The sight of a cat pausing mid-sniff, pulling back its upper lip, and holding an open-mouthed grimace after encountering a strong scent like human feet is a puzzling experience for many owners. This momentary “stinky face” looks like an expression of disgust, but it is actually a complex, involuntary biological action. Far from being repulsed, the cat is engaging a specialized sensory system designed to gather a deep, chemical analysis of odorant molecules. This behavior extracts detailed information that the cat’s primary sense of smell cannot access.
The Flehmen Response: Analyzing Scents
This distinctive facial contortion is formally known as the Flehmen response, a term derived from a German verb meaning “to curl the upper lip.” When a cat performs this action, it actively manipulates its facial muscles to draw air and non-volatile scent compounds into a specific duct. This grimace is a functional movement that creates a suction effect, pulling large scent molecules off the air or a surface and into the oral cavity. The Flehmen response is not unique to domestic cats; many other mammals, including lions, horses, and tigers, use this action to process complex environmental smells. The purpose is to transport these trapped chemical signals directly to a secondary sensory organ for further scrutiny.
The Vomeronasal Organ: Specialized Sensory Hardware
The destination for these collected scent particles is the Vomeronasal Organ (VNO), also known as Jacobson’s Organ. The VNO is an auxiliary olfactory structure located in the roof of the cat’s mouth, situated just behind the upper incisor teeth. This organ is a separate chemosensory pathway, distinct from the cat’s primary nasal olfactory system, which detects everyday volatile odors. The VNO is equipped with receptors designed to detect large-molecule, non-volatile chemical signals, which are often pheromones. These complex signals communicate information about other animals, such as reproductive status, territorial boundaries, and individual identity. The Flehmen response is physically necessary because the scent molecules are too heavy to be inhaled through normal sniffing and must be “pumped” through the ducts on the palate to reach the VNO’s sensory cells.
Why Feet and Strong Odors Trigger Deep Analysis
Human feet are potent sources of the complex, information-rich odors that prompt the Flehmen response. The strong smell associated with feet is a result of sweat, skin oils, and the bacterial degradation of amino acids, which produces volatile fatty acids, such as isovaleric acid. While humans perceive this mixture as merely an unpleasant smell, a cat perceives it as a concentrated chemical signature. These compounds, along with human pheromones or unique individual chemical signals present in the sweat, represent an information-dense sample. The cat is attempting to analyze this chemical cocktail to discern who or what has been contacted, where they have been, and potentially their physiological state. This deep analysis of a strong, novel, or complex chemical profile contrasts with the simple, rapid detection of airborne odors handled by the main olfactory system.