15.3. Tooth Replacement

In many mammals the infants have a set of teeth that fall out and are replaced by adult teeth. These infant teeth are called deciduous teeth, primary teeth, baby teeth or milk teeth. The animal has a primary dentition until the first permanent tooth erupts. Its dentition is then categorized as mixed until the last deciduous tooth defoliates (falls). From then on, the animals has permanent dentition.

The dentition of animals with two sets of teeth (deciduous, permanent) is referred to as diphyodont, while the dentition of animals with only one set of teeth throughout life is monophyodont. The dentition of animals in which the teeth are continuously discarded and replaced throughout life is termed polyphyodont.

Manatees are polyphyodont with mandibular molars developing separately from the jaw and are encased in a bony shell separated by soft tissue. Some of elephant teeth are similar to those in manatees, and it is notable that elephants are believed to have undergone an aquatic phase in their evolution. Elephants have four molars, one on each side of the upper and lower jaw. Until age 40, these are replaced by larger molars, up to five times each. The new molars shift forward from the back of the jaw as the old wear down. The last set of molars can last for about twenty years.

Rabbits and other lagomorphs usually shed their deciduous teeth before (or very shortly after) their birth, and are usually born with their permanent teeth. The teeth of rabbits complement their diet, which consist of a wide range of vegetation. Since many of the foods are abrasive enough to cause attrition, rabbit teeth grow continuously throughout life. Rabbits have a total of 6 incisors, three upper premolars, three upper molars, two lower premolars, and two lower molars on each side. There are no canines. Three to four millimeters of tooth are worn away from the incisors every week, whereas the posterior teeth require a month to wear away the same amount. The incisors and cheek teeth of rabbits are aradicular (lack roots). These teeth grow or erupt continuously. The growth or eruption is held in balance by dental abrasion from chewing a diet high in fiber.

Rodents' incisors grow continuously throughout their lives and are also aradicular. Unlike humans whose ameloblasts die after tooth development, rodents continually produce enamel and must wear down their teeth by gnawing on various materials.These teeth are used for cutting wood, biting through the skin of fruit, or for defense. The teeth have enamel on the outside and exposed dentin on the inside. Since dentin is softer and erodes more easily than enamel, regular use and gnawing naturally produce a sharp edge on the tooth's anterior surface. There is variation in the dentition of the rodents, but generally, rodents lack canines and premolars, and have a space between their incisors and molars, called the diastema.

Figure credits

Figure 1 by OpenStax College – Anatomy and Physiology. Chapter 23.3 Fig. 4.

Figure 2 by Christopher T Cooper, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=18904839

Figure 3 by Alexey Krasavin - originally posted to Flickr as Зубке (Tooth), CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=3875368