Jacobson’s organ, formally called the vomeronasal organ (VNO), is a specialized chemical-sensing structure found in the noses of most reptiles, amphibians, and mammals. It functions as a second, parallel smell system, tuned primarily to detect pheromones and other nonvolatile chemical signals that the main olfactory system handles poorly or not at all. The organ sits in the roof of the mouth or on the nasal septum depending on the species, and its biology spans everything from snake tongue-flicking to the way a stallion curls its upper lip. Whether humans still have a working version is one of the more contested questions in sensory anatomy.
Where It Sits and What It Looks Like
The vomeronasal organ is a tubular, typically blind-ended pouch lined with sensory tissue. In most mammals it sits along the base of the nasal septum, enclosed in a protective capsule of cartilage. In camels, for example, histological work shows the organ as a tube in the roof of the nasal cavity, surrounded by a J-shaped shell of hyaline cartilage.1PubMed. Structure of vomeronasal organ (Jacobson organ) in male Camelus Domesticus Var. dromedaris persica In cats, the cartilage takes a U-shaped form and wraps around a crescent-shaped internal lumen. The organ’s interior is lined on one side by sensory receptor epithelium packed with receptor cells and microvilli, and on the other side by ordinary respiratory-type epithelium. That asymmetry is consistent across species: one wall senses, the other wall just moves air or fluid.
The organ connects to the outside world through a narrow duct. In many mammals, this duct opens into the nasopalatine canal, a tiny passage linking the nasal cavity to the roof of the mouth, which is why the organ can sample chemicals from both inhaled air and substances in the mouth. In snakes and lizards, the openings sit in the palate, directly aligned with the tips of the forked tongue.
How the Organ Detects Chemical Signals
The sensory cells lining the VNO are neurons, and they work through a signaling chain that is distinct from the one used by the main olfactory system. When a chemical molecule lands on a receptor on one of these neurons, it triggers an enzyme called phospholipase C. That enzyme produces a fatty messenger molecule called diacylglycerol, or DAG. DAG then opens a specific ion channel called TRPC2, allowing charged particles to flood into the cell and generate an electrical signal.2PubMed Central. Odors activate dual pathways, a TRPC2 and a AA-dependent pathway, in mouse vomeronasal neurons Experiments in mice confirmed that knocking out the gene for TRPC2 severely cripples this channel, showing that TRPC2 is the primary gateway for turning a pheromone signal into a nerve impulse.3PubMed. A diacylglycerol-gated cation channel in vomeronasal neuron dendrites is impaired in TRPC2 mutant mice
The receptors themselves belong to two large gene families, called V1R and V2R. The size of these families varies wildly between species. Mice have around 187 functional V1R genes, rats about 102, cows roughly 32, and dogs only about 8.4PubMed Central. Dramatic variation of the vomeronasal pheromone receptor gene repertoire among five orders of placental and marsupial mammals That more than 23-fold range among mammals with working VNOs tells you that the organ’s importance is not uniform across species. A mouse lives in a world saturated with pheromone signals; a dog, with its extraordinarily powerful main olfactory system, apparently offloads much of that work to its regular nose.
What Happens When TRPC2 Is Missing
Mice engineered to lack the TRPC2 gene display striking behavioral changes that reveal just how much the VNO normally controls. Male mice without functional TRPC2 fail to distinguish male from female mice by scent, attempting to mate with both. They also show dramatically reduced aggression toward intruding males. Female mice missing TRPC2 display male-typical mounting behavior.5PubMed Central. TRICK or TRP? What Trpc2(-/-) mice tell us about vomeronasal organ mediated innate behaviors These behavioral shifts are more dramatic than what you see when researchers simply remove the VNO surgically, which suggests the TRPC2 channel may have some additional signaling roles beyond the VNO itself. Regardless, the overall picture is clear: disabling this one ion channel dismantles an entire layer of social and sexual behavior.
How Snakes Use Their Tongues to Feed the Organ
When a snake flicks its tongue, it is not tasting the air in the way we think of taste. The forked tongue collects chemical particles from the environment, and then delivers them to the paired openings of Jacobson’s organ in the roof of the mouth. High-speed film analysis of rat snakes showed the mechanics in detail: as the tongue retracts into its sheath, small structures called anterior processes elevate to meet it. The underside of the tongue presses against these processes, which are positioned directly beneath the VNO openings.6Canadian Journal of Zoology. Snake tongue-flicking: transfer mechanics to Jacobson’s organ Under an electron microscope, the surface of these processes is covered in ridged textures that increase surface area, presumably to help grab chemical molecules off the tongue.
The functional importance was confirmed with a simple experiment: when researchers surgically removed these anterior processes from garter snakes, the snakes could no longer detect food odors in an open field. The tongue-to-organ relay is not optional. It is the primary way snakes chemically navigate their environment, find prey, follow trails, and detect predators. Because the tongue is forked, each tine can deliver a slightly different chemical sample to each side of the paired organ, potentially giving the snake directional information about where a scent is coming from.
