Flehmen Response in Dogs: What That Funny Face Really Means
What is the flehmen response in dogs? Learn the science behind this strange facial expression, how the vomeronasal organ works, and what it means for your dog.
Quick Answer: What Is the Flehmen Response in Dogs?
The flehmen response is a behavior where a dog curls back its upper lip, lifts its head, and appears to "grimace" or sneer after sniffing something intensely. This is not aggression, disgust, or a smile — it is a deliberate sensory action that routes scent molecules into the vomeronasal organ (also called Jacobson's organ), a specialized chemosensory structure located on the roof of the mouth. Through this mechanism, dogs analyze pheromones and complex chemical signals that the standard nasal olfactory system cannot fully process. The flehmen response is most commonly triggered by urine marks, reproductive scents, and the scent of other animals.
The Science Behind the Flehmen Response in Dogs
If you have ever watched your dog sniff a fire hydrant, tilt their head back, and pull their lips into what looks like a bizarre grin, you have witnessed the flehmen response firsthand. The word "flehmen" comes from the German verb meaning "to bare the upper lip," and it was first documented in large cats before researchers identified it across dozens of mammal species, including dogs, horses, goats, and tigers.
The Vomeronasal Organ (Jacobson's Organ)
At the center of the flehmen response is a small but powerful structure called the vomeronasal organ (VNO). Located bilaterally on the floor of the nasal cavity, opening into the roof of the mouth via the incisive papilla (the small bump behind the upper front teeth), the VNO is lined with receptor cells that are distinct from those in the main olfactory epithelium.
While a dog's main nasal cavity contains approximately 220–300 million olfactory receptors (compared to roughly 5–6 million in humans), the VNO operates as a parallel detection system. It is tuned specifically to non-volatile chemical compounds — heavy molecules like pheromones that do not easily become airborne. When a dog performs the flehmen response, the curled lip and head elevation create a slight pressure change that draws these molecules through the incisive ducts and into contact with VNO receptor neurons.
Research published in the journal Applied Animal Behaviour Science has demonstrated that the VNO sends signals to the accessory olfactory bulb, which then routes information directly to the amygdala and hypothalamus — brain regions governing emotional responses, social behavior, and reproductive drives. This is why the flehmen response is so tightly linked to social and sexual scent analysis rather than general odor detection.
The pathway is remarkably efficient: scent molecules enter through the incisive papilla, contact VNO receptor cells, trigger neural signals through the vomeronasal nerve to the accessory olfactory bulb, and arrive at the hypothalamus within milliseconds. The dog is essentially running a chemical analysis of the social and reproductive status of whatever animal left the scent they are investigating.
For a deeper look at canine olfactory anatomy, the Merck Veterinary Manual's overview of nasal structure in dogs provides detailed diagrams of how these interconnected systems function.
Evolutionary Origins: Why Ancestral Dogs Developed This Trait
From Wolf Pack Survival to Modern Dog Behavior
The flehmen response is an ancestral trait shared across the order Carnivora, meaning it predates the domestication of dogs by millions of years. In wild canids — wolves, coyotes, foxes, and jackals — the ability to chemically analyze scent marks is critical for survival and social cohesion.
Key evolutionary functions in ancestral canids:
- Territory assessment: Wolves use urine marks to define pack boundaries. The flehmen response allows a wolf to determine whether a mark was left by a pack member, a rival pack, or a solitary animal — and how recently.
- Reproductive timing: Male wolves detect the estrus status of females through pheromones in urine. The VNO can identify hormonal metabolites that indicate whether a female is approaching ovulation, critical information for breeding synchronization in seasonal breeders.
- Social hierarchy maintenance: Dominant wolves mark more frequently and in more prominent locations. Subordinate animals use flehmen to "read" these marks, maintaining social order without physical confrontation.
- Predator/prey identification: Chemical signatures in urine and feces reveal species, health status, and even stress levels of nearby animals.
Domestication has softened some of these drives, but the neurological hardware remains intact. Your Labrador performing the flehmen response over a neighbor dog's urine mark is running the same chemical analysis program that a wolf would use to assess a territorial boundary — just in a suburban context.
What Triggers the Flehmen Response in Dogs?
