Ever walked out to the paddock and seen your horse with mouth open, neck stretched out, and upper lip curled back like they just smelled something truly offensive? It’s a classic look. Most people laugh and pull out their phones to grab a photo for Instagram. But honestly, that goofy expression—the Flehmen response—is actually a high-tech biological data-gathering mission. It's not just about weird smells, though.
If you see a horse with mouth open while you're riding, or if they’re doing it incessantly while standing alone, the vibe changes completely. It stops being funny. It starts being a signal. Sometimes it's a plea for help. Other times, it's just a horse being a horse. Understanding the nuance between a "stinky smell" face and a "my stomach is in knots" face is basically Horse Ownership 101.
The Flehmen Response: Nature’s Chemistry Lab
Let's talk about that lip curl. It’s technically called the Flehmen response. When a horse encounters a particularly interesting scent—think pheromones, new grass, or even your weirdly scented hand sanitizer—they trap those scent molecules. They pull their top lip up to close the nostrils and push the air toward the vomeronasal organ (VNO), also known as Jacobson’s organ. This is located at the base of the nasal cavity.
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It’s basically like they’re "tasting" the air. Stallions do it constantly to check if a mare is in heat. But even geldings and mares use it to process unfamiliar smells. It’s a sensory deep dive. You'll see it a lot with foals exploring their world for the first time. It's totally normal. Nothing to fix here.
When the open mouth means physical distress
If the horse isn't curling their lip but is just standing there with a horse with mouth open posture, your internal alarm should probably go off. Horses are "obligate nasal breathers." That’s a fancy way of saying they are biologically hardwired to breathe only through their noses. Unlike you or your dog, a horse can’t just pant to cool down after a long run.
If a horse is breathing through its mouth, it is a genuine medical emergency.
Usually, this points to a severe obstruction in the upper airway or extreme respiratory distress. We’re talking about things like "Roaring" (Laryngeal Hemiplegia) or a massive allergic reaction. Dr. Elizabeth Davidson at the University of Pennsylvania’s New Bolton Center has spent years looking at equine sports medicine, and the consensus is clear: a horse struggling for air will do anything to widen the passage, including gaping the jaw. If you see foam, hear a whistling sound, or see flared nostrils alongside an open mouth, call the vet. Immediately. Don't wait.
The silent scream of Gastric Ulcers
Horses are stoic. They evolved to hide pain because, in the wild, looking weak gets you eaten by a mountain lion. But ulcers? Ulcers are hard to hide. A horse with mouth open might be performing what behaviorists call "air chewing" or "empty mastication."
They aren't eating anything. They’re just moving their jaw, gaping the mouth, and maybe grinding their teeth (bruxism). This is often a response to the burning sensation of excess gastric acid. Since horses produce acid 24/7, if they don't have forage in their stomach to buffer it, the lining gets eaten away.
Think about how you'd look if you had a massive case of heartburn but no Tums. You’d probably be grimacing too. If your horse does this specifically around feeding time or when you tighten the girth, you’re likely looking at a digestive issue. Studies suggest up to 90% of racehorses and 60% of show horses deal with ulcers at some point. It’s a massive problem in the industry.
Fighting the bit: Resistance and dental pain
Now, if the horse with mouth open behavior only happens when you’re in the saddle, we have to look at the gear. Or the hands. Most horses gape their mouths to escape the pressure of the bit.
It’s a mechanical flight response.
If the rider’s hands are too heavy, or if the bit is hitting the sensitive bars of the mouth or the palate, the horse will open its mouth to say, "Stop it." This is why you see so many crank nosebands in dressage—it’s often a way to force the mouth shut so the judge doesn't see the resistance. But hiding the symptom doesn't fix the pain.
- Sharp enamel points: Horses' teeth grow continuously. They develop razor-sharp edges on the outside of the upper teeth and inside of the lower teeth. These can slice into the cheeks.
- Wolf teeth: These tiny, vestigial teeth sit right where the bit rests. If they’re present, the bit can bang against them, causing sharp, shooting pain.
- Improper bit fit: A bit that’s too wide will slide back and forth, while one that’s too narrow will pinch the corners of the mouth like a pair of pliers.
Dr. Jack Easley, a renowned equine dental specialist, often points out that many "behavioral" problems are actually just dental problems in disguise. If your horse starts tossing their head and opening their mouth the second you pick up the reins, get a float.
Choke: Not what you think it is
In humans, "choke" means something is stuck in the windpipe. In horses, choke means something is stuck in the esophagus. It’s a food blockage.
A horse with choke will often stand with its neck extended, mouth slightly open, and may have saliva or bits of food coming out of its nose. They look panicked. They might try to "swallow" the air, leading to that open-mouthed, gagging appearance. It looks terrifying because it is. While they can still breathe, the risk of aspiration pneumonia—where they inhale food particles into their lungs—is sky-high.
If you suspect choke, take the food away. Take the water away. Keep them calm and wait for the vet to pass a nasogastric tube to clear the blockage.
Practical steps for the concerned owner
So, your horse is standing there with its mouth hanging open. What do you actually do? Don't panic, but do be systematic. You have to be a bit of a detective.
First, check the context. Is it a hot day? Is there a weird smell? If they’re just Flehmening, let them be. It’s just "horse TV."
Second, look at the "rest of the horse." Are the ears pinned? Is the tail swishing aggressively? Is the horse "tucking" its belly? These are signs of colic or ulcers. Check the capillary refill time by pressing on their gums. They should turn from white back to pink in under two seconds. If the gums are purple or ghostly white, that's a "call the vet now" situation.
Third, look at the teeth. When was the last time a professional looked in there? Not just a quick peek, but a full exam with a speculum. If it’s been more than six months and you’re seeing an open mouth under saddle, that’s your first phone call.
Fourth, evaluate your tack. If you're using a harsh bit or a tight noseband, try riding in a sidepull or a simple bitless bridle for a day. If the horse suddenly stops opening its mouth and starts relaxing its neck, you’ve found your answer. It wasn't a "bad" horse; it was a "loud" bit.
Ultimately, a horse with mouth open is communicating. They don't have words, so they use their bodies. Whether it's a sensory exploration, a reaction to a sharp tooth, or a sign of systemic pain, it’s a signal that deserves more than a laugh. Pay attention to the "when" and "how" of the behavior. Most of the time, the horse is telling you exactly what’s wrong if you're willing to look past the goofy expression.