Article: What makes a rodent a rodent?

Tequila, the pet mouse, peeks out of a "log"

I take my pet capybara Caplin everywhere. She goes out to lunch, to various flooring stores looking for new vinyl for the laundry room, to get new tires on my truck, to buy paint brushes. It’s surprising how many places you can take a capybara, maybe because she’s so unusual and people just don’t know what to think.

Wherever we go, Caplin gets a lot of attention. People want to know what she is. Of course, “capybara” doesn’t mean much so I have to further explain that capybaras are the world’s largest rodent. “Oh,” many people respond, “a giant rat.” I don’t have anything against rats, they’re smart, cute and they make great pets, but Caplin is not a rat. Nevertheless this common reaction led me to wonder exactly what it is that makes a rodent a rodent.

Over one quarter of all mammal species are rodents and several rodent species are commonly kept as pets. A hamster or a gerbil is often a child’s introduction to the responsibilities of pet ownership. When I was growing up we kept guinea pigs. Other rodents are prairie dogs, nutria, groundhogs, squirrels, marmots, gophers, beavers, lemmings, chinchillas, chipmunks and porcupines.

Most rodents are very social and very vocal, although much of their vocalization is outside the range of human hearing. The prairie dog is thought to have the most sophisticated animal language known. Naked mole rats, such as the one depicted in the cartoon “Kim Possible,” are the only eusocial mammal. Like ants and termites, naked mole rats are born into “castes” for which they develop unique physical traits. There is a colony of naked mole rats at the Houston Zoo and it is well worth a visit.

The main identifying characteristic of rodents is their teeth. Rodents have four very large teeth at the front of their mouths, two on the bottom and two on the top. These incisors grow throughout their lives and rodents must gnaw on things to wear them down. The teeth retain their edge due to a natural sharpening process. Thick enamel on the front but not on the back results in a wear pattern that constantly sharpens the teeth. Note that although rabbits have similar looking teeth, they are not rodents and belong to the Lagomorpha.

Another characteristic of rodents is that they are able to digest cellulose, the tough polysaccharide that makes up plant cell walls. They do this through a symbiotic relationship with bacteria. Rodents have a specialized adaptation of the large intestine called the caecum where cellulose digestion takes place. Because cellulose breakdown occurs in the large intestine while absorption takes place in the stomach, rodents first eliminate the partially digested plant material in the form of pellets. The rodents then practice coprophagy which entails eating the passed pellets and returning them to the stomach for further digestion.

Most rodents are herbivores although rats and mice are omnivorous and a few species are specialized carnivores. The large incisors can be used to crack open seeds, cut tough plant stalks (in the case of beavers this even includes trees) and gain access to well-hidden human food stores.

Rodents association with food supplies and their ability to carry human disease has probably led to much of their negative image. It is common knowledge that the “Black Death,” the great bubonic plague epidemic that led to the Industrial Revolution, was spread by fleas that spent part of their lives on rats. Recently hanta virus has given the adorable prairie dog a bad name. On the bright side, rodents neither get nor carry the rabies virus.

I’m not sure how any of this information is going to help me explain Caplin to people who meet her. I’m sure they’re not going to want to know about the disease or the coprophagy aspects of rodent life. Maybe I’ll just mention she’s related to beavers. Or chipmunks, who can resist those little cuties?

Photo: Tequila's babies at the "hopper" stage.Photo: Caplin the capybara demonstrating the distinctive rodent teeth.
Photo: Prairie dogs, like this one from Big Spring, TX, have a complex social system and a language to match.
Photo: This groundhog lived in a burrow in a cemetery in Maine.
Photo: Chipmunks, or ground squirrels, are brightly colored and cute. This one lives in Yosemite National Park in California.
Photo: Squirrels, like most rodents, are herbivores. This one is eating a pear from a tree in my backyard.
Photo: There are more species of rodents than any other mammals. Notice this squirrel from Yosemite National Park looks quite different from the one from Texas.