A horse exhibiting the flehmen response will curl his upper lip, raise his head, maybe turn his head from side to side, and make a bit of a snuffling sound. | Photo: Alexandra Beckstett/The Horse

A: This is one of my favorite behaviors! This is called the flehmen response. It is fun to see, but it is actually a mechanism to initiate some really cool activity in the brain.

If you watch a horse do a full out flehmen, he will curl his upper lip, raise his head, maybe turn his head from side to side, make a bit of a snuffling sound, and afterward there will be clear, watery droplets from the nostrils. In adult horses it’s seen most often in stallions when they’re testing vaginal secretions from mares or sniffing urine and manure.

The flehmen response is supposed to help move compounds such as pheromones, hormones, and others into the vomeronasal organ. The vomeronasal organ, or VNO, is a neat little structure that sits under the floor of the nasal cavity. Most animals have this, and humans seem to have a vestigial (functionless) one. In the horse access to the VNO is from a duct in the nasal passages (in other species the opening is at the roof of the mouth)