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Why do cats open their mouths when they smell?

Why do cats open their mouths when they smell?

I don’t know about you, but I smell with my nose. But does your cat ever appear to be smelling something with her mouth?


I bet you’ve seen this before: your cat is catching a scent the usual way, with his nose, when he suddenly seems to go into a kind of trance. Maybe he curls his lips, and wrinkles his little nose. He might leave his mouth hanging open for a few seconds, and make a huffing sound, like he’s sucking in air.


It almost appears as if your cat is disgusted by whatever she’s smelling. But it’s actually just the opposite. Your cat is intensely interested in what she’s smelling…except it’s not smelling that she’s doing, exactly.


A cat who leaves his mouth open while “smelling,” is actually performing what is called a flehmen response. It’s a way of capturing a certain kind of scent, except it’s not really a scent, and so it’s not really smelling. It’s more of a smelling/tasting kind of thing. But for simplicity's sake (and for lack of a better one), we'll use the term "scent" in this post.


But what the heck is it?


What is the flehmen response?

A director of a German zoo in the 1930s, who was an expert on big cats, gave this behavior its name. Flehmen, in German, means “to bare the upper teeth.” It’s also similar to another German word, flemmen, which means “to look spiteful.”[1] It’s a good description of the behavior either way.


A cat has a vomeronasal organ, also called a Jacobson’s organ


Before we can talk about why cats flehmen, we have to talk about a special organ that they have. It’s called the vomeronasal organ, also known as the Jacobson’s organ. This organ is made of two fluid-filled sacs that live behind the roof of a cat’s mouth. This organ has nerves that send information about certain scents directly to a cat’s brain.


There are actually a couple of little pathways from the cat’s mouth to this organ. Two tiny ducts are situated right behind a cat’s incisor teeth. (These are the little teeth in front, between the canines.)


So, a cat who is opening her mouth is allowing air, filled with scent particles, to travel up those ducts to the Jacobson’s organ. A cat who is huffing, or sucking in a bit of air, is actively trying to draw this scent-laden air in.


The importance of the blep


cat blep

You may think the “blep,” in which a cat doesn’t quite retract her tongue all the way back into her mouth, is just your cat being extra cute.


It’s likely that a blepping cat is seriously trying to analyze some fascinating chemical in the environment. Her extended tongue is gathering up as many scent particles as possible, and when she pulls her tongue back in, she’ll drag those collected particles across the ducts for further analysis.


And you thought she was just being adorable.


What is a cat trying to “smell” with his Jacobson’s organ?


The vomeronasal organ isn’t for everyday sniffing. Your cat probably isn’t going to engage this special organ to smell the clean laundry in the basket, or the junk mail you left on the end table.


The scents that are being processed by the Jacobson’s organ are pheromones.


What are pheromones?


cat flehmen response

Pheromones are chemical substances that every animal emits to communicate specific things with others of its own kind. If you’ve ever watched a line of ants following a scent trail, you’ve witnessed pheromones at work.[2]


Cats have special glands all around their bodies that secrete pheromones: on their foreheads, cheeks, chins, earspaw padstails, and inside their anuses. They use these glands to leave scent messages for other cats, and sometimes just for themselves.


A cat can tell a lot about another cat from his pheromones. Every cat has his own scent, and other cats can identify a cat from the pheromone “calling card” he leaves behind.


One cat can probably tell the mood of another cat from his pheromones, the sex of the other cat, and even if the other cat is healthy or unwell. There may be more, but we just don’t know enough about what cats know.[3]


If you’ve ever been head butted by a cat, or had your lap kneaded by a cat, you’ve probably been the recipient of a deposit of cat pheromones. If you’ve ever had a cat stick her butt in your face, you’ve been invited to smell that cat’s pheromones. If a cat has ever sprayed urine on a wall of your house, or scratched your furniture, there was probably some intent on the part of the cat to leave pheromones behind.


When does a cat flehmen?


Because the flehmen behavior is a response to pheromones, a cat is only going to flehmen under certain circumstances.


Cats will flehmen when sniffing the urine or feces of another cat, since pee and poop contain pheromones. Male cats will use the flehmen response to pick up on pheromones from female cats who are in heat. A mother cat may flehmen to keep track of her kittens. Kittens use their Jacobson’s organs to find their mother’s milk.[4]


The ability to detect pheromones is really a sixth sense


cat flehmening

We like to think that there are only five senses, because those are the ones we use every day: sight, hearing, touch, smell, and taste.


But the ability to detect chemicals through the flehmen response is a kind of sixth sense.


It’s not the only additional sense that animals (and some humans) enjoy. Echolocation, the ability to map one’s surroundings using reflected sound, could be considered an additional sense.


The ability to sense electric or magnetic fields might also be considered additional senses.


It's like there's a whole world out there that we're just not that tuned into.


Do other animals have Jacobson’s organs?

Cats are not the only animals who have this special organ and who flehmen.


In fact, all snakes and lizards, and many mammals, including dogs, cows, pigs, and horses, have vomeronasal organs, and use them in similar ways.


Snakes and lizards don’t have the special ductwork that cats have; their vomeronasal organs open directly to the roof of their mouths. A snake continuously flicks its tongue to carry chemical particles from the outside world directly to that organ.[5], [6]


Do humans have a Jacobson’s organ?