Not every scent warrants a flehmen response. Dogs reserve this behavior for chemically complex stimuli that require deeper analysis than routine sniffing provides. Based on observational studies and veterinary behavioral research, the most common triggers include:
| Trigger Category | Specific Examples | Why It Triggers Flehmen |
|---|---|---|
| Urine marks | Other dogs' urine on posts, hydrants, trees | Contains pheromones signaling sex, reproductive status, identity, and stress hormones |
| Feces | Other dogs' or wild animals' feces | Anal gland secretions carry individual chemical signatures |
| Reproductive scents | Intact female in estrus, vaginal discharge | Estrus pheromones (methyl p-hydroxybenzoate and others) are VNO-specific triggers |
| Human scents | Sweaty clothing, shoes, bedding | Apocrine gland secretions contain complex hormonal information |
| Novel animal scents | Cat bedding, wildlife trails, livestock areas | Unfamiliar species signatures trigger deeper chemosensory investigation |
| Strong organic compounds | Certain plants, compost, decomposing matter | Complex volatile organic compounds may stimulate VNO receptors |
Male dogs tend to display the flehmen response more frequently and more intensely than females, particularly in the presence of urine marks from intact females. This aligns with the behavior's evolutionary role in reproductive assessment, though neutered males still exhibit it — the neural pathway does not disappear when reproductive hormones decline.
How Dogs Process Scent: Flehmen vs. Normal Sniffing
Understanding the flehmen response requires distinguishing it from standard canine olfaction. Dogs actually operate two separate but complementary scent-processing systems:
Main Olfactory System (Regular Sniffing)
- Receptor count: 220–300 million (breed-dependent)
- Location: Nasal cavity olfactory epithelium
- Detects: Volatile airborne molecules
- Brain pathway: Main olfactory bulb → olfactory cortex
- Function: General environmental odor identification (food, objects, locations)
- Visible behavior: Rapid nasal inhalation, nose to ground, nostril flaring
Vomeronasal System (Flehmen Response)
- Receptor count: Distinct VNO receptor family (V1R and V2R types)
- Location: Vomeronasal organ, roof of mouth
- Detects: Non-volatile pheromones and heavy molecules
- Brain pathway: Accessory olfactory bulb → amygdala → hypothalamus
- Function: Social, sexual, and territorial chemical analysis
- Visible behavior: Lip curl, head elevation, frozen posture, chattering teeth
This dual-system architecture is why dogs can smell a treat in your pocket (main olfaction) and also decode the hormonal status of the dog that peed on your front bush (vomeronasal). The two systems work simultaneously during many scent investigations, but the flehmen response specifically indicates that the VNO is being recruited for deeper chemical analysis.
Is the Flehmen Response Normal? Breed Variations and Common Misconceptions
The short answer: yes, the flehmen response is entirely normal and healthy. It is a hardwired neurological behavior, not a learned trick or a sign of illness. However, its frequency and intensity vary significantly.
Myth vs. Fact: The Flehmen Response
| Myth | Fact |
|---|---|
| "My dog is disgusted by the smell." | The grimace-like expression is mechanical, not emotional. The lip curl opens the incisive ducts to allow scent molecules into the VNO. Dogs do not find urine or pheromone scents repulsive — they find them information-rich. |
| "My dog is being aggressive or snarling." | A flehmen response can look similar to an aggressive snarl, but the context is different. During flehmen, the dog's body is typically relaxed, the ears are neutral or slightly back, and the dog is focused on a scent source rather than a perceived threat. Aggressive snarling involves direct eye contact, raised hackles, and body tension. |
| "Only male dogs do it." | Both sexes perform the flehmen response. Males do it more frequently, especially in response to urine marks, but females also use VNO analysis — particularly intact females assessing potential mates or evaluating the hormonal status of other dogs. |
| "Neutered dogs stop doing it." | Gonadectomy reduces the hormonal drive behind some scent-related behaviors, but the neurological pathway remains. Neutered dogs still display flehmen, just often at lower frequency. |
| "It means my dog has a dental problem." | Occasional flehmen is normal. However, if a dog repeatedly holds their mouth open with a lip curl without a scent trigger, or if the behavior is accompanied by drooling, pawing at the mouth, or reluctance to eat, a veterinary dental exam is warranted. |
Breed Variations in Flehmen Frequency
While all dogs possess a vomeronasal organ, certain breed groups show more pronounced flehmen behavior due to selective breeding emphasis on scent processing:
- Scent hounds (Bloodhounds, Basset Hounds, Beagles): Bred for centuries to track complex scent trails, these breeds have highly developed VNO systems and display frequent, intense flehmen responses. Bloodhounds have approximately 300 million olfactory receptors — the highest count of any breed.
