The Wild Side of Dental Care

Introduction

For most exotic animal veterinarians, dental problems commonly observed include malocclusion in rabbits and rodents, or tartar buildup in ferrets (Mustela putorius furo) and African hedgehogs (Atelerix albiventris). Veterinarians that provide care in zoological parks or private wildlife collections frequently encounter unique dental challenges as they care for species from the full spectrum of the animal kingdom. This article will describe the wild side of dental care in exotic mammals, illustrating just some of the exiting differences in dental anatomy and the most common dental problems of various species.

 

Carnivorous mammals

Most carnivorous mammals, such as exotic felids, canids, ursids (bears), and procyonids, including raccoons, red pandas, and coatimundis, exhibit the dental anatomy and dental formula typical of all carnivores:

2 x (I2-3/3, C1/1, P2-3, M2-3/3-4)

Carnivores possess a variable number of small incisors, large fang-like canines, varying numbers of small pointed premolars, and larger pointed and scissor-like working molars (Fig 1). The long canines allow carnivores to catch and hold on to prey, while the scissor-like molars are used to rip and tear.

Common dental problems encountered in carnivores housed at zoos and wildlife parks are fractured canines, premolars, or molars due to chewing on rock, metal bars, or fences. Subsequently, dental extractions and root canals were often performed on zoo carnivores in the past. Fortunately, the introduction of environmental and behavioral enrichment has reduced the need for dental procedures in zoo carnivores. Dietary changes have further improved overall dental health by reducing tartar formation, periodontal disease, and excessive dental wear.

Hyenas

Some carnivorous species like the hyena are considered food specialists. The spotted hyena (Crocuta crocuta), the striped hyena (Hyaena hyaena), and the brown hyena (Hyaena brunnea) are the “bone crunchers” of the carnivore world (Fig 2). Thus, their premolars and molars are larger and stronger than those of any other carnivorous species (Fig 3). The most common dental problems encountered in hyenas are tartar formation, fractured teeth due to trauma, or excessive wear in older animals.

 

 

Aardwolves

Although the aardwolf (Proteles cristata) also belongs to family Hyenidae (Fig 4), this species has developed an insectivorous nutritional strategy. Aardwolves use a long, sticky tongue with prominent nobs on its surface to pick up termites and larvae.

Therefore, aardwolf teeth are reduced in size or are even missing when compared to the typical carnivore (Fig 5). The aardwolf dental formula is:

2 x (I3/3, C1/1, P2/2, M2/0)

Dental problems are rather uncommon in this species.

Herbivorous mammals

Many members of order Artiodactyla, such as antelopes, gazelles, exotic sheep and goats, have the same dentition and similar dental problems as their domestic counterparts. Some herbivores have developed specialized dentition depending on their evolutionary development, habitat, and social function.

Giraffes

The giraffe (Giraffa camelopardalis), a close relative to bovine species, has dentition similar to that seen in other ruminants:

2 x (I0/3, C0/1, P2-3/3, M3/3)

Giraffe are specialized browsers, feeding primarily on acacia leaves with the help of premolars and molars that are low-crowned with rough enamel (Fig 6). Giraffe premolars and molars are rather sensitive to the consumption of roughage containing silicates, which makes excess dental wear a major problem in captivity.

Another minor difference between giraffe and bovid teeth, is that giraffe possess lobed, lower incisiform canines. The canine teeth of other ruminants usually cannot be differentiated from the incisors.

Deer

The dental formula of deer includes the absence of upper incisors as well as the presence of incisor-like lower canines:

2 x (I0/3, C0-1/1, P3/3, M3/3)

Among deer, unique dentition is observed in musk deer and chevrotains.

• Musk deer are either placed in subfamily Moschinae of the deer family Cervidae or they are classified as a separate family entirely.
• Chevrotains, also known as mouse deer, belong to a primitive ruminant family called the Tragulidae, and are more closely related to pigs than deer.

Musk deer and chevrotains mainly differ from “normal” deer (family Cervidae) by the absence of antlers and facial scent glands. Both male and female musk deer and chevrotains also possess elongated, upper canines that develop into visible sharp tusks in the male (Fig 7, Fig 8). Tusk fracture with or without exposure of the pulp is a very common dental problem in captivity.

Macropods

Belonging to order Diprodontia, family Macropodidae consists of kangaroos and wallabies. Macropods are grazers and browsers. They possess only one mandibular incisor on each side and up to three maxillary incisors per side. The lower incisors are elongated and more horizontally aligned (Fig 9). The remainder of the dentition is similar to that seen in ungulates (see deer above). The macropod dental formula is:

2 x (I1-3/1, C0/0, P2-3/3, M3/3)

Lumpy jaw is a very common problem in macropods. Caused by opportunistic pathogens like Fusobacterium necrophorum and Bacteroides spp., infection usually begins with local trauma to the oral mucosa. Invasion of these opportunistic pathogens causes progressive infection and necrosis of the soft tissue. Tooth roots and associated bone tissue are often secondarily affected and can cause bone resorption, jaw fracture, and tooth loss. Lumpy jaw is extremely difficult to manage, however treatment often involves removal of loose teeth, aggressive wound management, and systemic antibiotic and anti-inflammatory therapy.

Babirusas

Despite a fairly standard warty pig diet of leaves, roots, fruit, and animal matter, the babirusa (Babyrousa babyrussa) has unique dentition within family Suidae:

• The upper canines of the male grow through the hard palate and skin forming typical backwards-curved facial structures that can reach up to 30 cm in length (Fig 10, Fig 11). The size of these canines can influence the boar’s status, but are not used as weapons as they are very fragile.
• As in other pig species, the lower canines form strong, visible tusks that instead are used for fighting.