It’s the 21st century and you’d think a question about whether humans have a particular body part or not would be easy to answer. Incredibly, it’s not.


person with mouth open

The controversy about humans and whether or not we have a Jacobson’s organ started in 1703, and hasn’t yet been fully resolved. Frederik Ruysch, the Dutch botanist who discovered the Jacobson’s organ, claims to have seen it in a human cadaver he was dissecting. A hundred years later, Ludwig Jacobson, the Danish surgeon for whom this body part was named, insisted humans do not have one.


The problem was that Jacobson put his conclusions in a paper that was printed in Gothic script, in Danish. It didn’t get translated (into French) until 1950, and even then, only 150 copies of the translation were made. So, his work didn’t really get discovered until 1998, when it was found in the library at the Agricultural University of Copenhagen.[7] It’s part of the reason this controversy has been raging for several hundred years.


Today we have tools like endoscopy, CT scans, and MRIs to really look inside a human body. Current research suggests that about 1/3 of the population has a vomeronasal organ, but that unlike most animals who have a matched set, most of the people who have a Jacobson’s organ only have one, usually on the left side.


Other studies suggest that kids and young people are more likely than older people to have this organ, and some other researchers contend that almost all newborn babies have two.


The bigger question (if you can believe it) is whether the Jacobson’s organs in humans who have them even work. Some tests do show that there are working receptors inside the human version. Other tests claim the receptors aren’t functioning at all. One research paper referred to this question as a “hot controversial topic.”[8] I wouldn’t want to get in the middle of that fight between scientists at a bar.


It’s possible that in humans, a working vomeronasal organ could account for a woman’s heightened sense of smell during pregnancy and possibly even contribute to morning sickness.[9] But since we don’t know who has these organs and whether they function or not, it’s still just conjecture.


When should you be worried about a cat with an open mouth or heavy breathing?


Sometimes, flehmening isn’t really flehmening. An open mouth and/or huffing can sometimes indicate a health problem.


Respiratory distress. If your cat is panting, or breathing with an open mouth, it could be a sign of asthma or another respiratory problem.


Count the number of breaths your cat is taking per minute while she’s resting. If it’s consistently more than 30, call your veterinarian.[10]


Heat stroke or heat exhaustion. Even though cats evolved from desert animals, they don’t have a good way of dealing with excess heat. They can only pant or sweat through their paw pads. If a cat’s body temperature gets too high, a cat can experience organ damage or death.


If your cat has been stuck somewhere without access to water or shade, it’s more likely that any panting is an early sign of heat stress rather than flehmening. Remove the cat to a shaded area, provide water, and call your vet immediately for advice.


Poisoning. Cats exposed to dangerous toxins can pant as a response to the poisoning. Remember that foods that are safe for humans are not necessarily safe for cats. Review this list of foods that are toxic to cats.


Essential oils are another potential toxin for cats. Essential oils can be found in products you probably didn’t even realize contain essential oils: bathroom spray, candles, perfumes, and even certain foods. Read all about cats and essential oils here.


Hyperthyroidism. Thyroid hormones help manage a cat’s metabolism. If a cat’s body produces too much thyroid hormone, his metabolism will speed up, causing all manner of health problems.


Panting is not the only sign of hyperthyroidism in cats. Your cat’s appetite may increase while her weight drops. She may seem nervous or restless. She might drink more and urinate more, too. Vomiting, anxiety, nighttime yowling, and confusion are also signs.[11]


Luckily, hyperthyroidism is a very treatable disease. Contact your vet if you suspect your cat’s panting is unrelated to flehmening.


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why do cats open their mouths when they smell? 


DAwn and Timmy
Dawn LaFontaine

Dawn LaFontaine is a lifelong animal lover who always seems to have a little pet hair in her keyboard. Her blog, Kitty Contemplations, helps cat guardians better understand and care for the special beings they share their lives and homes with. Her cat-products business, Cat in the Box, sells beautiful, well-made, and award-winning products that she designed to meet the biological needs of cats.




[1] “Flehmen Response.” Wikipedia, Wikimedia Foundation, 30 May 2022,


[2] “Pheromone.” Wikipedia, Wikimedia Foundation, 29 Jan. 2023,


[3] Cinelli, Elisa. “The Reason Your Cat Sticks Their Butt in Your Face Is Surprisingly Simple.” Yahoo!, Yahoo!, 28 Oct. 2020,


[4] “Why Cats Sniff Rear Ends: VCA Animal Hospital.” Vca,


[5] “Jacobson's Organ.” Encyclopædia Britannica, Encyclopædia Britannica, Inc.,


[6] Helmenstine, Anne Marie, Ph.D. "Jacobson's Organ and the Sixth Sense." ThoughtCo, Aug. 25, 2020,


[7] Stoyanov GS, Matev BK, Valchanov P, Sapundzhiev N, Young JR. The Human Vomeronasal (Jacobson's) Organ: A Short Review of Current Conceptions, With an English Translation of Potiquet's Original Text. Cureus. 2018 May 17;10(5):e2643. doi: 10.7759/cureus.2643. PMID: 30034965; PMCID: PMC6050168.


[8] Stoyanov GS, et al.


[9] Helmenstine.


[10] “Home Breathing Rate Evaluation: VCA Animal Hospital.” Vca,


[11] “Top 10 Signs of Hyperthyroidism in Cats.” Vetco,


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