That dopey face your cat makes—its mouth half-open, its lips curled awkwardly away from its teeth—has a name. It’s called the flehmen response, and yes, it looks ridiculous. But for many mammals, it’s a critical part of their sex life.
Photo Credit: tambako via flickr | CC BY-ND 2.0
Although the expression practically invites you to stare, it’s incidental to the real sensory work that’s going on. Flehmening animals, male and female both, are sucking in air to force it into their vomeronasal organ (VNO), a structure that detects chemical signatures of social status and sexual readiness.
The VNO sits in a tiny capsule embedded inside the roof of the mouth. It’s isolated from the rest of the nose, accessible only by a small duct that opens into the nasal cavity (or in some mammals, into the mouth). It’s filled with fluid, and flanked by blood vessels that may produce a pumping action to pull air through the ducts. In short, regular breathing and sniffing won’t get air into the VNO; it takes some serious chuffing—and, well, that face.
Like the nose, the wall of the VNO is lined with chemoreceptors, sensory structures sensitive to the presence of specific chemicals. But where the olfactory tissues in the nose detect an enormous variety of chemical types, in the case of the VNO the only chemicals that matter are pheromones.
Pheromones are chemicals that trigger social responses. Mammals secrete them in their urine, sweat, and tears, creating a cloud of individualized scent that other members of their species can probe for information. The regular olfactory tissues inside the nose can detect pheromones as well, but when these chemicals bind to the receptors inside the VNO, they trigger a unique set of neural pathways in the brain.
Instead of traveling to the olfactory bulb of the brain, where signals from odors are typically sorted out, VNO neurons send information to a separate processing hub where they connect to neurons from the amygdala and hypothalamus, parts of the brain that control sexual response. Some pheromones work through the VNO to amplify sexual interest, other pheromones can damp it down.
Humans, as it turns out, don’t have a working VNO. Neither do most of the other Old World primates like apes and baboons. So if pheromones have any role in our daily lives, they have to fight for attention in our noses with all the other odors out there, rather than tickling the main line to the sexual brain.