Why do shih tzus have underbites

Overview

Shih Tzus, known for their distinctive flat faces and flowing coats, have a documented history dating back to Tibetan monasteries around 800 AD, where they were bred as companion dogs. These "lion dogs" were later refined in Chinese imperial courts during the Ming (1368-1644) and Qing (1644-1912) dynasties, where selective breeding emphasized their brachycephalic (short-headed) features. The modern Shih Tzu emerged in the 1930s when the breed was introduced to Europe and America, with the American Kennel Club officially recognizing it in 1969. This centuries-long breeding history has fixed the underbite as a breed characteristic, with kennel club standards worldwide accepting a slight undershot jaw as normal for the breed. The Shih Tzu's unique facial structure represents one of the most extreme examples of brachycephaly among dog breeds, resulting from human-directed selection over approximately 1,200 years.

How It Works

The underbite in Shih Tzus results from their brachycephalic skull morphology, where selective breeding has shortened the maxilla (upper jaw) while maintaining normal mandible (lower jaw) length. This creates a dental malocclusion where the lower incisors sit in front of the upper incisors when the mouth is closed. The condition develops during embryonic growth around days 20-35 of gestation when facial bones form, with genetic factors determining the degree of mismatch. Specifically, mutations in genes like BMP3 and MSX2 that regulate craniofacial development contribute to the shortened maxilla. The typical Shih Tzu underbite measures 1-3 millimeters of protrusion, though severe cases can reach 5+ millimeters. This jaw structure affects dental alignment, often causing the lower canine teeth to rub against the hard palate, which can lead to periodontal issues in approximately 15-20% of affected dogs without proper dental care.

Why It Matters

The Shih Tzu's underbite matters because it represents both a breed characteristic and a potential health concern. As the 20th most popular dog breed in the United States according to 2022 AKC registration statistics, understanding this trait helps owners provide appropriate care. While mild underbites generally cause no problems, severe cases can lead to dental disease, difficulty eating, or oral injuries requiring veterinary intervention. This knowledge informs breeding practices, as responsible breeders avoid exaggerating the trait to prevent health issues. The underbite also serves as an example of how centuries of selective breeding can fix physical traits, offering insights into canine genetics and the ethics of breed standards. For prospective owners, awareness helps in choosing healthy puppies and maintaining proper dental hygiene throughout the dog's 10-16 year lifespan.