The Flehmen Response in Mammals
If you have ever seen a horse, cat, or goat curl back its upper lip with its head raised and nostrils flared, you have watched the flehmen response. This behavior channels air and dissolved chemicals over the nasopalatine canal and into the VNO. Cats do a version where they hold their mouth slightly open with a blank, almost grimacing expression after sniffing another cat’s urine or a novel scent. Stallions do it after smelling a mare’s urine, especially when checking for signs of reproductive readiness.
In cats, the VNO’s internal anatomy supports this function elegantly. The organ’s receptor epithelium is richly supplied with capillary blood vessels and bundles of vomeronasal nerve fibers that carry signals toward the brain. Vomeronasal glands within the organ produce secretions that help dissolve incoming chemical signals and bathe the receptor surface, keeping it primed for detection. The organ acts as a kind of pump: the autonomic nervous system controls blood flow to the tissue surrounding the VNO, and changes in blood vessel engorgement create a suction effect that draws fluid into the lumen. In cats, experiments showed that stimulating sympathetic nerve fibers caused fluid to be sucked into the organ, while stimulating parasympathetic fibers caused secretion to drip out.7Physiology & Behavior. Autonomic innervation of the vomeronasal organ of the cat This vascular pumping mechanism is how the organ actively samples its environment rather than passively waiting for chemicals to diffuse in.
Pheromones, Pregnancy, and Social Behavior
The VNO does not just detect pheromones passively. It feeds into neural circuits that drive powerful behavioral and hormonal responses. In mice, the VNO is involved in controlling aggression, mating, parental behavior, suckling, and the ability to distinguish members of your own species from predators.8PubMed Central. Mammalian pheromones
One of the most dramatic examples is the Bruce effect: a pregnant mouse exposed to the scent of an unfamiliar male will spontaneously abort her pregnancy. The mechanism runs through the VNO. The organ detects specific peptides from the immune system (MHC class I peptides) in the unfamiliar male’s urine. These peptides trigger a signaling cascade inside VNO neurons that produces a molecule called inositol trisphosphate, which in turn alters hormonal output from the brain and ultimately terminates the pregnancy.9PubMed. Pregnancy block by MHC class I peptides is mediated via the production of inositol 1,4,5-trisphosphate in the mouse vomeronasal organ The female’s VNO essentially “remembers” the chemical identity of the male who sired the litter. If the scent profile doesn’t match, the pregnancy is scrapped. Urine from castrated males or juvenile males of a different genetic background can trigger the same effect, confirming that the cue is chemical, not behavioral.
The neural wiring behind these responses begins during fetal development. The VNO forms early in embryonic life, and nerve fibers grow from it toward the brain. Cells that will eventually produce GnRH, the hormone that governs the entire reproductive axis, actually originate in the same tissue as the VNO and migrate along vomeronasal nerve fibers to reach the hypothalamus.10European Annals of Otorhinolaryngology, Head and Neck Diseases. Update Vomeronasal organ and human pheromones This migration is why the organ’s developmental biology is intertwined with reproductive function from the very beginning.
Evolutionary Origins in Water, Not on Land
An older textbook assumption held that the VNO evolved as an adaptation to terrestrial life, helping animals detect airborne chemicals after they left the water. That story has not held up. The VNO is present in fully aquatic salamanders, and because the last common ancestor of amphibians and amniotes (the group that includes reptiles, birds, and mammals) was itself fully aquatic, the organ almost certainly arose in water-dwelling animals.11PubMed Central. Presence of the vomeronasal system in aquatic salamanders In frogs, the VNO is located in a set of interconnected cavities anterior to the main olfactory chamber, and its sensory lining contains both ciliated cells and microvillous receptor cells. When the frog is submerged, water enters through the external nostril and flows directly over the vomeronasal sensory surface, allowing it to sample waterborne chemicals. When the frog is on land, the main olfactory organ handles airborne smells instead.12Acta Zoologica. Functional Architecture of the Vomeronasal Organ of the Frog (Genus Rana) This dual arrangement in an amphibious animal neatly illustrates that the VNO was not an invention for life on land but rather a pre-existing system that continued to be useful after the transition.
Across vertebrate evolution, the VNO has been lost multiple times independently. It is absent or nonfunctional in birds, crocodilians, whales and dolphins, some bats, and some salamanders.13Current Biology. What Is a Jacobson’s Organ and How Does It Work? Each loss appears to have happened separately, which makes the VNO a case study in convergent regression: when a lineage no longer benefits from the organ, it degrades over evolutionary time rather than being actively maintained.
Do Humans Have a Working Jacobson’s Organ?
