The fantastic sense of smell of our pets is often mentioned. Most people know that they are in an exceptional position both in terms of smell receptors and in the area of the brain responsible for processing smells. However, fewer may know that they also have a special organ that supports their sniffing ability, with which they are even able to identify scent samples underwater without needing to inhale them.

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In the following, we will discuss the dog’s special olfactory organ, also known as the vomeronasal or Jacobson’s organ.

Photo: welovedogz.hu

What is the Jacobson’s organ and where is it located

The vomeronasal or Jacobson’s organ (VNO) is a paired supplementary olfactory organ which connects directly to the dog’s brain. The former name comes from the nearby vomer bone, which is found in the skull of animals, and the latter from Ludvig Levin Jacobson, who studied this organ in several species in 1811. This organ is present in all snakes and lizards, as well as – without aiming for completeness – in mammals such as cats, mice, and cows. Its primary function is to detect and interpret pheromones emitted by other animals – but it is not only suited for that; we will detail this next.

In dogs, the vomeronasal organ is located in the soft tissues of the nasal septum, in the nasal cavity, directly above the hard palate. Behind the canine teeth, its bump-like entrance is clearly visible. If we look closely into a dog’s mouth, we can often see it. When looking at such photos on the internet, people often comment that these are simply bumps from growing teeth, but this is an incorrect assumption.

The entrance to the vomeronasal organ on a dog’s palate:

What do dogs use the Jacobson’s organ for?

Dogs use the Jacobson’s organ to detect pheromones, as well as volatile compounds, although the primary olfactory system is responsible for detecting the latter. The pheromones may come from prey animals, predators, or their own kind. The activation of the vomeronasal organ triggers appropriate behavioural responses in the presence of one of these three stimuli – we will also immediately address what exactly this reaction is.

Since dogs do not have such developed sound-based communication as humans, they use scents to understand what is happening around them. For example, dogs often sniff each other’s rear ends to gather information. In the anal area, under the skin, there are pheromone glands that reveal, for instance, the dog's sex, health status, mood, and much more.

Bottom sniffing is information gathering / Photo: welovedogz.hu

The Flehmen response

As mentioned earlier, while perceiving pheromones – or stimuli – we can observe a signal in dogs called the Flehmen response, which is accompanied by very characteristic external signs. In this case, the animal pulls up their upper lip to reveal their incisors, inhales air, and their nostrils close – as if they were grimacing. They usually remain in this posture for a few moments. During the reaction, they are able to channel air directly into the vomeronasal organ using this technique for extremely fast and efficient information processing.

Can dogs identify scent trails underwater?

The answer is yes, although exactly how is not yet fully understood. However, it is already well known that the vomeronasal organ plays a role in the process. Underwater, the dog is able to activate the vomeronasal organ by opening their mouth, which, as mentioned, is suitable not only for detecting pheromones but also volatile compounds – and it works effectively in water as well.

Thanks to tests and studies carried out with cadaver-detection dogs, it is also suspected that dogs taste the water while searching, which probably also helps them to localise the sample they are looking for.

In films, for example, it is often shown that when fleeing prisoners wade across a river, the dogs will then lose the scent trail – but this is not true for the reasons detailed above.

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Do humans have a vomeronasal organ?

Numerous studies have tried to determine whether the vomeronasal organ exists in adult humans. According to one estimate, about 92% of their subjects had at least one intact vomeronasal organ. However, researchers of another study claimed that the structure of the VNO disappears during foetal development, as it does in some other primates. Refuting this, another research group stated that their colleagues simply failed to notice the structure in older foetuses.

So the question cannot yet be answered with full certainty, but in the current situation, it is most likely that even if we do possess the organ, there are no active sensory neurons in the VNO, and there is also no evidence that any sensory receptor cells potentially present in adult humans are connected to the brain. Most experts in the field are sceptical about the existence of a functioning vomeronasal organ in humans.