Dogs With an Underbite: The Science, Breeds, and Health Implications
Everything about dogs with an underbite: the genetics, evolutionary origins, breed predispositions, dental health risks, and what owners need to know.
Quick Answer: Is an Underbite in Dogs Normal?
Yes — in certain breeds, an underbite (mandibular prognathism) is a normal, breed-standard trait. Brachycephalic breeds like Bulldogs, Pugs, and Boxers are genetically predisposed to undershot jaws, where the lower jaw extends beyond the upper jaw. However, in breeds where an underbite is not expected, it can signal a developmental abnormality, dental misalignment, or skeletal growth issue that warrants veterinary evaluation. An underbite itself is not inherently painful, but it can lead to secondary dental problems including abnormal tooth wear, soft tissue trauma, and periodontal disease if not monitored.
The Genetics and Developmental Science Behind Canine Underbites
An underbite — clinically termed mandibular prognathism or a Class III malocclusion — occurs when the mandible (lower jaw) grows longer than the maxilla (upper jaw), causing the lower incisors to sit in front of the upper incisors when the mouth is closed. This is fundamentally a skeletal discrepancy, not merely a dental one.
How Malocclusion Develops in the Canine Skull
During embryonic and postnatal development, the maxilla and mandible grow at different rates, guided by distinct genetic signaling pathways. Research published in the Journal of Veterinary Dentistry has identified that malocclusions in dogs result from disruptions in the timing and magnitude of jaw growth rather than from tooth positioning alone. The mandible continues growing until approximately 10-12 months of age in most breeds, while the maxilla reaches near-adult size slightly earlier.
Key developmental factors include:
- Growth plate activity: The mandibular condyle contains a secondary cartilaginous growth center that remains active longer than maxillary growth plates, making the lower jaw susceptible to prolonged growth.
- Bone remodeling rates: Osteoblast and osteoclast activity differ between the maxilla and mandible, influenced by mechanical loading from chewing forces.
- Tooth eruption timing: Deciduous (baby) teeth erupt at 3-6 weeks; permanent teeth replace them between 3-7 months. If deciduous teeth are retained, they can alter jaw alignment during this critical growth window.
The heritability of malocclusion has been studied in several breeds. According to the Merck Veterinary Manual, mandibular prognathism is considered a polygenic trait — meaning multiple genes influence jaw length, rather than a single dominant or recessive gene. This is why two dogs with normal bites can produce offspring with underbites, and why the severity varies even within a single litter.
Evolutionary Context: Why Shorter Jaws Persisted in Canids
From Wolf to Bulldog: The Brachycephalic Revolution
The gray wolf (Canis lupus) has a dolichocephalic (long-narrow) skull with a perfectly aligned scissor bite — upper incisors just overlapping lower incisors. This bite configuration is biomechanically optimal for gripping, tearing, and shearing prey. So how did underbites become common in domestic dogs?
The answer lies in artificial selection for brachycephaly — shortened skull length — which began thousands of years ago. The genetic mutation responsible is primarily found in the BMP3 gene (Bone Morphogenetic Protein 3). A 2010 study published in PLoS Biology identified that a variant of BMP3 is strongly associated with brachycephalic skull shape across multiple dog breeds.
When breeders selected for shorter muzzles, they inadvertently compressed the maxilla more than the mandible. The maxilla is more directly affected by the brachycephalic mutation because it is part of the facial skeleton that undergoes the most dramatic shortening. The mandible, being a separate bone with its own growth regulation, does not always shorten proportionally. The result: a jaw mismatch where the lower jaw protrudes beyond the shortened upper jaw.
From an evolutionary standpoint, this trait offers no survival advantage in the wild. Wolves or feral dogs with significant underbites would struggle to efficiently capture and process food. The trait persists entirely because humans selected for the accompanying flat-faced appearance and maintained these breeds through controlled breeding programs.
Breed Variations: Which Dogs Commonly Have Underbites?
Underbites range from a breed-standard expectation to an unexpected fault depending on the breed. The American Kennel Club breed standards explicitly describe acceptable bite configurations for each recognized breed.
| Breed | Underbite Status | Typical Severity | Breed Standard Notes |
|---|---|---|---|
| English Bulldog | Breed standard (expected) | Moderate to severe (up to 1-2 inches / 2.5-5 cm protrusion) | AKC standard calls for a "turn-up" lower jaw; undershot bite required |
| Pug | Breed standard (expected) | Mild to moderate | Slightly undershot bite preferred |
| Boxer | Breed standard (expected) | Mild (lower incisors just ahead of upper) | Undershot bite specified; excessive underbite is faulted |
| Shih Tzu | Breed standard (expected) | Mild | Undershot or level bite acceptable |
| Boston Terrier | Breed standard (expected) | Mild | Even or slightly undershot bite |
| Pekingese | Breed standard (expected) | Moderate | Undershot with wide lower jaw |
| Lhasa Apso | Breed standard (acceptable) | Mild | Prefer slightly undershot; level bite acceptable |
| German Shepherd | Fault (not expected) | Variable | Scissor bite required; underbite is a disqualifying fault |
| Labrador Retriever | Fault (not expected) | Variable | Scissor bite or level bite acceptable; underbite faulted |
| Any mixed breed | Variable | Variable | Depends on genetic contribution; may inherit brachycephalic traits |
It's worth noting that mixed-breed dogs with brachycephalic ancestry — such as Bulldog or Pug mixes — frequently inherit underbites. If your rescue dog has a flat face and a protruding lower jaw, this is likely a genetic trait rather than a pathology, though a veterinary dental exam is still recommended.
