Canine Underbite Explained: The Science, Breeds, and Health Impact
What causes a canine underbite? Explore the genetics, evolutionary basis, breed predispositions, and health implications of mandibular prognathism in dogs.
Quick Answer: A canine underbite (mandibular prognathism) occurs when the lower jaw extends beyond the upper jaw, causing the bottom teeth to sit in front of the top teeth. It is a hereditary skeletal trait caused by polygenic inheritance — multiple genes controlling jaw growth rate and timing. In brachycephalic breeds like Bulldogs and Pugs, an underbite is a breed-standard trait selected over centuries. In other breeds, it may signal a developmental mismatch or dental problem requiring veterinary evaluation.
The Genetics and Physiology Behind Canine Underbite
What Is Mandibular Prognathism?
The clinical term for an underbite in dogs is mandibular prognathism — the lower jaw (mandible) grows longer or faster than the upper jaw (maxilla). The reverse condition, where the upper jaw protrudes, is called maxillary prognathism or an overbite (mandibular brachygnathism).
During normal canine development, puppies are born with a slight relative mandibular shortening. As they grow, the upper and lower jaws lengthen at coordinated rates to achieve a scissors bite — where the upper incisors slightly overlap the lower incisors and the lower canines fit into the diastema (gap) between the upper third incisor and canine tooth.
When this coordination fails, malocclusion results. According to the Merck Veterinary Manual, malocclusions in dogs can arise from skeletal discrepancies (jaw length mismatch), dental discrepancies (tooth angulation problems), or a combination of both.
Polygenic Inheritance and Jaw Growth
Canine underbite is not controlled by a single gene. Research published in PLOS Genetics has identified that skull shape in dogs — including jaw proportions — is governed by a complex interaction of multiple genes, including BMP3, MSRB3, and SMOC2. The BMP3 gene variant, in particular, is strongly associated with brachycephalic skull conformation, which inherently shortens the maxilla while the mandible retains ancestral proportions.
The mechanism works like this: during embryonic development, neural crest cells migrate to form the facial skeleton. Growth factors (particularly bone morphogenetic proteins) regulate how rapidly and how far each jaw bone extends. When selective breeding favors a shortened muzzle, breeders are selecting for reduced maxillary growth — but the mandible may continue growing at a more ancestral rate, producing the characteristic underbite.
Evolutionary Basis: Why Some Dogs Developed Shorter Upper Jaws
Evolutionary Context: Wolves and wild canids universally possess a dolichocephalic (long-muzzled) skull with a perfect scissors bite. This jaw configuration maximizes bite force distribution across the carnassial teeth for shearing meat and crushing bone. No wild canid naturally exhibits an underbite.
The underbite in domestic dogs is entirely a product of artificial selection. As humans began breeding dogs for specific functions — and later for appearance — skull shape became one of the most dramatically altered traits. A 2017 study in Scientific Reports demonstrated that brachycephalic skull morphology arose independently in multiple dog lineages through selection on the same genetic pathways, a phenomenon known as convergent evolution at the molecular level.
The shortened muzzle served no survival advantage in the wild. In fact, brachycephalic dogs with severe underbites would struggle with prey capture and thermoregulation (panting efficiency depends on muzzle surface area). These traits persisted because humans provided food, shelter, and selective breeding that prioritized neotenic (juvenile) facial features — flat faces, large eyes, and compact skulls — which trigger caregiving responses in humans.
Breed Variations: Which Dogs Are Predisposed to Underbites?
