Rattus Biologicus:
A Gnawing Issue - Rat Teeth

Caryl Hilscher-Conklin
From the Nov/Dec 1996 Rat & Mouse Gazette

When most people think of rats, they envision long, yellow teeth chewing away at everything in their path. The word "rodent" is derived from the Latin word rodere, which means "to gnaw," a name well-deserved by our little friends. It is a fact that rats can cause great destruction with their powerful teeth and jaws -- jaws so powerful that they can bite through lead!

Wild rats are so notorious for their devastation that "rat-proofed" buildings have became a necessity in order to prevent their entrance into human habitat. "Rat-proofing" involves using special non-chewable structural materials for walls and doors at rat height so that they cannot chew through small openings to enter buildings. So, just what is it about the rat's tooth structure that makes it such an effective shredder?

Rats are monophyodont, which means that they have one set of teeth during their lifetime. In comparison, humans are polyphyodont, because we lose one set, then grow another. Rats have only two types of teeth in their mouth -- molars and incisors. They lack the pointed canines, which function to tear food (and enemies!) in other mammals like dogs, cats and humans. Rat molars are very similar to human molars. They begin to erupt around 16 days after birth, take about 125 days to completely develop, and are composed of a layer of enamel on the exterior. Enamel is the hardest substance in the body. Underneath the thin layer of enamel is dentin, which is a material very similar to bone. A layer of cementum covers the dentin at the level of the root. Finally, in the interior of the tooth is the pulp, which contains blood vessels and nerves. Just like humans, rats are susceptible to dental caries -- but since rats cannot brush their teeth, they rely on us to feed them healthy meals that are low in the sugar on which bacteria thrive!

Oh, the lovely rat incisor, with its elegant curve and smooth lines ... what?! Believe it or not, the shape of the rat's front teeth is literally a work of art. The morphology of the rat incisor was long ago described by a biologist as "a shape...with almost geometric precision...of a segment of a spiral." The incisors of a rat are very different from those of other mammals, and this difference is what makes the rat such a good chewer. The pulp cavity for the incisors remains open and no roots are developed; as a result, the rat's incisors grow continuously. There are two sets of incisors: one upper and one lower, both erupting at about 8-10 days of age. They are covered by enamel only on the labial side (facing the lips); on the interior surface of the tooth is dentin. This dichotomy in structure, with the tooth being harder on one side than the other, gives rise to the specialized shape and function of the incisors.

Attrition is the normal wearing of the teeth due to functional activity. The back and forth movement of the jaws during gnawing rapidly wears away the softer dentin on the interior surface of the incisor more so than the hard enamel on the labial side. As a result, a sharp, chisel-like edge is formed. The incisors grow at a rate of about 5 inches a year, and are continuously worn away, to be completely replaced every 40 to 50 days. There even appears to be a circadian rhythm associated with the eruption rate, with slower eruption taking place at night when the incisors are in more continual use, and thus experiencing more frequent functional occlusion. In the event of improper wear due to fracture of the opposing tooth or misalignment of the jaw, the formation of a screw-like helix may occur. If left untrimmed, the overgrown incisor will likely penetrate the rat's palate. Unfortunately, this often results in the animal's death due to its inability to eat.

Contrary to its appearance, the yellow/orange color of the rat incisor is not a sign of poor hygiene -- it is perfectly normal. Rather, the color of the incisor enamel is due to the presence of an iron-containing pigment, and the shade of orange deepens with age starting about 21 days after birth. This is in contrast to the situation in other mammals such as humans, cattle and pigs, where a brownish color of the teeth is characteristic of the rare bone disease porphyria.

Remember the mob of rats chewing through the wall in the movie "Willard?" Do rats actually eat through barriers, ingesting everything in their path? No, they are much more sophisticated than that -- rats are adapted to get where they need to go without ending up with a stomach full of splinters. Between the incisors and the molars is a space called the diastema. Folds of the rat's inner cheek can extend into this area, serving to separate the incisors (gnawing apparatus) from the rear part of the mouth cavity. Hence, the rat can chew away and let the shreds of wood and cardboard fall to the wayside. The bite load of a rat is actually quite strong, generating up to 10 pounds! With that knowledge in mind, let us give thanks in this holiday season that our rat friends are such sweet and docile creatures.