Observe: Your cat crouches with his nose down, nuzzling the ground, before glancing up with his mouth slightly (and comically) open. So, why do cats open their mouths after smelling something? It turns out this is a behavior called the flehmen response. Let’s take a look at what exactly the flehmen response in cats is!
Why do cats open their mouth when they smell?
Although it may appear that your cat is staring in open-mouthed disbelief, there’s actually a reason he pulls this funny face: Your cat is analyzing a new or strange smell—but instead of doing so through the nostrils, he is “scent-sucking” through the roof of his mouth.
This is called the flehmen response—or, depending on who you ask, the cat stinky face.
What is the flehmen response in cats?
The flehmen response opens up two small ducts on the roof of your cat’s mouth behind the incisors. This allows the scent to travel to the vomeronasal organ, also called Jacobson’s organ, inside of the nasal cavity. By taking in smells via this method, cats can detect chemicals by using nerves that lead directly to the brain—even odorless or undetectable smells!
The flehmen response is a voluntary action and is considered a sense that lies somewhere between smell and taste.
Most cats will show the flehmen response now and then, especially when they come across strong scents like urine markings, unfamiliar objects, or even human sweat. If you’ve ever seen your cat sniff something, then suddenly make that funny “grimace,” that’s exactly what’s happening! Some cats do it a lot, while others might not show it as obviously—it all depends on their personality and what scents they’re exposed to.
What does the cat flehmen response look like?
In Feline Behavioral Health and Welfare, Theresa L. DePorter notes that “cats display a response called the ‘gape’ to gather pheromones into the passageways. The gape expression follows olfactory investigation and is characterized by a tongue lick to the nose, followed by the cat’s gazing in a thoughtful, preoccupied fashion while the upper lips are raised slightly and fluffed with the mouth slightly open.”
Common causes for the flehmen response
Male cats, particularly unneutered ones, exhibit this behavior more frequently. This increased occurrence is linked to their heightened sensitivity to pheromones related to mating.
Cats exhibit the flehmen response most often when they encounter:
- Urine or scent markings – Particularly from other cats, as they use pheromones to communicate.
- Female cat pheromones – Males commonly use it when detecting the pheromones of a female in heat.
- Mother cats tracking their kittens – They use it to identify and monitor their young through scent.
- New or strong-smelling objects – Such as certain foods, plants, or even human sweat. Catnip is especially famous for triggering the flehmen response in cats as it produces a chemical response in a cat’s brain when inhaled or ingested.
- Household smells – If something carries a particularly intriguing or unfamiliar scent.
Cats will most often exhibit the flehmen response in areas where scents are concentrated, such as:
- Litter boxes – This is a hotspot for pheromones, making it one of the most common places to see the response.
- Doorways and walls – If another cat has rubbed or marked a spot, your cat may detect it.
- Shoes or bags – Items that have been outside can carry interesting smells for your cat to analyze.
- Furniture or blankets – If new scents are present, your cat may use the flehmen response to investigate.
Recognizing the flehmen response
When a cat exhibits the flehmen response, it may appear to be:
- Slightly opening its mouth while pausing mid-sniff.
- Curling back its upper lip, exposing some teeth.
- Squinting or widening its eyes as it focuses on the scent.
- Holding the pose for a few seconds, almost as if in deep thought.
The flehmen response typically lasts just a few seconds before your cat returns to normal. Some cats do it frequently, especially in multi-cat households where scent communication is common. Others may only do it occasionally, particularly if they are less reactive to scents.
Sometimes, cat owners confuse the flehmen response with other actions.
Don’t confuse the flehmen response in cats for open-mouth breathing
The flehmen response in cats is nothing to worry about—it is a perfectly normal behavior. However, don’t confuse it with other signs from your cat that may actually be cause for concern.
If your cat is breathing with the mouth open, it may be a sign that she is experiencing some sort of respiratory distress. If your cat is breathing with mouth open, it may be a sign of:
- Asthma. If you notice these symptoms consistently or a respiratory rate of over 50 breaths/minute, call your vet or make a trip to the emergency clinic. (Hint: Count the number of breaths in 15 seconds and multiply by 4 to get the total breaths per minute.)
- Heat stroke. Panting can be a sign of heat exhaustion. Even if it’s not a hot summer day, keep in mind that your cat could be experiencing issues related to heat after becoming trapped in a clothes dryer or in a shed or other structure with unregulated temperatures; after being left in a confined space without access to water or shade, such as a sunroom; or after being left in a cage in direct sunlight.
- Poisoning such as after consuming garlic or onion or being exposed to harmful essential oils.
- Hyperthyroidism. Hyperthyroidism is often seen in middle-aged to geriatric cats (around 8+ years of age). When in doubt, as your cat ages, make sure to monitor him for the clinical signs of this common endocrine disease.
Finally, panting after playing can be a sign of hyperthyroidism in cats. When in doubt, call your veterinarian or head to an emergency clinic to get your cat checked out!
Here’s how to tell the difference between these conditions and the flehmen response:
| Behavior | Description | Key Differences |
|---|---|---|
| Flehmen Response | Mouth slightly open, lip curled, “sniffing” intensely | Happens after sniffing something specific, short-lived, and normal behavior |
| Panting | Mouth open, rapid breathing | Can be due to stress, heat, or overexertion; not scent-related |
| Sneezing | Sudden, forceful exhale through the nose | Happens due to dust, allergies, or illness |
| Yawning | Wide mouth opening, stretching jaws | Associated with tiredness or relaxation |
| Teeth Chattering | Rapid jaw movement, clicking sounds | Usually seen when watching birds or prey animals (linked to hunting instinct) |
| Asthma | Labored breathing, wheezing, coughing | Can occur suddenly, often during activity or stress; may worsen over time |
| Heat Stroke | Excessive panting, drooling, lethargy, vomiting | Caused by overheating, requires immediate cooling and vet attention |
| Poisoning | Drooling, vomiting, seizures, lethargy | Usually follows ingestion of a toxic substance; needs urgent vet care |
| Hyperthyroidism | Increased thirst, weight loss despite eating more, hyperactivity | Long-term condition, usually affects older cats |
Understanding these differences helps you recognize when your cat is actually engaging in the flehmen response rather than another behavior.
Which other animals exhibit the flehmen response?
Cats are just one of dozens of animals that exhibit the flehmen response. Dogs, camels, llamas, goats, horses, tapirs, rhinos, rams, elk, giraffes, buffalos, hedgehogs, and more have the flehmen response. Even your cat’s wild relatives—lions and tigers—show this behavior!
Although most people think that dogs have the upper hand when it comes to sense of smell, cats actually have a stronger flehmen response. According to Bradshaw, the average tabby has 30 different types of receptors in the vomeronasal organ while a hound dog has just 9.
Once upon a time, humans may have had the flehmen response—but no more. There is evidence of the vomeronasal organ seen during human fetal development. However, when a baby is born, “the only remaining clues are a pair of pits at the bottom of our nostrils where the ducts used to connect to the organ.”
The more you know
So, now you know why your cat makes that funny open-mouth face after smelling something. Spread the knowledge: The flehmen response in cats (and many other animals) is an intriguing factoid that you can impress other pet parents with!
Sources:
- Use of Pheromones in Feline Practice
- Why Do Dogs, Cats, Camels, and Llamas Make That Weird Face?
- Why Cats Sniff Rear Ends
