Radical Rodents

Degus are members of an amazing group of mammals called rodents. This group includes all sorts of familiar furry faces. Rats, mice, hamsters, guinea pigs, squirrels, chipmunks, porcupines, beavers, gophers . . . I could go one all day, but there are over 2,000 species. In fact, rodents make up over 40% of all known mammals! They inhabit every continent except Antarctica and occupy just about every ecological niche imaginable. Some rodents are tiny, weighing a handful of ounces, while the capybara can be as heavy, or heavier, than a German shepherd. The prehistoric Josephoartigasia monesi was the size of a small car, making it the largest rodent of all time. There are rodents that live underground, in trees, in or near water, and even in human environments. The can swim, hop, climb, burrow, run, and glide. In many ecosystems, rodents serve as keystone species and numerous predators rely on them for survival. Rodents have spread across the globe along with humans. We have used them as research animals, pets, and commercial livestock, and some varieties are considered pests or invasive species. Everywhere you look, there are rodents and few other mammal groups have achieved such evolutionary success. Rodents owe tall of this to the one trait that unites them all, their teeth.

Degu Fact Sheet

Buck Toothed Beauties

The most distinguishing feature of rodents is their teeth, which are unique in the mammal world. Take a look in the mouth of a degu and the first thing one notices is the big buck teeth that defines the group. Most mammals have serval pairs of incisors in each jaw, but rodents have only one. Rodent incisors are coated with thick layers of enamel on the front, but have none on the back. Because their incisors grow continuously throughout their lifetime and are not replaced like in other mammals, rodents must gnaw on objects to keep their teeth healthy. The term rodent is derived from the Greek word rodere, which means “to gnaw.” If not kept trim the incisors can become overgrown, preventing the animal from eating and possibly causing death by curling into the skull to pierce the brain. By constantly wearing down the dentine, the incisors maintain a razor-sharp edge. Coupled with powerful jaw muscles rodents can chew their way into anything, from grass to nuts to insects. Rodent incisors are less prone to breaking due to their high concentration of iron, which strengthens the enamel and causes the teeth to be a distinctive orange color. Rabbits also have continuously-growing incisors, but they belong to a different group, the lagomorphs, and can be distinguished from rodents due to the presence of four upper incisors rather than two.

The skull of O degus (in front)

Other Dental Details

Incisors aren’t the only unique features in a rodent’s mouth. The also lack canines or premolars. A small gap called a diastema separates the front teeth from the back molars. Diastemas are typically associated with herbivory, as the reduced dentition allows the tongue a greater range of motion while manipulated food during chewing. Since carnivores tend to bolt their food down in big chunks or swallow it whole, they tend to have more crowded teeth. Most rodents are indeed herbivores, but unlike ungulates their tongues are not particularly mobile and cannot be protruded past the teeth. The gap allows them to suck in their cheeks to create a seal when gnawing to prevent debris going down their throats. Rodents also have molars with elaborate ridges that permit food to be ground into particularly fine particles before swallowing, thus allowing them to extract as much nutritious goodness from whatever foodstuff is available. The molars of degus are shaped like a figure eight, hence the genus name of Octodon. Different families of rodents of rodents have different arrangements of their jaw musculature, which gives them specialized bites based on their dietary preferences. Degus are known as hystricomorphs, which means they posses an extra opening in the skull below each eye that allows attachment of jaw muscles that allow the lateral jaw movement required to efficiently process fibrous plant material.

The skull of O degus (from belew)