This is the question people ask most, and the honest answer is: we have the structure, but it almost certainly does not function as a chemical sensor. Clinical examinations of more than 200 people found paired vomeronasal pits on the nasal septum in every single case. Biopsies showed that each pit leads to a closed tube, roughly 2 to 8 mm long, lined by a unique pseudostratified epithelium found nowhere else in the human body.14The Journal of Steroid Biochemistry and Molecular Biology. The vomeronasal (Jacobson’s) organ in man: ultrastructure and frequency of occurence The lining contains distinct cell types, including large, clear “light” cells packed with Golgi stacks and membrane-bound vesicles, which at first glance look like they could be doing something secretory or sensory.
But the critical piece is missing. Although the fetal human VNO contains neurons and nerve fibers that project toward the brain, multiple studies have failed to find neurons or nerve bundles in the adult human version.15PubMed Central. The clinical significance of the human vomeronasal organ Without neurons, the organ cannot transmit signals to the brain. It is an anatomical remnant, present in the vast majority of adults but disconnected from the nervous system.
The genetic evidence lines up with the anatomy. TRPC2, the ion channel gene essential for VNO signal transduction in mice, is a nonfunctional pseudogene in humans. Analysis of TRPC2 sequences across 15 primate species showed that the gene was likely still functional in the common ancestor of New World monkeys and Old World monkeys. But in the lineage leading to Old World monkeys and apes (the group that includes us), the gene accumulated disabling mutations and became vestigial.16PubMed Central. Relaxed selective pressure on an essential component of pheromone transduction in primate evolution The timing roughly coincides with the evolution of full trichromatic color vision in Old World primates, leading some researchers to speculate that enhanced visual signaling replaced chemical signaling in social and sexual communication.
Can Humans Still Respond to Pheromones Without the Organ?
The loss of a functional VNO does not necessarily mean humans are completely blind to pheromone-like chemicals. Neuroimaging studies have shown that when healthy men smell estratetraenol (EST), a steroid compound found in female urine that has been proposed as a candidate human pheromone, it activates the anterior hypothalamus, a brain region involved in reproductive and hormonal regulation. Men with anosmia (a complete loss of smell due to nasal polyps) did not show this hypothalamic activation when exposed to EST, even though their trigeminal nerve responses to irritating chemicals remained intact.17PubMed Central. Pheromone signal transduction in humans: what can be learned from olfactory loss This finding suggests that if EST does act as a pheromone-like signal in humans, the route is through the main olfactory system, not through a vestigial VNO.
This is an important distinction. In many animals, the VNO and the main olfactory system handle different chemical classes with different receptor families. But these systems are not perfectly segregated even in rodents. Some pheromone responses in mice persist after VNO removal, mediated by the main olfactory epithelium. In humans, it appears that whatever remnant sensitivity to social chemosignals might exist is routed entirely through the conventional smell pathway. The VNO itself is a bystander.
Who Actually Discovered It
The organ is named after Ludwig Levin Jacobson, a Danish surgeon who described it in nonhuman mammals in 1811. But Jacobson himself reported that the organ was absent in humans, and the common story that the Dutch anatomist Frederick Ruysch discovered the human version in the early 1700s is shaky. Ruysch illustrated the nasal septum of a two-year-old child, but it is unclear from his drawing whether he was pointing at the VNO, its opening, or an unrelated duct. A historical re-examination credits the Swiss anatomist Albert von Kölliker, who described the human VNO in 1877, as the true discoverer of the structure in our species.18PubMed. The human vomeronasal organ. V. An interpretation of its discovery by Ruysch, Jacobson, or Kölliker, with an English translation of Kölliker (1877) Naming conventions in anatomy do not always track with priority of discovery, and Jacobson’s organ is a good example: the person it is named after did not think humans had one.
Clinical Relevance in Nasal Surgery
Even though the human VNO is nonfunctional, its presence on the nasal septum has practical implications for surgeons. Septal surgery, rhinoplasty, and procedures to correct a deviated septum can involve the anterior third of the septum where the vomeronasal pits sit. Damage to the area is unlikely to cause any sensory loss, given the organ’s lack of neural connections. However, the vomeronasal pits can occasionally be mistaken for pathological openings or cysts during endoscopic examination, leading to unnecessary biopsies or concern. Awareness that these small bilateral depressions are a normal anatomical feature, present in virtually everyone, saves both the surgeon and the patient unnecessary worry.15PubMed Central. The clinical significance of the human vomeronasal organ
There is also an indirect clinical connection through embryology. Because the GnRH-producing neurons that control the reproductive hormone axis migrate along vomeronasal nerve fibers during fetal development, disruptions to this migration pathway can result in Kallmann syndrome, a condition characterized by both an absent or reduced sense of smell and failure to undergo puberty. The developmental link between the VNO and the reproductive system is a vestige of the organ’s ancient functional importance, even though the organ itself no longer works in adults.