Teeth & Jaw
| There
are well over 2000 species
or rodent worldwide, and are usually singled out from other mammals for
their
constantly-growing opposing pairs of incisors and their owner's
ever-present need to wear them
down by gnawing hard surfaces! These four front teeth are maintained at
a constant
length at all times. The jaws have flat articulation (joints) so they
can grind better with their molars and pre-molars, commonly called
"cheek teeth". The word rodent comes from the Latin rodere, which means "to gnaw" and dent, which means "tooth".  Pictured to the right are the upper and lower incisors and left mandible of North America's largest living rodent, the Canadian beaver (Castor canadensis). A Canadian dime lays beside the teeth to give you a better idea of scale. The two upper incisors (the shorter teeth) are called orthodonts, and the two lower incisors (the longer teeth) are called pro-odonts. As you can see, the exposed incisor is just the 'tip of the iceberg" in a sense - much of the tooth lies within the tooth canal! The thin, yellow-brown area of the incisor is a very hard enamel. This enamel is harder than iron - in fact, the reason why the incisors are this colour is due to the iron content in the enamel! The enamal is also composed of calcium, phosphorus and other mineral salts. Because of its high mineral content, tooth enamal is brittle. The much thicker, cream-coloured area of the tooth is a soft dentin (or dentine) material. The soft area wears far more easily than the hard enamel, and this is why the incisors are always razor-sharp! The incisors are separated from the grinding pre-molars and molars or "cheek teeth" by a large space called a diastema. This is true for all rodents and lagomorphs (rabbits, picas and hares). The flying squirrel's incisors are able to rip open hard mast coverings, but also cut much of their food into smaller pieces. There are no carnassial or canine (cuspid) teeth in a rodent's jaw. Just incisors and molars. Once the incisors have finished their work, the food is passed down to the pre-molars and molars. The circular grinding action of the jaw's molars pulverizes the food so that it is ready to be swallowed. Flying squirrels have 22 teeth (for comparison, humans have 32). The dental formula for a flying squirrel is:  The formula is really quite simple. The top set of numbers represents the upper jaw's teeth on one side (double the number when totalling the number of upper jaw teeth). The first number (1) represents the number of incisor teeth. The second number (0) represents the number of canine teeth. The third number (2) represents the number of pre-molars, and the fourth number (3) represents the number of molars. The same rules apply for the bottom jaw's teeth. Not just for eating A flying squirrel's teeth are not used solely for eating! They'll use their teeth to hold on to food while gliding or running. They will use their teeth to groom fellow squirrels in a shared nest. They will use their teeth to pound food into crevices for storage. And they will use their teeth to strip bark from trees for nesting material, to pick up their young pups and move them to another nest, and for defence. The author has been bitten many a time by wild flying squirrels while processing them in the field. Abnormalities Because the incisors are constantly growing, a rodent must gnaw to wear down the teeth, otherwise there will be consequences. Let's say a rodent develops a gum disease and finds it too painful to use its incisors - resulting in starvation. A condition called "wolfing", or more accurately "malocclusion", can kill a rodent fairly quickly. It can often be a hereditary condition. Malocclusion is the misalignment of teeth, and in rodents it normally involves only the incisors. When the incisors grow at funny angles, it means that the squirrel cannot grind the incisors down...and the incisors continue to grow and grow and grow. In flying squirels, malocclusion usually results in the death of the animal, either due to starvation or because the teeth grew into the brain case. |