The Flehmen Response in Horses: Why They Curl Their Lip

Every horse person has seen it. That goofy, lip-curling face where the horse raises its head, peels back the upper lip, and looks like it just tasted the worst thing in the world. People laugh. They grab their phones. They caption it "when Monday hits." But behind that ridiculous expression is a sophisticated sensory system that most mammals, humans included, have lost. Or at least mostly lost.

It's called the flehmen response, and your horse is not making a face. He's analyzing chemistry.

The Mechanics of Flehmen

When a horse performs the flehmen response, it curls the upper lip upward and extends the neck, sometimes holding the position for several seconds. What's happening isn't just dramatic posing. The horse is trapping air and volatile chemical compounds against the roof of its mouth, specifically directing them toward a structure called the vomeronasal organ, also known as Jacobson's organ.

This organ sits in a pair of fluid-filled sacs along the nasal septum, just above the hard palate. It has its own dedicated neural pathway to the brain, separate from the regular olfactory system. While the nose handles general smell, the vomeronasal organ specializes in detecting pheromones and other non-volatile chemical signals. It's basically a second nose, but calibrated for social and reproductive chemistry specifically.

Texas A&M's anatomy department has documented the vomeronasal organ in equine cadavers at lengths of 10 to 12 centimeters, substantially larger relative to body size than what you'd find in many domestic species. That's not a vestigial afterthought. That's a fully operational chemical laboratory sitting right there in the horse's skull, running analyses that would require a mass spectrometer for us to replicate.

By curling the lip and pressing the tongue against the palate, the horse creates a kind of pump that pushes air and scent molecules into the vomeronasal ducts. The sensory neurons there send signals directly to the accessory olfactory bulb, which connects to brain regions involved in reproductive and social behavior. It's an elegant system, and horses are far from the only animals that use it.

Who Does It and Why

Stallions are the most frequent and enthusiastic practitioners of flehmen. If you've ever been around a stallion when a mare in heat walks by, you've seen the full production: sniffing the mare's urine or genital area, then throwing the head up with that exaggerated lip curl. He's not being dramatic (well, maybe a little). He's running a chemical analysis on her reproductive status. The pheromones in a mare's urine change throughout her estrous cycle, and the vomeronasal organ lets the stallion detect exactly where she is in that cycle. The information is remarkably precise, conveying not just whether she's cycling but approximately when ovulation will occur.

But stallions don't own this behavior. Mares do it too, particularly around unfamiliar scents or new horses. Geldings flehmen. Foals flehmen, sometimes repeatedly as they encounter new things in their environment. It's part of how young horses learn about their world, categorizing smells and building a chemical map of their surroundings. Any horse exploring an unusual smell might throw a flehmen, which is why you'll sometimes see it after a horse sniffs a new supplement, a strange dog, or your lunch.

Some common triggers beyond reproductive contexts:

• Novel odors. Anything new and interesting. A new feed, a different horse's manure, cleaning chemicals, perfume on a visitor. Novelty is the key trigger here.
• Taste. Some horses flehmen after tasting something unusual. That bitter dewormer? Flehmen city. A particularly pungent mineral supplement? Same deal.
• Pain or discomfort. Here's one people miss. Horses sometimes flehmen when experiencing colic or abdominal discomfort. If your horse is flehmening repeatedly without an obvious scent trigger, pay attention to what else is going on. Check for signs of digestive distress. Look at the overall picture: is the horse also pawing, looking at its flanks, or refusing feed?
• Social information. A new horse moved into the barn? Every horse on the property will be sniffing that stall and flehmening for days. They're reading the newcomer's chemical resume, learning its sex, reproductive status, health condition, and probably things we haven't even identified yet.

The Vomeronasal Organ: A Closer Look

Jacobson's organ was first described in humans by Danish anatomist Ludwig Jacobson in 1813, though it was later identified across a huge range of vertebrates. In horses, the paired organs are relatively well-developed compared to many species. Each one is an elongated, cigar-shaped structure enclosed in cartilage, lined with sensory epithelium packed with receptor neurons.

These neurons express a different family of receptors than the ones in the main olfactory epithelium. While your regular smell receptors (in the nose) detect a broad range of airborne molecules, vomeronasal receptors are tuned specifically to larger, less volatile molecules, the kind that carry social and reproductive information. Pheromones, basically, though the definition of "pheromone" in mammals is still debated among researchers with considerable vigor.

The neural pathway is fascinating. Signals from the vomeronasal organ don't go to the same brain regions as regular smell. They route through the accessory olfactory bulb to the amygdala and hypothalamus, areas involved in emotional responses, reproductive behavior, and hormonal regulation. This is a direct chemical-to-behavior pipeline. The stallion doesn't "think about" what he's smelling. The information goes straight to the parts of his brain that control mating behavior. Conscious processing isn't required. The chemistry triggers the response.