- Working/guardian breeds (German Shepherds, Belgian Malinois, Rottweilers): These breeds rely on environmental assessment and territorial awareness, making them responsive to pheromone-laden scent marks.
- Sporting breeds (Labrador Retrievers, Golden Retrievers, Spaniels): Moderate flehmen frequency; their scent abilities are strong but were selected more for locating game than for social scent analysis.
- Brachycephalic breeds (Pugs, Bulldogs, Shih Tzus): May display the flehmen response less visibly due to shortened facial anatomy that can alter the mechanics of lip curling and the geometry of the incisive ducts. The VNO still functions, but the external expression may be subtler.
- Sighthounds (Greyhounds, Whippets, Afghan Hounds): Primarily rely on vision rather than scent. They possess a functional VNO but tend to show less frequent flehmen behavior compared to scent-specialized breeds.
Practical Implications: What the Flehmen Response Means for Dog Owners
What This Means for Your Daily Life With Your Dog
1. Allow sniffing time on walks. When your dog stops to intensely sniff a spot and then displays the flehmen response, they are gathering critical social information about the dogs in your neighborhood. Rushing them away is equivalent to someone snatching a newspaper out of your hands mid-sentence. Aim for at least 15–20 minutes of decompression sniffing per walk, particularly in areas with high dog traffic.
2. Do not punish the behavior. The flehmen response is involuntary and self-reinforcing. Scolding your dog for "making a weird face" or pulling them away harshly creates negative associations with normal exploratory behavior. If the behavior occurs in an inconvenient moment, use a gentle redirect cue like "let's go" paired with a treat — positive reinforcement methods are the only recommended approach.
3. Use it as a training and enrichment tool. Because scent investigation is intrinsically rewarding for dogs, you can leverage the flehmen response in nosework activities. Hide treats or scent articles (cotton swabs rubbed with anise, birch, or clove essential oils — dog-safe nosework scents available from suppliers like AKC Scent Work programs) and let your dog use their full olfactory toolkit to find them. Nosework classes typically cost $25–$50 per session and provide exceptional mental stimulation.
4. Understand multi-dog household dynamics. If you have multiple dogs, you may notice them performing the flehmen response after sniffing each other, particularly around the genital and facial areas. This is normal social information gathering. Dogs use VNO analysis to assess each other's stress levels, health status, and hormonal state. Allow these greeting rituals to proceed naturally unless one dog shows signs of discomfort.
5. Recognize when scent exposure matters. If your dog has been rolling in or intensely investigating the waste of wild animals (raccoons, coyotes, deer), monitor for signs of illness in the following days. While the flehmen response itself is harmless, exposure to parasite-laden feces or urine from wildlife can transmit diseases like leptospirosis or intestinal parasites.
When the Flehmen Response Might Signal a Problem
While the flehmen response is normal, certain patterns warrant veterinary attention. Distinguishing between healthy chemosensory behavior and a symptom of an underlying condition requires attention to context, frequency, and accompanying signs.
Red Flags: When to Consult Your Veterinarian
Contact your veterinarian if you observe any of the following alongside or instead of normal flehmen behavior:
- Persistent open-mouth posture without a scent trigger, especially if the dog seems unable to close their mouth normally
- Excessive drooling or saliva strands hanging from the lips during or after the lip-curl behavior
- Pawing at the mouth or face repeatedly, which may indicate dental pain, an oral foreign body, or an abscess
- Foul breath combined with the lip-curling behavior — this can indicate periodontal disease, oral tumors, or kidney disease
- Reluctance to eat or dropping food while eating, which may signal oral pain that is causing abnormal mouth postures
- Nasal discharge (especially unilateral or bloody) alongside frequent sniffing and flehmen-like behavior, which could indicate a nasal tumor, fungal infection, or foreign body
- Sudden increase in flehmen frequency in a senior dog, which may indicate cognitive changes or a new environmental stressor
Always consult your veterinarian if you are unsure whether a behavior is normal. A thorough oral exam and, if needed, dental radiographs can rule out painful conditions that mimic the flehmen response.
