An underbite in a dog is a distinctive facial feature where the lower jaw extends noticeably past the upper jaw. This misalignment gives the dog a characteristic “undershot” appearance, often revealing the lower teeth. While this look is celebrated in certain breeds, it represents a deviation from the typical canine bite structure.
Understanding the Canine Underbite
The technical term for a canine underbite is mandibular prognathism, which veterinarians classify as a Class 3 malocclusion. This condition occurs because the lower jaw (mandible) is disproportionately long compared to the upper jaw (maxilla). The standard bite for most dog breeds is a scissor bite, where the upper incisors slightly overlap the lower incisors.
A slight underbite, where the upper and lower incisors meet edge-to-edge, is sometimes referred to as a level bite. A more pronounced underbite results in the lower teeth protruding significantly in front of the upper teeth. The severity of this malocclusion determines whether it is a harmless cosmetic trait or a condition requiring veterinary attention to prevent oral trauma.
Breeds Commonly Featuring an Underbite
The underbite is a defining and expected trait in many brachycephalic dog breeds. The English Bulldog is perhaps the most recognized example, where the broad, square jaw and protruding lower incisors are a hallmark of the breed.
The Boxer also commonly exhibits a pronounced underbite, which contributes to its powerful, square-jawed appearance. Similarly, the French Bulldog, a smaller relative of the English Bulldog, is expected to have a slightly undershot jaw. These breeds have been selectively bred for their unique head shape, which inherently includes the jaw misalignment.
Smaller companion breeds also frequently display this trait due to their compact skull structure. The Pug is known for its deeply wrinkled face and a jaw that is distinctly undershot. The Shih Tzu and the Lhasa Apso, both ancient breeds, are also commonly seen with a Class 3 malocclusion.
The Genetic and Structural Basis
The presence of an underbite is directly linked to the brachycephalic skull structure, which is characterized by a shortened muzzle. This condition is inherited, resulting from a genetic predisposition that affects the growth rate of the upper and lower jaws. The genetic selection for a flat face effectively shortens the maxilla, but the mandible’s growth is not equally restricted.
This differential growth results in the lower jaw being too long relative to the upper jaw, causing the lower teeth to sit forward. This structural imbalance is the biological mechanism behind the underbite in these specific breeds.
Health and Management of an Underbite
While an underbite is normal for certain breeds, a severe malocclusion can lead to health concerns. The most common issue is trauma to the soft tissues of the mouth, such as the gums or the roof of the mouth, caused by misaligned teeth digging into the tissue. This trauma can result in chronic discomfort, sores, or ulcers.
Misaligned teeth can also lead to abnormal tooth-on-tooth wear, potentially exposing the sensitive pulp inside the tooth. Furthermore, the crowding and rotation of teeth that often accompany a shortened upper jaw can predispose the dog to periodontal disease. Regular veterinary dental checks are necessary to monitor these issues, and management may involve daily brushing, tooth extractions, or orthodontic devices.