Geldings seem to flehmen the least. In fact, it's theorized that the process of gelding seems to compromise a male's ability to detect and analyze pheromones, making him sexually ineffectual in more ways than one.But while sex pheromones are definitely the most likely flehmen trigger, they're not the only ones. Occasionally, horses will react with an upper lip curl when they come in contact with an unusually strange or pungent inorganic odor--smoke from a fire or fresh paint, for example.
Most of us, at one time or another, have seen a horse tilt up his head and curl his upper lip in a "horse laugh." Although the expression is amusing, it actually has a practical purpose, writes Karen Briggs in the February edition of The Horse. The posture is called "flehmen" (roughly translated it means "testing"), and it appears to help horses trap pheromone scents in the vomeronasal organs (VNOs) so they can be analyzed more closely. Pheromones are the chemical signals emanating from other horses (and on occasion from humans).After a horse draws in the organic odor (by several seconds of olfactory investigation), he curls up his lip to temporarily close the nasal passages and hold the particles inside. Then an upward head tilt seems to help the airborne molecules linger in the VNOs, which are located under the floor of the horse's nasal cavity.Stallions are, by far, the most enthusiastic equine practitioners of the flehmen posture. In the presence of a mare in estrus, for example, they might flehmen several times an hour. Mares also will flehmen, although not as frequently; the smell of birthing fluids on a newborn foal triggers the response.