Health Implications: What Owners Need to Monitor
While an underbite in a predisposed breed is not inherently a medical emergency, it creates anatomical conditions that increase the risk of several dental and oral health problems. Understanding these risks allows owners to intervene early.
Common Secondary Problems
Myths vs. Facts About Dog Underbites
| Myth | Fact |
|---|---|
| "An underbite always causes pain." | Many dogs with mild to moderate underbites experience no discomfort. Pain occurs only when secondary issues develop (tooth-on-tissue contact, fractures, infections). |
| "Underbites can be fixed with braces." | While veterinary orthodontics exists, it is rarely pursued for cosmetic reasons. Treatment is reserved for cases causing pain or functional impairment, and costs typically range from $1,500 to $5,000+ depending on complexity. |
| "Puppies will outgrow an underbite." | Some mild puppy malocclusions self-correct during jaw growth (particularly between 3-6 months), but true skeletal mandibular prognathism does not resolve on its own. A veterinary dentist can assess whether the malocclusion is dental or skeletal. |
| "Underbited dogs can't eat normally." | Most dogs with underbites eat kibble and wet food without difficulty. Severe cases may benefit from smaller kibble sizes or softened food, but total feeding inability is rare. |
| "Breeding two underbited dogs guarantees underbited puppies." | Because mandibular prognathism is polygenic, severity and expression vary unpredictably. Even two heavily underbited Bulldogs may produce puppies with milder or different jaw configurations. |
Dental Disease Risk
The most significant concern for dogs with underbites is accelerated periodontal disease. Misaligned teeth create abnormal contact points and open spaces where food debris and bacteria accumulate. According to data from the American Veterinary Dental College, dogs with malocclusions develop periodontal disease earlier and more severely than dogs with normal occlusion.
Specific risks include:
- Labioversion of mandibular canines: Lower canine teeth angled outward can puncture the upper lip or gingiva, causing chronic ulceration and pain. This is one of the most common reasons for veterinary dental intervention in underbited breeds.
- Abnormal attrition: Teeth that contact at abnormal angles wear unevenly, potentially exposing the pulp chamber and leading to tooth root abscesses.
- Plaque retention: Crowded or rotated teeth — common alongside underbites — create pockets where plaque mineralizes into calculus (tartar) within 24-48 hours, initiating gingivitis.
- Temporomandibular joint (TMJ) stress: Severe underbites can alter the biomechanics of jaw opening and closing, placing uneven stress on the TMJ over time.
Practical Dental Care Protocol for Underbited Dogs
Based on veterinary dental recommendations, owners of dogs with underbites should follow an enhanced oral care routine:
- Daily tooth brushing using a VOHC (Veterinary Oral Health Council)-approved enzymatic toothpaste. Focus on the outer surfaces of the lower canines and incisors where plaque accumulates most in underbited dogs.
- Biannual veterinary dental exams (every 6 months rather than the standard annual check) to catch early signs of tooth wear, soft tissue trauma, and periodontal pockets.
- Professional dental cleanings under general anesthesia as recommended by your veterinarian — typically annually for brachycephalic breeds, starting by age 2-3 years. Expect costs of $300-$800 per cleaning depending on your region and whether extractions are needed.
- VOHC-approved dental chews such as Purina Dentalife or Greenies, sized appropriately for your dog's weight. These reduce calculus accumulation by approximately 30-45% based on VOHC testing data.
- Avoid hard chew toys (antlers, nylon bones, real bones) that can fracture already-stressed teeth. Opt for rubber-based toys like Kong Classic (approximately $12-$18 depending on size) or softer dental-specific toys.
When an Underbite Signals a Problem
Red Flags That Require Veterinary Attention
While many underbites are benign breed traits, certain signs indicate that the malocclusion is causing active harm:
- Visible sores or puncture wounds on the upper lip, palate, or gums where lower teeth are making contact
- Drooling more than usual or saliva tinged with blood
- Reluctance to eat, dropping food, chewing on only one side, or sudden preference for soft food
- Bad breath (halitosis) that worsens — often the first sign of periodontal disease or tooth root infection
- Visible tooth fractures or teeth that appear discolored (gray or pink-tinged), suggesting pulp death
- Swelling below the eye or along the jawline, indicating a possible tooth root abscess
- Difficulty closing the mouth or an obvious change in bite alignment that develops suddenly (not present since puppyhood)
If your dog exhibits any of these signs, schedule a veterinary dental examination promptly. Left untreated, dental infections can spread systemically, affecting the heart valves (endocarditis), kidneys, and liver.