Underbites are so common in certain breeds that they appear in the AKC breed standards as acceptable or even preferred traits. In other breeds, an underbite is considered a fault or disqualification.
| Breed | Underbite Status | Typical Severity | Associated Concerns |
|---|---|---|---|
| English Bulldog | Breed standard (required) | Severe — mandible significantly protrudes | BOAS, dental crowding, lip-fold dermatitis |
| Pug | Breed standard (common) | Mild to moderate | Dental overcrowding, periodontal disease |
| Boxer | Breed standard (required — slight undershot) | Mild | Minimal if slight; moderate if severe |
| Shih Tzu | Breed standard (acceptable) | Mild to moderate | Retained deciduous teeth, dental tartar |
| French Bulldog | Breed standard (common) | Mild to moderate | BOAS, dental disease, eating difficulty |
| Boston Terrier | Breed standard (acceptable) | Mild | Dental misalignment |
| Lhasa Apso | Breed standard (slight undershot acceptable) | Mild | Periodontal disease risk |
| Pekingese | Breed standard (common) | Moderate | Severe dental crowding, soft palate elongation |
| Non-brachycephalic breeds (Labrador, GSD, etc.) | Fault/disqualification | Variable | May indicate developmental abnormality |
Health Implications: When an Underbite Becomes a Problem
A mild underbite in a breed predisposed to the trait often causes no functional problems. However, moderate to severe underbites — or underbites in breeds where the trait is unexpected — can create significant health issues that require veterinary attention.
Dental Disease Acceleration
The most common consequence of an underbite is malocclusion-induced periodontal disease. When the teeth do not align properly:
- Abnormal tooth-to-tooth contact (attrition): Lower canines may strike the hard palate or upper gums, causing painful ulcers, oronasal fistulas, or enamel wear.
- Food trapping: Misaligned teeth create pockets where food debris accumulates, accelerating plaque mineralization into calculus (tartar).
- Gingival trauma: Teeth may impinge on opposing gum tissue, causing chronic inflammation and recession.
- Crowding: Brachycephalic dogs have the same 42 adult teeth as long-muzzled dogs but in a much smaller jaw, leading to rotated, overlapping, or impacted teeth.
Studies in the Journal of Veterinary Dentistry indicate that brachycephalic breeds develop periodontal disease at significantly younger ages — often by 2-3 years old — compared to mesocephalic breeds where onset typically occurs around 5-7 years.
Functional and Comfort Issues
Severe underbites can affect:
- Chewing efficiency: The carnassial teeth (the primary shearing teeth) fail to meet properly, reducing the dog's ability to process food.
- Prenhension: Difficulty picking up food or toys, particularly in Bulldogs with extreme mandibular protrusion.
- Lip and skin fold infections: The protruding lower jaw creates skin folds prone to moisture accumulation and bacterial/fungal dermatitis (lip-fold pyoderma).
- Temporomandibular joint (TMJ) stress: Altered jaw mechanics can place abnormal forces on the TMJ, potentially contributing to pain or degenerative changes over time.
When to See a Veterinary Dentist: Consult a veterinarian or board-certified veterinary dentist if you observe any of the following:
- Your dog drops food frequently while eating or shows reluctance to chew hard kibble or toys
- Visible ulcers, redness, or sores on the gums or hard palate
- Excessive drooling or blood-tinged saliva
- A sudden change in bite alignment (may indicate jaw fracture, tumor, or TMJ luxation)
- Pawing at the mouth or facial swelling
- Foul breath beyond normal (may indicate advanced periodontal disease or oronasal fistula)
Always consult your veterinarian for a proper diagnosis. Dental procedures in dogs require general anesthesia and should only be performed by licensed veterinary professionals.