Related Conditions That Can Mimic Flehmen Behavior
Several medical conditions produce facial expressions or mouth movements that untrained observers may confuse with the flehmen response:
- Nausea: Dogs experiencing nausea often lick their lips repeatedly, drool, and hold their mouths slightly open. This can resemble the flehmen posture but is typically accompanied by swallowing motions, decreased appetite, and sometimes vomiting.
- Dental disease: Tooth root abscesses, fractured teeth, and severe gingivitis cause dogs to hold their mouths in unusual positions. Unlike the brief, scent-triggered flehmen response, dental pain produces a more constant abnormal posture.
- Neurological conditions: Focal seizures, trigeminal neuropathy, or facial nerve paralysis can cause involuntary lip movements. These episodes are typically not triggered by scents and may involve other neurological signs like head tilting, circling, or altered consciousness.
- Oral foreign bodies: A stick fragment, bone shard, or foxtail lodged in the roof of the mouth or between teeth can cause a dog to hold their mouth open and curl their lip — mimicking flehmen but driven by discomfort rather than scent analysis.
Frequently Asked Questions About the Flehmen Response
Why do dogs do the flehmen response?
Dogs perform the flehmen response to direct non-volatile scent molecules — particularly pheromones — into the vomeronasal organ (Jacobson's organ) on the roof of the mouth. This allows them to analyze complex chemical information about other animals' identity, sex, reproductive status, and emotional state that the main olfactory system cannot fully decode.
Can dogs detect pheromones from other species?
Yes. The canine VNO can detect pheromones and chemical signals from other species, including cats, livestock, wildlife, and humans. This is why your dog may display the flehmen response after sniffing cat bedding, horse stalls, or even your gym clothes. The VNO is broadly tuned to a wide range of chemical signals, though species-specific pheromones elicit the strongest responses.
How do dogs process scent differently from humans?
Dogs possess approximately 40–60 times more olfactory receptors than humans (220–300 million vs. 5–6 million), a proportionally larger olfactory bulb in the brain, and a dedicated vomeronasal organ that humans lack in functional form. While humans have a vestigial VNO structure, research indicates it does not function as a chemosensory organ in adults. Dogs literally experience a richer, more layered chemical world than we do.
What does it mean when my dog curls their lip at me?
If your dog curls their lip after sniffing you — particularly after you have been around other dogs, after exercising, or after handling animals — they are likely performing a mild flehmen response. They are gathering chemical information about where you have been and what you have encountered. It is investigative, not aggressive. However, if the lip curl is accompanied by growling, stiff body posture, hard staring, or raised hackles, that is an aggressive signal and has a different behavioral context entirely.
Is the flehmen response normal in puppies?
The vomeronasal organ is functional at birth in puppies — it is essential for locating the mother's teats during nursing through pheromone detection. However, the mature flehmen response (the pronounced lip curl and head elevation) typically becomes more visible as puppies mature and begin exploring their environment more independently, usually around 3–6 months of age. Young puppies may show abbreviated or incomplete versions of the behavior.
How is the flehmen response different in dogs compared to cats?
Both dogs and cats display the flehmen response, but the feline version is generally more pronounced and easier to observe — cats often freeze with their mouths open for several seconds, creating the classic "stink face." In cats, the flehmen response is heavily tied to reproductive behavior, particularly males detecting females in estrus. Dogs display a wider range of contexts for the behavior (territorial, social, reproductive) and tend to have shorter-duration flehmen episodes. Additionally, cats have a more prominent incisive duct anatomy, making the physical mechanism more visually obvious.
References
- Salazar, I., Quinteiro, P., & Cifuentes, J.M. (1994). "The vomeronasal organ of the dog: A morphological and immunohistochemical study." Anatomical Record, 240(4), 544-553.
- Heywood, E.L. & Brunjes, P.C. (1993). "The vomeronasal organ of the dog: Morphology and development." Chemical Senses, 18(5), 560.
- American Kennel Club: How Dogs Smell — Understanding Canine Scent Abilities
- Merck Veterinary Manual: Normal Social Behavior and Communication in Dogs
Beth Carrasco
All our authors care for dogs every day — read more of their work on the authors page.
