The flehmen response is a behavior in which an animal curls back its upper lip, raises its head, and holds its mouth slightly open to draw chemical signals toward a specialized scent organ in the roof of its mouth. You’ve probably seen it in horses, cats, or livestock and thought the animal was making a strange face or reacting to a bad smell. What’s actually happening is a sophisticated form of chemical analysis that gives the animal detailed information about other animals nearby.
How the Flehmen Response Works
The behavior serves one main purpose: getting chemical compounds to an organ called the vomeronasal organ (also known as Jacobson’s organ), a small sensory structure located in the roof of the mouth. This organ is separate from the regular sense of smell. While the nose handles airborne scents, the vomeronasal organ specializes in detecting pheromones, the chemical signals animals release through urine, feces, saliva, and glandular secretions to communicate with members of the same species.
When an animal performs the flehmen response, the lip curl and open mouth create airflow that pushes non-volatile chemical compounds up through a duct connecting the mouth to the vomeronasal organ. Once those compounds reach the organ, specialized sensory cells act as both receptors and nerve cells. They detect the pheromone and relay the signal directly to the brain, where it triggers the release of a reproductive hormone called gonadotropin-releasing hormone. This is why the flehmen response is so tightly linked to mating behavior: the organ doesn’t just identify a smell, it kicks off a hormonal chain reaction.
What Animals Are “Reading”
The information packed into pheromones is remarkably specific. A male animal investigating a female’s urine through flehmen can pick up signals about whether she is in heat, how far along in her reproductive cycle she is, and whether she’s ready to breed. In cattle, researchers have identified specific compounds in the saliva, urine, and feces of cows in estrus that directly increase flehmen frequency and mating behavior in bulls. Bulls consistently show higher rates of flehmen when exposed to estrus urine compared to non-estrus urine.
But reproduction isn’t the only thing animals investigate this way. The vomeronasal system also processes information about social hierarchies, species recognition, maternal bonding, and even potential predators. Jaguars, for example, use their vomeronasal organ to decode scent cues in their environment more broadly. The flehmen response is essentially a way of “reading” the chemical landscape with far more precision than sniffing alone allows.
Which Animals Do It
The flehmen response shows up across a wide range of mammals, though it’s most recognizable in two groups: ungulates (hoofed animals) and felids (cats). Horses, cattle, goats, sheep, deer, elk, and giraffes all perform it regularly. Among cats, both domestic cats and big cats like lions, tigers, and jaguars display the behavior. If you’ve ever seen your house cat sniff something intently, freeze with its mouth slightly open, and stare into the middle distance, that’s flehmen.
The behavior appears remarkably early in life. Welsh ponies have been observed performing the flehmen response as early as one day after birth, and calves in semi-wild herds develop it during infancy as well. This suggests the behavior is largely hardwired rather than learned, though what triggers it and how frequently it occurs can shift with experience and context.
Flehmen in Horses
Horses are the animal most people associate with the flehmen response, partly because the lip curl is so dramatic and visible on their long faces. Stallions perform it most often, typically after sniffing urine, feces, or the body of a mare. Interestingly, research on stallions in pasture with mares found that flehmen was most commonly preceded by nasal investigation rather than oral contact, meaning the horse sniffs first, then performs flehmen to get a deeper chemical read.
The stallion studies also revealed something surprising: after performing flehmen, stallions were more likely to follow up with marking behaviors (urinating or defecating over the same spot) than with direct courtship. This suggests flehmen isn’t an immediate prelude to mating. Instead, it functions more like ongoing surveillance. Stallions use it to monitor where mares are in their estrous cycles, chemically “priming” themselves for reproduction over time rather than reacting to a single moment of readiness. Their rate of flehmen varied with the mares’ cycles, rising and falling in sync, but it didn’t change based on time of day.
One complicating finding: some studies have shown that stallions don’t actually sniff or flehmen more to estrous urine or feces than to non-estrous samples. This contrasts with bulls, who clearly discriminate. It may mean horses rely on a broader set of chemical cues, or that flehmen in horses serves a wider monitoring purpose beyond simple estrus detection.
Do Humans Have This Ability?
Humans do not perform the flehmen response, and the organ it serves is almost certainly nonfunctional in our species. The vomeronasal organ is physically present in the vast majority of human adults as a tiny structure in the nasal septum. But that’s where the similarity ends. In adult humans, the organ lacks neurons and nerve fibers, meaning it has no way to send signals to the brain. Humans also lack the accessory olfactory bulb, the brain structure that processes vomeronasal input in other mammals. On top of that, the genes that code for vomeronasal receptor proteins and the ion channels needed for signal detection have mutated and are nonfunctional in humans.
The scientific consensus is that the human vomeronasal organ is vestigial, a leftover from an evolutionary past when our ancestors relied more heavily on chemical communication. Some earlier studies using electrical stimulation claimed to show receptor activity in the human organ, but the genetic evidence against functionality is strong. Surgeons performing nasal septum operations are not advised to take special care to preserve it, which speaks to how settled the question has become.
Why It Matters Beyond Curiosity
Understanding the flehmen response has practical value in animal breeding and veterinary medicine. In livestock operations, recognizing flehmen patterns in bulls or stallions can help handlers assess whether a male is detecting estrus in nearby females, which aids in timing breeding. The specific compounds that trigger flehmen in cattle have been identified and studied for potential use in reproductive management.
For pet owners, recognizing the flehmen response in cats or horses can prevent unnecessary worry. The behavior looks odd, sometimes as if the animal is in pain, disgusted, or smelling something terrible. In reality, the animal is simply gathering information. It’s one of the most refined sensory tools in the mammalian world, and when your cat freezes with its mouth hanging open after sniffing your shoe, it’s processing your chemical signature with a precision that the human nose can’t come close to matching.