Cornell researchers studying stallion breeding behavior found that flehmen frequency spiked dramatically, sometimes eight to twelve displays per minute, when stallions encountered urine from mares within 24 hours of ovulation. The precision is staggering. No blood test, no ultrasound, just a curled lip and a brain that evolved to decode the answer instantly.

UC Davis published work on the receptor proteins expressed in equine vomeronasal neurons, identifying receptor families (V1R and V2R types) that are completely absent from the main olfactory epithelium. These are specialist tools. The regular nose smells hay and manure and rain coming. The vomeronasal organ reads the social network.

Humans have a vestigial vomeronasal organ, or at least the structural remnants of one. Whether it's functional in adult humans is controversial, with most evidence suggesting it's not. The genes that code for vomeronasal receptors in humans appear to be largely pseudogenes, broken copies that no longer produce functional proteins. We've largely lost this chemical communication channel, which is probably why we find the flehmen response so amusing. We literally can't relate to what the horse is experiencing.

Flehmen Across the Animal Kingdom

Horses are hardly alone in this behavior. Cats do it; that weird open-mouthed stare your cat gives after sniffing your shoe is the same basic mechanism, different species. Big cats, ungulates (deer, goats, cattle), elephants, and many reptiles all perform variations of flehmen. Snakes constantly flicking their tongues are doing something functionally similar, delivering chemical information to their own vomeronasal organs through openings in the roof of the mouth.

In horses specifically, the response is particularly pronounced because of the anatomy of their upper lip. That mobile, flexible upper lip that's so useful for sorting through feed and manipulating objects also makes for a very dramatic flehmen display. It's one of many fascinating aspects of equine anatomy that reflects both form and function working in concert.

Interestingly, the frequency and intensity of flehmen varies between individual horses. Some are prolific flehmeners, reacting to every mildly interesting scent. Others are more reserved, saving the display for genuinely novel or important chemical signals. Whether this represents differences in vomeronasal sensitivity, personality, or both remains an open question in equine behavior research.

When Flehmen Signals a Problem

Most of the time, flehmen is completely normal and nothing to worry about. But context matters enormously. A horse that's flehmening repeatedly without exposure to novel smells, especially if it's also looking at its flanks, pawing, or showing other signs of discomfort, may be communicating pain. Horses with colic sometimes flehmen, possibly in response to the unusual sensations in their gut or to chemical changes in their own body that the vomeronasal organ detects. Since horses are obligate nasal breathers and can't vomit, abdominal discomfort has limited avenues of expression.

Post-foaling mares will flehmen over their newborns extensively, learning the foal's individual scent signature. This is part of the bonding process, and mares who are prevented from this olfactory investigation of their foals can have trouble bonding. The chemical imprinting that happens in those first hours is critical for the mare to recognize and accept her own offspring. Interfere with it at your peril.

Some horses flehmen in response to certain medications or topical products. Liniments with strong aromatic compounds, fly sprays, and even certain shampoos can trigger the response. This isn't a problem; it's just the horse investigating an unusual chemical signal. But if a horse that never used to flehmen starts doing it constantly, or if the behavior accompanies other changes like weight loss, lethargy, or behavioral shifts, mention it to your vet. Behavioral changes always deserve a second look.

The Merck Veterinary Manual's section on normal equine behavior lists flehmen as a standard behavioral repertoire item and explicitly distinguishes it from signs of respiratory distress or oral pain, which is worth knowing because panicked new horse owners occasionally call the vet thinking their horse is choking when it's just investigating a particularly fascinating pile of manure.

There's no medical condition called "excessive flehmen." If your horse does it a lot, he's probably just a curious individual with a lot of opinions about smells. But sudden changes in frequency, especially combined with other behavioral changes, warrant a closer look at the whole clinical picture.

The Takeaway

Next time your horse throws that ridiculous lip curl, resist the urge to just laugh (okay, laugh first, then think about it). That silly face is a window into a sensory world we can barely imagine, a world of chemical signals carrying information about identity, reproductive status, emotional state, and environmental conditions. Your horse is reading a newspaper you can't even see. And he's been doing it since long before newspapers existed.

It's one of those reminders that horses experience reality very differently than we do. Their world is richer in smell and chemical communication than ours, just as it's richer in peripheral vision and vibration detection. Understanding the flehmen response is a small piece of understanding how horses perceive and navigate their environment, and that understanding makes us better at working with them, reading their behavior, and respecting the complexity of what goes on between those ears.

Frequently Asked Questions

Sources

• "The Flehmen Response and Vomeronasal Organ in Horses" - Texas A&M Veterinary Medicine tamu.edu
• "Equine Behavior and the Flehmen Response" - UC Davis Center for Equine Health ucdavis.edu
• "Jacobson's Organ and the Vomeronasal System" - Cornell University College of Veterinary Medicine cornell.edu
• "Normal Behavior of Horses" - Merck Veterinary Manual merckvetmanual.com
• "Chemical Communication in Domestic Animals" - AAEP Convention Proceedings aaep.org