Important: If an underbite appears suddenly in an adult dog that previously had a normal bite, this is not a developmental trait — it may indicate a jaw fracture, TMJ luxation, or a growth (neoplasia) affecting the jaw bones. This requires immediate veterinary evaluation. Always consult your veterinarian for any sudden changes in your dog's bite or oral health.
How Is a Canine Underbite Different From Humans?
While both dogs and humans can develop Class III malocclusions (underbites), the underlying anatomy and clinical significance differ substantially:
- Skull architecture: Human underbites involve the relationship between the maxilla and mandible within a relatively flat facial plane. In dogs, especially brachycephalic breeds, the entire facial skeleton is compressed, creating a three-dimensional mismatch that is more complex than simple jaw length discrepancy.
- Dentition differences: Dogs have 42 permanent teeth (compared to 32 in humans) with specialized carnassial teeth (the upper fourth premolar and lower first molar) that must align precisely for shearing function. An underbite disrupts this carnassial alignment, which has no human equivalent.
- Treatment philosophy: In humans, underbites are routinely corrected with orthodontics or orthognathic surgery for both functional and cosmetic reasons. In veterinary medicine, treatment is pursued only when the malocclusion causes pain or functional impairment — cosmetic correction is considered ethically unnecessary.
- Adaptation: Dogs are remarkably adaptive eaters and can compensate for significant malocclusions using their tongue and altered chewing mechanics. Humans with untreated underbites typically experience more pronounced speech and mastication difficulties.
Frequently Asked Questions
Is an underbite normal in dogs?
In brachycephalic breeds like Bulldogs, Pugs, Boxers, and Shih Tzus, an underbite is a normal, breed-standard trait selected over centuries. In dolichocephalic or mesocephalic breeds (German Shepherds, Labradors, Greyhounds), an underbite is considered a fault and may indicate a developmental problem. Regardless of breed, any underbite should be evaluated by a veterinarian to assess dental health risk.
Why do some dogs have underbites?
Canine underbites are primarily genetic, caused by polygenic inheritance affecting jaw bone growth rates. The brachycephalic skull mutation (associated with the BMP3 gene) shortens the maxilla more than the mandible, creating a mismatch. Environmental factors like retained deciduous teeth, nutritional deficiencies during growth, or jaw trauma can also contribute, though these are less common causes.
Can dogs with underbites eat normally?
Most dogs with mild to moderate underbites eat standard kibble and wet food without difficulty. Dogs with severe underbites may benefit from smaller kibble pieces or slightly moistened food. If your dog is dropping food, chewing with head tilted, or avoiding hard food entirely, a veterinary dental exam is warranted to check for painful dental complications.
How do I know if my dog's underbite is causing problems?
Watch for behavioral and physical signs: reluctance to eat hard food, pawing at the mouth, excessive drooling, blood-tinged saliva, foul breath, visible sores on the lips or gums, and facial swelling. Annual (or biannual for underbited breeds) veterinary dental exams are the most reliable way to detect problems early, since dogs often mask oral pain until it becomes severe.
What does it mean when my dog has an underbite but isn't a brachycephalic breed?
In a non-brachycephalic dog, an underbite may be a developmental anomaly from retained baby teeth altering jaw growth, a genetic variation, or — if it appeared suddenly — a sign of trauma or disease. A veterinary dentist can determine the cause through oral examination and dental radiographs. In puppies, early intervention (extracting retained deciduous teeth by 4-5 months of age) can sometimes prevent worsening of the malocclusion.
How is an underbite different from an overbite in dogs?
An underbite (mandibular prognathism, Class III malocclusion) means the lower jaw extends past the upper jaw. An overbite (maxillary prognathism, Class II malocclusion) means the upper jaw extends significantly past the lower jaw — sometimes called "parrot mouth." Both are skeletal malocclusions, but overbites are more common in dolichocephalic breeds like Collies and Dachshunds. Both types require veterinary dental monitoring for secondary complications.
Should I avoid breeding a dog with an underbite?
If the underbite is a breed-standard trait (Bulldog, Pug, etc.), breeding is acceptable within responsible breeding practices that include health testing. If the underbite is unexpected for the breed, the dog should not be bred, as the polygenic nature of the trait means it can be passed to offspring. Responsible breeders screen for bite conformation and avoid breeding animals with non-standard malocclusions. Always consider adoption from breed-specific rescues as an equally valid option.
Tom Renshaw
All our authors care for dogs every day — read more of their work on the authors page.
