The flehmen response is a behavior in which an animal curls back its upper lip, exposing its front teeth, and inhales with its mouth open. It looks bizarre, almost like the animal is grimacing or laughing, but it serves a precise biological purpose: drawing scent molecules into a specialized sensory organ in the roof of the mouth. Horses, cats, lions, elephants, and dozens of other mammals do it, most often to analyze chemical signals from other animals.
How the Flehmen Response Works
The target of the whole performance is a structure called the vomeronasal organ (also known as Jacobson’s organ), a small pouch of sensory tissue located in the roof of the mouth or nasal cavity. This organ is essentially a second nose, but tuned to detect chemical signals that regular sniffing can miss. It sits inside a bony capsule, sealed off from normal airflow, so scent molecules can’t just drift in passively. They need to be actively delivered.
That’s what the lip curl accomplishes. When an animal raises its head, opens its mouth, and curls its upper lip, it’s creating a pathway for scent-laden air to reach the vomeronasal organ. In many species, a duct connects the mouth to this organ, forming a continuous track from the nostrils through the organ and into the oral cavity. Once molecules arrive, the organ works like a tiny pump, pulling chemical compounds into its sensory lining and then expelling them in a repetitive cycle. The chemicals need to dissolve in the nasal fluids before the organ can read them, which is why some animals (like male giraffes) will actually taste a female’s urine before performing the response.
The signals detected this way bypass the regular smell-processing pathways and feed directly into brain regions involved in reproductive and social behavior. It’s not just “smelling harder.” It’s routing specific chemical information through a completely separate sensory system.
Why Animals Do It
The primary function of the flehmen response is reading reproductive status. In cattle, bulls perform flehmen after sniffing a cow’s urine, saliva, feces, or even milk, and they do it far more intensely when the cow is in estrus (heat) than at other times in her cycle. Research on cattle shows that flehmen behavior ramps up about four days before estrus, peaks on the day of estrus itself, and then tapers off. Stallions show the same pattern, responding significantly more to mares in estrus than to those not in heat. Rams use it to confirm the reproductive state of ewes.
The chemical compounds responsible for triggering this response are pheromones, molecules released in body fluids that carry information about an animal’s hormonal state. During estrus, females produce specific pheromonal compounds that are absent or present in much lower concentrations at other times. When a male picks up these chemicals through flehmen, it kicks off a cascade of premating behaviors. In this sense, flehmen functions as a biological pregnancy-readiness detector.
But reproduction isn’t the only trigger. Young foals perform the flehmen response after investigating the urine of other horses, long before they’re sexually mature. Colts will sniff the urine or perineum of a mare in heat and flehmen without following up with any mating behavior. This suggests the response also plays a role in general social information gathering, helping animals learn about the identity, health, or emotional state of others in their group.
Which Animals Show Flehmen
The list is long and spans the mammal family tree. Ungulates (hoofed animals) are the most visible practitioners: horses, cattle, sheep, goats, bison, giraffes, tapirs, rhinoceroses, hippopotamuses, antelope, and kobs all display the response. Among predators, cats are especially well known for it. Lions, tigers, and domestic cats all perform flehmen, though the expression varies by species. Llamas, hedgehogs, giant pandas, and elephants round out the roster.
Elephants have their own variation on the technique. Rather than just curling a lip, they use the finger-like tip of their trunk to pick up chemical stimuli and transfer them directly to the vomeronasal opening in the roof of their mouth. It’s the same sensory system, just adapted to very different facial anatomy.
The Flehmen Response in Cats
If you’ve ever seen your cat freeze with its mouth hanging open and a vacant, slightly disgusted expression on its face, you’ve witnessed flehmen. Cat owners sometimes describe it as “stink face” or think something is wrong, but it’s completely normal. Cats have a particularly strong flehmen response, and they show it in reaction to a wide range of smells: another cat’s urine, anal gland secretions, facial pheromones rubbed on furniture, dirty laundry, or simply any odor that strikes them as new or interesting.
In cats, the vomeronasal organ connects to the mouth through a pair of small ducts just behind the front teeth. When your cat holds its mouth open in that characteristic gape, it’s pulling air across those ducts to sample whatever it just sniffed. The whole episode typically lasts only a few seconds. Cats of both sexes and all ages do it, not just intact males, which reinforces the idea that flehmen serves a broader sensory function beyond mate detection.
Why Humans Don’t Do It
Humans have a vomeronasal organ, or at least the remnant of one. Anatomical studies find it present in the nasal septum of almost all adults. But the scientific consensus is that it’s vestigial, a leftover from an evolutionary ancestor that no longer serves a sensory function. The neural connections that would relay signals from the organ to the brain appear to be absent or nonfunctional. Surgeons operating on the nasal septum don’t need to take any special care to avoid it, because damaging it has no known effect on smell or behavior.
This means humans have the anatomical echo of the system but none of the working hardware. We process pheromone-like chemicals (to whatever extent we do) through our main olfactory system instead, which is why you’ll never catch yourself involuntarily curling your upper lip after smelling someone’s sweaty gym clothes, no matter how much information your brain might be extracting from it.