The babirusa’s dental formula is:

2 x (I2/3, C1/1, P2/2, M3/3)

The most common dental problems of the babyrusa involve trauma to the fragile upper canines.

Hippopotami

The river hippopotamus (Hippopotamus amphibius) possesses long, strong lower incisors that are used like shovels to graze river bottoms as well as in defense (Fig 12). On land, hippos use their wide, powerful lips to tear off vegetation and their bovid-like molars to grind down small shoots, grasses, and reeds. The tusk-like canines of the male hippo are used to impress and deter potential territorial intruders (Fig 13, Fig 14). The hippopotamus dental formula is:

2 x (I2/2, C1/1, P3-4/3-4, M 3/3)

Elephants

Perhaps the most unique dental features of the herbivore world are found in the elephant:

2 x (I 1/0, C 0/0, P 0/0, M 6/6)

Elephants develop only two upper incisors. These incisors form long tusks of dentin or “ivory” that surround a pulp cavity (Fig 15). During early development, the tusks have an enamel cap that is rapidly worn away. While both male and female African elephants (Loxodonta africana) carry visible tusks, some, but not all, male Asian elephants (Elephas maximus) develop long, visible tusks. Female Asian elephants do not always develop incisors and when present, these small, oddly shaped tusks or “tushes” are usually hidden by the lips.

The molars consist of a series of ridges and vertical lamellae held together by cementum and covered by enamel. The diamond shape of lamellae in African elephants give Genus Loxodonta its name. ‘Loxo‘ from the Greek word for ‘slanting’ and ‘odonta‘ for tooth. Asian elephant lamellae have a flat, oval shape (Fig 16).

Elephants develop six sets of maxillary and mandibular molars throughout their life (Fig 17). Molars erupt one at a time and move towards the front of the mouth pushing the old molar forward (Fig 18). Therefore molars are worn first at their cranial end. Once the caudal end of the molar wears down, a new molar begins to erupt and push on the old molar. This pressure on the old tooth compromises its blood supply and the crown eventually fractures transversely. Old tooth fragments are spit out or swallowed and the roots are resorbed.

Juvenile elephants are born with two molars in each jaw. Each successive set of molars is progressively larger in size. The wear of one molar usually lasts over 10 years, giving the average elephant dental tools for about 60 years. After the sixth set of molars have worn off, the elephant is unable to feed and the animal will ultimately starve to death, usually somewhere between 50 to 70 years of age. Additional dental problems observed in captive elephants include abnormal development of the molars, such as excessive growth or malocclusion, dental caries, dental abscesses, and tusk fractures.

Sloths

Sloths belong to order Xenarthra, together with armadillos and anteaters (Fig 19). The former name of this order, Edentata, refers to the absence of incisors and canines in these species:

2 x (I0/0, C0/0, P2/3, M2-3/2-3)

The term “Edentata” or ‘toothless‘ was misleading, however, because these species still develop premolars and molars (Fig 20). The premolars of the sloth are triangular, razor-sharp teeth that resemble canines, and are used for defense and to rip foliage off of branches. The molars are used to grind down leaves and, if necessary, bark.

Insectivorous mammals

Bats

Bats, the flying mammals of order Chiroptera, usually have approximately 38 teeth, although vampire bats have the smallest number with only 20 teeth. All bats have long upper and lower canines, which give them the classic vampire appearance (Fig 21).

2 x (I2-3/0, C1/1, P0-2/0-2, M2-3/2-3)

The dental features seen in members of order Chiroptera reflect their nutritional strategy:

• Nectarivorous bats have less functional, very small, rootless premolars and molars. The lower incisors are missing to provide more space for the extended tongue.
• Frugivorous bats have large, flattened cheek teeth to facilitate mastication of fruits and leaves high in fiber (Fig 22).
• Insectivorous bats have sharp cheek teeth used to chew the exoskeletons of their prey. A callus-like, muscular gingival papilla in front of the lower premolars helps to break down and separate chitinous parts.

Neonatal bats have milk incisors, which are very sharp and curved backwards to ensure a secure grip on the nipple. The incisors are replaced and the premolars and molars appear at 3 to 6 weeks of age.

Aardvarks

Aardvarks (Orycteropus afer) are the only species belonging to order Tubulidentata (Fig 23). This very unique animal is myrmecophagous, specializing in the consumption of ants and termites. Aardvarks are born with conventional incisors and canines, but these teeth fall out in the embryo or infant and are not replaced (Fig 24). Adult aardvarks have only premolars and molars with a dental formula of:

2 x (I0/0, C0/0, P2/2, M3/3)

Aardvarks possess tubular or peg-like, rootless teeth, which are built from clusters of thin, upright, hexagonal-shaped tubes of vasodentin, a modified form of dentin. These tubes are arranged in parallel rows and are held together by cementum. Since each tube has its own individual pulp canal, a large, central pulp cavity is not present in aardvark teeth. The number of tubes or columns present depend on the size of the tooth, with the largest teeth containing up to 1500 tubes. Aardvark teeth also lack an enamel coating; they exhibit constant wear and continual growth similar to the hyposodont, open-rooted teeth of rabbits.

 

Summary

The dentition of zoo animals is as varied as the species housed in zoological parks, however a good understanding of the normal diet will generally provide important clues to the dental anatomy observed. Carnivorous species require dental tools for grasping and shredding, while herbivore teeth grind and cut. Insect-eaters also require a grinding surface to break down the chitinous exoskeleton, while fruit eaters require a large, flat grinding surface for fiber-rich foods.

 

Acknowledgement:

Special thanks to Joey Williams from the Museum of Osteology in Oklahoma City, Oklahoma for providing excellent pictures for this article.

 

References