Myth vs. Fact: Common Misconceptions About Canine Underbites
| Myth | Fact |
|---|---|
| "An underbite means a dog is aggressive or has a bad temperament." | Jaw alignment has zero correlation with temperament. Underbites are skeletal traits governed by bone-growth genes, not behavioral genes. Bulldogs and Pugs — breeds commonly having underbites — are among the most gentle companion breeds. |
| "Puppies will grow out of an underbite." | While jaw proportions shift slightly during growth (the "ugly duckling" phase between 3-6 months), a true skeletal underbite does not self-correct. If anything, mandibular growth may outpace maxillary growth during adolescence, worsening the discrepancy. Some puppies experience "wandering canines" where deciduous tooth eruption temporarily alters appearance, but underlying skeletal proportions remain. |
| "All underbites require surgery or braces." | Many mild underbites are purely cosmetic and require no intervention. Treatment is only necessary when malocclusion causes pain, tissue trauma, or functional impairment. Options range from tooth extraction (most common) to orthodontic movement, crown reduction with vital pulp therapy, or in rare cases, orthognathic surgery. A veterinary dentist determines the appropriate approach. |
| "You can fix an underbite by pushing on a puppy's jaw." | Manual manipulation of a puppy's jaw cannot alter genetically programmed bone growth. This folk remedy has no scientific basis and may cause TMJ injury or stress to the developing puppy. |
| "Underbites only happen in purebred dogs." | Mixed-breed dogs can inherit underbites, especially if brachycephalic breeds are in their lineage. Mixed-breed dogs with one brachycephalic parent may exhibit intermediate jaw proportions with mild underbites. |
How Is a Canine Underbite Different From a Human Underbite?
While both dogs and humans can develop mandibular prognathism, the underlying mechanisms and clinical significance differ substantially:
| Feature | Dogs | Humans |
|---|---|---|
| Primary cause | Selective breeding for brachycephalic skull shape; polygenic inheritance | Genetic inheritance, sometimes environmental (thumb-sucking, tongue thrust) |
| Prevalence | Extremely high in brachycephalic breeds (estimated >50% in Bulldogs) | Approximately 3-5% of human population (Class III malocclusion) |
| Treatment approach | Extraction, crown reduction, orthodontics only if causing pain/damage | Braces, headgear, orthognathic surgery (often for aesthetics + function) |
| Tooth count | 42 adult teeth in a compressed jaw | 32 adult teeth (with wisdom teeth) |
| Functional impact | Carnassial shear failure, prehension difficulty | Chewing difficulty, speech impediment, TMJ disorders |
A key physiological difference: dogs have anisognathic jaws — the upper jaw is naturally wider than the lower jaw, allowing the teeth to interlock for shearing. When an underbite disrupts this relationship, the carnassial teeth (upper fourth premolar and lower first molar) lose their scissor-like action, which has no human equivalent since humans rely on flat molar surfaces for grinding.
Practical Takeaways: Daily Care for Dogs With Underbites
Dental Hygiene Protocol
- Daily tooth brushing with a veterinary enzymatic toothpaste (e.g., C.E.T. Enzymatic Toothpaste, ~$8-12 per tube) using a dual-head brush designed for canine dentition. Focus on the crowding areas where teeth overlap.
- Professional dental cleanings every 6-12 months for brachycephalic breeds with underbites (vs. every 12-24 months for dogs with normal occlusion). Expect to pay $300-$800 per cleaning under general anesthesia, depending on your region and whether extractions are needed.
- Dental chews with VOHC (Veterinary Oral Health Council) approval — look for the VOHC seal on products like Purina Dental Pro Plan Dental Chews or Greenies. Avoid extremely hard chews (antlers, weight-bearing bones) that can fracture already stressed teeth.
Feeding Adjustments
- If your dog drops kibble or struggles with prehension, try larger kibble size (paradoxically, bigger pieces are easier to grip than tiny ones for underbite dogs) or a flat-faced breed formula (e.g., Royal Canin Brachycephalic Adult, ~$45-65 for a 10 lb bag) with specially shaped kibble designed for easier pickup.
- For dogs with severe underbites that impair eating, wet food or moistened kibble reduces the mechanical demand on misaligned jaws.
- Elevated feeders are generally not recommended for brachycephalic breeds due to increased bloat risk — feed at floor level unless your veterinarian advises otherwise.
Monitoring Schedule
- Puppy stage (8-16 weeks): Have your veterinarian assess jaw alignment at the first wellness visit. Note the deciduous (baby) tooth eruption pattern.
