Why Do Horses Do the Flehmen Response? The Science Behind the Silly Lip Curl

Image from Friese.

Your horse is not laughing at you. When a horse stretches its neck, lifts its head and rolls its top lip back over its nostrils, it is analysing a smell. The move is called the flehmen response, and it sends scent to a second smelling organ that most people don’t know horses have.

What You Are Looking At

The sequence is always much the same. The horse sniffs something, raises its head, stretches its neck out, curls the upper lip up and back until the front teeth and gums show, and holds the pose for a few seconds while breathing in. Then the lip drops and normal service resumes.

The name is German. Flehmen means roughly “to bare the upper teeth”, and it was introduced as a scientific term in 1930 by Karl Max Schneider, the director of Leipzig Zoo. English never came up with a better word, so we borrowed it.

The Second Nose

Tucked into the floor of the nasal cavity is the vomeronasal organ, also known as Jacobson’s organ. It is a chemical sensor, separate from the ordinary lining of the nose that handles everyday smells, and it has its own line to the brain: its signals go to a region called the accessory olfactory bulb, which feeds the parts of the brain concerned with reproduction and social behaviour.

It is a specialist. Where the main nose deals with airborne smells in general, the vomeronasal organ is tuned to pheromones and other heavy chemical signals, the sort found in urine, sweat and birth fluids.

The flehmen pose is how the sample gets delivered. According to veterinary behaviourist Sharon Crowell-Davis, who has studied flehmen in horses for decades, the posture helps move scent molecules into the organ. Curling the lip over the nostrils is thought to hold the air in while that happens.

In most animals that flehmen, the vomeronasal organ opens into the mouth. Horses are the exception: theirs connects only to the nasal passages, which fits an animal that can’t breathe through its mouth.

Who Does It, and When

Stallions do it most. The classic trigger is a mare’s urine or droppings: the stallion sniffs, then flehmens, sometimes several times in a row. It looks like part of courtship but it is better understood as research. Crowell-Davis describes it as chemical priming for reproductive behaviour and not an immediate part of mating. Sight plays a part too. In one experiment, stallions that could watch mares urinate flehmened more often than stallions whose view was blocked.

Mares do it less, with one big exception. In the first few hours after foaling a mare often flehmens repeatedly as she smells her newborn foal and the birth fluids. It is probably part of how she learns who her foal is. (If you are waiting on a foal, our gestation calculator will tell you when to start losing sleep.)

Foals do it from the first weeks of life, and colts seem to be the keenest of all. In a field study of Welsh pony foals, colts flehmened about once an hour during daylight and fillies about once every five hours. A later study that tested foals under controlled conditions found no real difference between the sexes, so the gap may have more to do with what young colts go around sniffing than with any difference in equipment.

Geldings do it as well. Castration doesn’t remove the organ or the curiosity.

Away from sex and foals, the trigger is usually anything new or strong: an unfamiliar feed, a wormer, liniment, your hand cream, a carrot that has been somewhere odd. Plenty of horses will flehmen on cue if you offer them a novel smell, which is the trick behind a great many “laughing horse” photographs. (We have a whole gallery of horse smiles.)

When the Lip Curl Is a Warning

Flehmen with an obvious cause is normal and harmless. Flehmen with no smell to explain it can be a sign of discomfort.

Both the Merck Veterinary Manual and the American College of Veterinary Surgeons list curling of the upper lip among the signs of colic. On its own it means little. Take it seriously if the horse is also doing any of these:

• pawing, or turning to look at its flank
• lying down and getting up again, or rolling
• refusing feed
• sweating without having worked
• passing fewer droppings than usual

That combination means a call to the vet, promptly. Because horses can’t vomit, a painful belly can turn serious faster than you would expect. We have a separate guide to telling a happy roll from a colicky one.

Horses Are Not the Only Ones

Flehmen turns up across the mammals. Cats do a subtler version, with the mouth hanging slightly open and an expression of mild disgust. Goats, sheep, giraffes, llamas, tapirs, rhinos, lions and tigers all do it, and elephants have their own variation. Among the horse’s closer relatives, zebras and donkeys do it too.

Humans don’t. In adults the organ is generally considered a vestige with no working connection to the brain, which may be why we find the whole performance so funny.

The Short Version

• Flehmen is a way of sampling a smell, not an expression of emotion.
• It delivers scent to the vomeronasal organ, a second chemical sensor in the nose.
• Stallions do it most, mares do it around foaling, and foals of both sexes start young.
• New or pungent smells set it off in any horse.
• Repeated lip curling with no smell to blame, alongside other signs of pain, is a reason to call the vet.

Horses read a good deal more through their noses than we give them credit for, including us. For that, see the research on horses smelling human emotions.