Flehmen Response — Cat Behaviour Glossary

Flehmen response

Flehmen response — a behaviour in which a cat curls back its upper lip, opens its mouth slightly, and inhales. The grimace-like expression routes scent molecules to the vomeronasal organ (also called Jacobson's organ), a specialised chemical-sensing structure in the roof of the mouth. It is an information-gathering behaviour, not an expression of disgust.

The vomeronasal organ

The vomeronasal organ (VNO) is a pair of fluid-filled sacs that open into the roof of the mouth via small ducts just behind the upper incisors. Unlike the olfactory epithelium of the nose, which detects airborne volatile molecules, the VNO specialises in non-volatile compounds — particularly pheromones and chemosignals in urine, glandular secretions, and saliva.

When a cat performs the flehmen response, it is essentially drawing a scent sample directly into the VNO for chemical analysis. The information is processed by a separate neural pathway to the accessory olfactory bulb, distinct from the main olfactory system. Research on this organ is reviewed in detail in veterinary physiology texts such as Dukes' Physiology of Domestic Animals.

What triggers it

The flehmen response is most reliably triggered by:

Urine from other cats — especially that of intact males and females in oestrus
Glandular secretions from cheeks, paws, or the perineal area
Certain plant compounds — valerian, catnip metabolites, and some essential oils
Human sweat or personal-care products that contain steroid-like compounds

Not the same as a sniff. If you see your cat sniff something and then look briefly vacant with its mouth slightly open — that is the flehmen response. The momentary still expression is the cat processing the chemical information. It typically lasts only a few seconds.

Role in social and reproductive behaviour

The primary evolutionary function of the VNO in mammals is reproductive and social communication. Male cats use it heavily to assess whether a female is in oestrus; females use it to evaluate rival females' territorial signals. In neutered domestic cats, the response still occurs — cats continue to investigate and interpret chemical signals from other animals even without a reproductive drive. This is why even neutered cats may show flehmen in response to urine from an intact cat, or from an unfamiliar cat entering their territory.

The International Cat Care resource on feline communication places the flehmen response within the broader picture of how cats gather social information through chemical channels.

Frequently asked questions

Is the flehmen response the same as smelling?

Not exactly. Regular sniffing uses the olfactory epithelium in the nose. The flehmen response routes scent molecules to the vomeronasal organ (VNO), a separate chemical-sensing organ in the roof of the mouth. The VNO specialises in detecting pheromones and chemosignals that are largely invisible to ordinary smell. See our pheromones glossary entry for how these chemicals work.

Why does my cat pull a face at certain smells?

The curled-back lip and open-mouthed grimace is what the flehmen response looks like from the outside. It is not a sign of disgust — the cat is simply maximising the scent sample being delivered to the vomeronasal organ. It is most commonly triggered by other cats' urine, mating-related scents, or unusual or intense odours.

Do all cats show the flehmen response?

All domestic cats are capable of the flehmen response — it is an anatomical behaviour, not a learned one. However, some individuals show it more readily than others. It is also seen in lions, tigers, horses, goats, deer, and many other mammals. The biology is described in detail in veterinary anatomy references.

Can I trigger the flehmen response deliberately?

Cats are most likely to show it in response to strong animal-derived scents — other cats' urine or gland secretions, certain essential oils, or Valerian root (which contains compounds similar to cat pheromones). It is not harmful to let your cat investigate these smells.

Is the flehmen response related to aggression?

Not directly. The response is informational — the cat is analysing a scent, not reacting emotionally to it. What the cat does after the analysis may include aggression, if the scent indicates a rival male, but the flehmen response itself is neutral.