- Adolescent stage (4-7 months): Monitor for retained deciduous teeth — brachycephalic breeds are at high risk. Retained baby teeth alongside erupting adult teeth worsen crowding and should be extracted promptly ($50-$150 per tooth extraction at time of spay/neuter anesthesia).
- Adult stage (1 year+): Annual oral examination with full-mouth dental radiographs when possible. Radiographs reveal hidden root resorption, bone loss, and impacted teeth not visible above the gum line.
Canine Underbite FAQ
Why do some dogs have underbites while others don't?
Underbites result from selective breeding that altered skull proportions. Breeds selected for flat, brachycephalic faces have genetically shortened maxillas (upper jaws) while the mandible retains more ancestral proportions. The genes controlling jaw growth — particularly BMP3 and related developmental pathways — were selected for neotenic appearance, producing the underbite as a secondary consequence. Non-brachycephalic breeds retain the ancestral dolichocephalic skull shape with a normal scissors bite.
Can dogs with underbites eat normally?
Most dogs with mild to moderate underbites eat normally. Dogs with severe underbites may have difficulty picking up small kibble pieces or may drop food due to misaligned incisors. Switching to a brachycephalic breed kibble shape, moistening food, or using a wet diet can help. If your dog consistently struggles to eat, a veterinary dental evaluation is warranted to rule out painful tooth-to-palate contact or TMJ issues.
Is a canine underbite normal?
It depends on the breed. In English Bulldogs, Boxers, Pugs, Shih Tzus, and several other brachycephalic breeds, an underbite is a breed-standard trait and is considered normal for that breed. In a German Shepherd, Labrador Retriever, or other dolichocephalic/mesocephalic breed, an underbite is abnormal and may indicate a developmental problem, trauma, or genetic anomaly that warrants veterinary investigation.
What does it mean when my dog suddenly develops an underbite?
An underbite that appears suddenly in an adult dog who previously had normal jaw alignment is a medical concern. Possible causes include jaw fracture (from trauma), temporomandibular joint (TMJ) luxation or dysplasia, oral tumors displacing the jaw, or severe periodontal bone loss altering tooth position. Schedule a veterinary appointment promptly — this is not a cosmetic issue and may cause significant pain.
How do dogs process food differently with an underbite?
Dogs with normal occlusion use their incisors to grasp food, canines to hold and tear, and carnassial teeth (upper P4 and lower M1) to shear food into swallowable pieces. With an underbite, the incisor grasp is weakened because the lower incisors protrude past the uppers. The carnassial shear may be partially or completely lost depending on severity. Most dogs compensate by using more head-tossing motions to position food and may swallow larger pieces. This doesn't typically cause digestive problems since dogs have highly acidic stomachs (pH 1-2) capable of processing large food chunks, but it does increase the risk of choking in extreme cases.
Can an underbite in a puppy correct itself as they grow?
Rarely. While jaw proportions shift slightly during the transition from deciduous to permanent dentition (approximately 3-7 months of age), a true skeletal underbite does not self-correct. The "interlocking" phase of puppy teeth can sometimes create a temporary appearance of misalignment that resolves as adult teeth erupt. However, if the underlying jaw bone lengths are disproportionate, the underbite will persist and may become more pronounced as the mandible continues to grow until skeletal maturity (typically 10-14 months in most breeds).
References
- Merck Veterinary Manual — Developmental and Congenital Disorders of the Mouth and Teeth in Dogs
- American Kennel Club — Dog Dental Care: How to Brush Your Dog's Teeth
- Schoenebeck, J.J. et al. (2012). "BMP3 variant is associated with brachycephalic skull shape in dogs." PLOS Genetics.
- American Veterinary Medical Association (AVMA) — Pet Dental Care
Jonas Cole
All our authors care for dogs every day — read more of their work on the authors page.
