors found in many animals and often located in the mouth. One of their primary functions is to pick up molecules of chemicals — called pheromones — that animals use to communicate and to find potential mates. Many reptiles use their tongues to carry scents to the Jacobson's organ, located in pits inside the mouth, while mammals often have specialized behaviors to convey the scents there. In humans, these organs stop development before the time of birth, and scientists have not identified a function for them in adults.
Many vertebrate animals, from reptiles to mammals, possess vomeronasal organs. These organs operate within the olfactory system that mediates the sense of smell and are chemoreceptors that detect small molecules of certain chemicals in the air. Most of the animal species with developed vomeronasal organs use them to detect pheromones released by other animals within their species for communication, particularly to interact with potential mates during reproductive season. Because of its close relation to the sense of smell, the Jacobson's organ is said to be an accessory olfactory sense organ.
Discovered by Ludwig Jacobson in the early 19th century, the vomeronasal organs develop in the embryos of most vertebrates, including humans. During gestation, the organs regress to the point of being nonfunctional by the time of birth. The presence and function of these organs in adult humans are controversial among scientists, but there is evidence that hormonal changes, such as those occurring in pregnancy, can trigger limited function within them. Some scientists have linked the heightened sensitivity of pregnant women to scents to the activation of their vomeronasal organs.
Certain reptiles and amphibians detect molecules with their tongues, which they use to carry the scent to their Jacobson's organ. They have specially adapted vomeronasal organs located within their mouths to identify an odor, often the scent of prey animals. Snakes and lizards have shifted most of their regular sense of smell to their tongues, which may be forked so that the molecules on the tongue's surface can be flicked against two sensory pits on either side of the mouth. Some adult mammals, such as deer, use their Jacobson's organ to detect the pheromones of potential mates during mating season.
Mammals have a wide variety of methods that they employ in transporting substances to their accessory olfactory system, which is complementary to their nasal odor receptors and does not replace it. Elephants will carry scent molecules on their trunks and apply these to their vomeronasal organs. Members of the cat family have a facial contortion or tick they employ to briefly close their nasal passages, pause their breathing, and open their mouth to detect odorants orally.