The vomeronasal organ (VNO), also known as Jacobson’s organ, is a small chemosensory structure located in the nasal cavity that detects pheromones and other chemical cues in many animal species. In humans, the question is more complicated. The anatomical structure is present in most adults, often visible as a small pit on either side of the nasal septum, but whether it functions the way the animal version does is one of the most contested questions in the human pheromone literature.
The short version: animals have a clearly functional VNO that detects pheromones through dedicated receptor types and dedicated neural pathways. Humans have a structurally recognizable VNO that may or may not connect to the brain in any meaningful way, depending on which research you read.
What this means for the existence question is less dramatic than the public conversation makes it sound. Even if the human VNO turns out to be vestigial, pheromones can still be detected through other receptor pathways in the main olfactory system, which several lines of recent research have documented in detail.
The questions this page answers, in order: what is the vomeronasal organ, why is it also called Jacobson’s organ, how does the animal version work, do humans have a functional VNO, and how does human pheromone detection actually happen whether the VNO is involved or not.
What The Vomeronasal Organ (VNO / Jacobson’s Organ) Actually Is
The vomeronasal organ is a small tubular structure located near the base of the nasal septum, separated from the main olfactory system by a thin wall of tissue. In most species that have a functional VNO, it consists of two narrow chambers, one on each side of the septum, lined with chemosensory receptor cells that respond to specific compounds.
The structure was first described in detail in 1813 by the Danish anatomist Ludvig Levin Jacobson, which is why it is often referred to as Jacobson’s organ. Jacobson identified the structure in mammals, including humans, and noted its connection to the broader olfactory system. The name has stuck across two centuries of research, and modern literature uses “Jacobson’s organ” and “vomeronasal organ” interchangeably depending on context.
The term “vomeronasal” comes from the structure’s anatomical position. It sits next to both the vomer bone (the small bone forming the lower part of the nasal septum) and the nasal cavity itself, and the name combines the two locations into one descriptive label.
In animals where the VNO is functional, the chemosensory cells inside it project to a separate brain region called the accessory olfactory bulb, which is distinct from the main olfactory bulb that handles ordinary smell. This separation is part of why the VNO is often described as a “second nose”: it operates on a different neural pathway from conscious smell and processes chemical information that the main olfactory system would not necessarily pick up.
The size and prominence of the VNO varies dramatically across species. Snakes have a particularly well-developed Jacobson’s organ that they use constantly, flicking the tongue to deliver chemical information directly to the receptors. Cats and dogs have functional VNOs that activate during specific behaviors, particularly the flehmen response, where the animal lifts its upper lip to channel air toward the structure. Mice and rats use their VNOs heavily for social and reproductive cue detection.
Humans have something. The question is what.
How The VNO Works In Animals
In animals with a fully functional vomeronasal organ, the system runs on its own dedicated machinery. Different receptor types, different neural pathway, different brain regions. Understanding the animal version helps explain why the human VNO question matters and why it has been so hard to resolve.
The receptor side is where the differentiation starts. The main olfactory system uses a family of receptors called olfactory receptors, with around 400 functional types in humans and similar numbers across most mammals. The VNO uses a separate family of pheromone receptors, primarily V1R and V2R types, which are tuned to different kinds of compounds than the main olfactory receptors. V1R receptors generally respond to small volatile molecules. V2R receptors respond to larger peptide and protein molecules that the main olfactory system would struggle to detect.
When a chemical reaches the VNO and binds to one of these receptors, it triggers a neural impulse that travels along its own dedicated nerve, the vomeronasal nerve, to a specialized brain region called the accessory olfactory bulb. From there, the information routes to areas of the limbic system involved in emotional, hormonal, and reproductive processing, particularly the amygdala and hypothalamus.
What the system never does is route to conscious smell. Animals processing pheromones through the VNO often show no behavioral indication of having “smelled” anything, even as their hormonal or behavioral state shifts in response to what they detected. The system operates below the level of conscious perception by design.
This pathway is what most animal pheromone research is built on. The clean releaser effects in moths, the hormonal primer effects in mice, the territorial responses in cats. Most of these flow through the VNO or VNO-equivalent structures. The reason animal pheromone research produces such clean results is partly that the animals being studied have a dedicated detection system specifically built for chemical cues, separate from ordinary smell.
The human pheromone story gets complicated because the human version of this dedicated system might not exist in any practical sense, even though the structural remnant of it does.
Do Humans Have A Vomeronasal Organ?
Yes, anatomically. Whether it does anything is the harder question.
The human vomeronasal organ develops during fetal life as a clearly identifiable structure, with the same basic architecture as the VNO in other mammals. Studies of fetal tissue have documented the developmental sequence in detail. By birth, most humans have a structurally recognizable VNO, and adult anatomical surveys have confirmed that around seventy to ninety percent of adults still have a visible vomeronasal pit on at least one side of the nasal septum, often visible t