Horses aren’t actually smiling. That curled-lip, teeth-baring expression that looks like a goofy grin is called the Flehmen response, and it’s all about smell. When a horse raises its head and peels back its upper lip, it’s pushing scent molecules toward a specialized organ in its nasal cavity that gives it a kind of “super smell” beyond what its nostrils alone can detect.
What the Flehmen Response Actually Does
Horses have a sensory structure called the vomeronasal organ (sometimes called Jacobson’s organ) tucked inside their nasal passages. This organ connects to both the nasal and oral cavities through a small duct called the incisive duct. When a horse curls its upper lip and stretches its neck upward, it’s physically directing airborne chemical signals, especially pheromones, into this organ.
Once those scent molecules reach the vomeronasal organ, signals travel along a dedicated nerve pathway to a separate processing area in the brain, distinct from the regular sense of smell. This system is tuned to detect chemical information that ordinary sniffing might miss, particularly hormonal cues from other horses. Think of it as a second, more specialized nose built for decoding social and reproductive chemistry.
Why Stallions Do It Most Often
Stallions are the most frequent “smilers.” Research shows that stallions use the Flehmen response to monitor the reproductive cycles of nearby mares. Their rate of lip curling rises and falls in sync with the mares’ hormonal cycles, and they perform the behavior more frequently when standing near a mare in heat compared to one that isn’t.
Interestingly, studies have found that stallions don’t necessarily flehmen more when sniffing urine or manure from a mare in heat versus one that’s not cycling. The response seems more strongly triggered by the full sensory picture of being near the mare herself, not just her waste products. Researchers believe the Flehmen response helps stallions stay chemically “primed” for breeding over time rather than acting as a simple on/off signal for mating behavior. One specific pheromone horses can detect this way is androstenone, a steroidal compound that was the first mammalian pheromone ever identified. Horses have a dedicated receptor for it in both their vomeronasal organ and their regular nasal lining.
Foals and Mares Do It Too
The Flehmen response isn’t limited to stallions checking on mares. Foals do it frequently, often more than adults. Young horses appear to use the behavior as a way to investigate their environment, curling their lips at unfamiliar smells as they learn to categorize the world around them. Since foals have no reproductive reason for the behavior, this suggests the Flehmen response serves a broader sensory function beyond breeding.
Mares and geldings also flehmen, though less often than stallions. Any strong, unusual, or interesting scent can trigger it. New feeds, unfamiliar objects, cleaning products, another animal’s scent on your clothing: all of these can prompt a horse to raise its head and curl its lip for a deeper chemical read.
When It Signals Pain Instead of Curiosity
There’s one important exception to the “just smelling things” explanation. A Flehmen-like lip curl can also be a sign of abdominal pain. The American Quarter Horse Association lists lip curling as one of the common signs of colic, a serious and potentially life-threatening digestive condition in horses.
The difference is usually in context. A horse that flehmen once after sniffing something and then goes back to normal behavior is just processing a scent. A horse that repeatedly curls its lip, especially alongside other signs like pawing at the ground, looking at its flanks, refusing to eat, lying down and getting up repeatedly, or sweating without exertion, may be in distress. If the lip curling comes with any of those other behaviors, it’s not a smile and it’s not casual sniffing. It’s a pain response that needs attention.
Why It Looks So Much Like a Smile
The reason the Flehmen response reads as a “smile” to humans is pure coincidence of anatomy. Horses pull back their upper lip to seal off the nostrils and redirect airflow toward the incisive duct that leads to the vomeronasal organ. That mechanical action happens to expose the upper teeth and gums in a way that, to human eyes, looks like a delighted grin. We’re hardwired to read facial expressions through a human lens, so a flash of teeth registers as happiness.
Horses do express positive emotions, but not through their “smile.” Relaxed ears, soft eyes, a lowered head, and gentle nickering are better indicators that a horse is content. The big toothy face, charming as it looks, is the horse equivalent of taking a long, deep sniff of something really interesting.