Dental Anatomy of Rabbits and Rodents

Key Points

  • Prolonged chewing of tough abrasive diets causes rapid tooth wear in rabbits and herbivorous rodents (chinchillas, degus and guinea pigs). To compensate for this, these species have permanent teeth that grow and erupt continuously, never producing anatomical roots.
  • Omnivorous rodents such as the Old World rat, mouse, and gerbil have small, short crowned molars with anatomical roots. Only the large, chisel-shaped incisors used for gnawing grow continuously in these rodents.
  • Rabbits and rodents lack canine teeth, a long diastema or gap occurring between the incisors and the premolars.
  • The premolars and molars are commonly referred to as “cheek teeth” since they function as a single unit and tend to have similar structures.

Introduction

The rabbit (Oryctolagus cuniculus), chinchilla (Chinchilla spp.), degu (Octodon degu), and guinea pig (Cavia porcellus) are true herbivores. The natural diet of the rabbit consists primarily of grasses, supplemented with low growing leafy vegetation. When these items are scarce, rabbits also eat roots and tree or shrub bark. The chinchilla originates from mountainous semi-desert areas in South America where vegetation tends to be largely tough and fibrous monocotyledon species, requiring thorough chewing. Whilst the guinea pig and degu also originating in South America they tend to inhabit less extreme habitats, degus living in dry grassland areas where they become agricultural pests, whilst guinea pigs frequent clearings in jungle and forest habitats, the predominant natural food of both being grasses. The smaller old-world rodents (rats, mice, gerbils, and hamsters) tend to have a higher energy, less abrasive diet that includes seeds.

The dental anatomy and physiology of rabbits and herbivorous rodents has evolved for efficient prehension and chewing of their bulky natural diets. Monocotyledenous plants such as grasses contain large numbers of phytoliths, highly abrasive silicate deposits which cause marked wear of the teeth. Since grass plants are low to moderate sources of dietary energy, large quantities must be eaten for animals to survive. The combination of fibrous structure and their abrasive nature requires prolonged chewing, promoting tooth wear. Lost tooth substance must be replaced to maintain chewing efficiency, and in rabbits, chinchillas, degus and guinea pigs this is possible because the permanent teeth remain in a life-long growth phase. These continuously growing teeth do not form anatomical roots, so the unerupted part of the teeth are like the “reserve crowns” of larger herbivores.

 

Dental Anatomy

Dental anatomy of rabbits and rodents is incredibly complex and only a basic review is provided here. The reader is encouraged to consult the references listed below for additional information.

 

Rabbits

Rabbits are diphyodont, having two recognizable sets of teeth. A set of small or deciduous teeth, which erupt in utero, are replaced by a fuller set of larger teeth by about one month of age. The deciduous dental formula is: 2 x 2/1 0/0 3/2= 16. The deciduous incisors are often shed before or around the time of birth, whilst the last of the deciduous premolars are replaced by permanent teeth by Day 35.

The adult rabbit dental formula is: 2 x 2/1 0/0 3/2 3/3 = 28.

Lagomorphs (rabbits, hares, and pikas) all possess two sets of maxillary incisor teeth: large first incisors and smaller peg-like incisors positioned directly behind to the first ones. Rabbits and rodents lack canine teeth. A long diastema or gap sits between the incisors and premolars in both rabbits and rodents. The cheeks fold in behind the incisors separating the front of the oral cavity from the more caudal section, thereby permitting separate function of the incisors and back teeth.

Premolars and molars are commonly referred to as “cheek teeth” since they tend to have similar structures and work together as a functional unit. The cheek teeth consist of three maxillary and two mandibular premolar teeth on each side, with three molars in each jaw quadrant (Fig 1). They are aligned to form nearly straight dental arcades, with oral surfaces of adjacent teeth contacting to create a continuous chewing surface (Fig 2). This chewing surface wears to create a serrated pattern due to the folded structure of the teeth, parallel layers of enamel and dentine in upper and lower teeth wearing at different rates as the jaws are moved side to side during chewing. Adjacent teeth are kept in contact by the converging arrangement of the teeth, those towards the extremities of the arcades tending to move inwards. This creates problems when a tooth is lost or extracted from the middle of an arcade, as the adjacent teeth tip into the defect creating gaps and irregularities in the occlusal surface. These gaps become packed with food leading to progressive periodontal disease, and the occlusal surfaces no longer wear normally promoting malocclusion. Since the teeth do not meet one on one, extraction of opposing teeth will make matters worse not better.

Rabbit cheek teeth

Figure 1. Endoscopic view of “cheek teeth” in a rabbit. Click image to enlarge.

Rabbit oral cavity

Figure 2. Endoscopic view of the rabbit oral cavity. Premolar and molar teeth are aligned to form nearly straight dental arcades. The oral surfaces of adjacent teeth contact to create a continuous chewing surface. Click image to enlarge.

All of a rabbit’s permanent teeth grow and erupt continuously. The mandibular cheek teeth of rabbits grow and erupt at approximately 3 to 4 mm per month. Maxillary teeth grow and erupt at a slightly slower rate. This wear is dramatically faster than occurs in large grazing animal such as the horse whose cheek teeth wear and erupt approximately 3 mm per year.

 

Rodents

The order Rodentia is vast, containing over 1700 species. The anatomy of these varies considerably, but rodents are united in having a single continuously growing incisor in each jaw quadrant, with most species being considered monophyodont with a single set of teeth without precursors or successors, though a few species do develop recognizable deciduous premolar teeth.

Guinea pigs, degus and chinchillas

These herbivorous rodents have elodont or aradicular hypsodont teeth with a long continuously growing crown and no anatomical root. All their teeth grow continuously to compensate for the constant wear that occurs when eating their natural diets.

These three S. American species share the same dental formula: 2 x 1/1 0/0 1/1 3/3 = 20. In most rodents the superficial enamel layer on the incisors contains a yellow-orange iron-based pigment (Fig 3), but this coloration is not present in guinea pigs. As in rabbits, a long diastema or gap sits between the incisor and premolar teeth and the cheek teeth have evolved to provide a large grinding surface with one premolar and three molars in each quadrant of the jaw. The different folding pattern of tooth structure and a more front to back propalineal chewing movement does not result in distinct ridging of the occlusal surfaces. The mandibular cheek teeth of chinchillas grow and erupt at approximately 3 to 4 mm per month, maxillary teeth being somewhat slower. No accurate figures are available regarding guinea pig or degu cheek teeth.

dental anatomy touch up

Rats, mice, hamsters

In their natural environment, rats, mice, and hamsters feed largely on seeds, grains, and tubers. These are high-energy foods with minimal abrasive properties that require little chewing so the cheek teeth are subjected to little wear. As a result, the cheek teeth of these species have a relatively small chewing surface that does not need constant replacement. They typically have three small brachydont or short crowned molars in each quadrant of the jaw, which possess anatomical roots and stop growing once they are fully erupted.

As with most other rodents, they possess a single, large, chisel-shaped incisor tooth in each quadrant of the mouth, these teeth generally having yellow enamel. The incisors can be used for biting but have evolved for gnawing using a forward scraping action of the lower teeth. Gnawing and grinding the teeth together to maintain their shape, wears away tooth substance that must be replaced, so incisors continue to grow throughout the animal’s life, never developing anatomical roots. The dental formula of Old World rats and mice is 2 x 1/1 0/0 0/0 3/3 = 16.

 

Conclusion

Rabbit and rodent dentistry is still at an early stage of development, however with an understanding of the underlying oral anatomy and physiology it is possible to devise an appropriate treatment regime for most dental problems.

References