According to Grandpa, any wild mammal that invades your home or garden is a rodent, often lovingly referred to as a "varmint." This turns out to be not quite so. We know that mice, rats, and hamsters are rodents, but other animals that resemble mice or rats may or may not belong to the order Rodentia.
Image by Flickr user HocusFocusClick.
Moles are not rodents. Most moles belong to the order Soricomorpha. They eat insects and earthworms. However, there are several species called mole rats which are blind rodents of the genus Spalax. Mole rats were so named because they live underground like moles, but are not related to them.
Image by Flickr user ice.bluess.
A porcupine is a rodent. There are a couple of dozen species of porcupine belonging to two families. What they have in common are the quills they use to hide or defend themselves. Folklore says that they can shoot these quills at will, but that isn't so. However, the quills do detach from the porcupine easily, so a dog that messes with a "quill pig" may come home with its face embedded. Porcupines are herbivores like other rodents.
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A hedgehog is not a rodent. Their order is Erinaceomorpha, which means they are not related to porcupines at all! Hedgehogs share more of their lineage with shrews. They do, however, sport hair that is modified into quills for defense. Unlike porcupines, hedgehogs will eat anything, including insects, frogs, snails, eggs, roadkill, and plants.
Image by Flickr use Sharon Mollerus.
Squirrels are rodents belonging to the family Sciuridae. This is a large family of species that include chipmunks, prairie dogs, groundhogs (also called woodchucks), marmots, and flying squirrels. Which takes care of many rodents you might wonder about. *Note: Many people confuse flying squirrels with sugar gliders. While flying squirrels are rodents, sugar gliders are marsupials.
Image by Flickr user Michael Scheltgen.
The raccoon is not a rodent. It belongs to the Carnivora order, meaning it's a meat-eater, although a raccoon will eat anything available, including whatever is growing in your garden or sitting in your garbage can. Raccoons are members of the Procyonid family, which also includes ringtail cats, coatis of South America, and kinkajous of Central and South America. Taxonomists once placed the animal in the same genus as bears (Ursus), but later gave raccoons their own genus (Procyon). Raccoons are native to North America, but due to human intervention (both deliberate and accidental), they can now be found roaming Europe and Japan.
Image by Flickr user Tim Sträter.
The capybara is a rodent, despite being the size of a St. Bernard. The original R.O.U.S. is native to South America. The capybara is the only animal of the Hydrochaeridae family, of the suborder Hystricomorpha, which also includes nutrias, chinchillas, porcupines, and guinea pigs.
Image by Flickr user Zeke Franco.
Opossums are not rodents. They are an order unto themselves, part of the "infraclass" known as marsupials. The label distinguishes marsupials from placental mammals, such as rodents. Like Australian marsupials (kangaroo, koala, possum, and wombat), the opossums of North and South America are born quite undeveloped, and must live in a pouch made of the mother's skin and nurse until it is able to produce enough heat to survive. And get this: Australia is known for its marsupials because they flourished there, but those species are descended from South American marsupials.
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A beaver is a rodent, belonging to the genus Castor. It is the largest rodent in North America, and can weigh up to 60 pounds! Beavers have specialized guts that can digest tree bark, and can build a nest better than even the most clever pack rat.
Image by Flickr user Kahunapule Michael Johnson.
Bats are not rodents. The flying mammals have their own order called Chiroptera. They were once classified in the superorder Archonta which also includes primates, but they are now in the superorder Laurasiatheria along with whales and hoofed animals, but not rodents.
Image by Flickr user Snork Maiden.
Rabbits are not rodents, despite what you may have been taught in school. They look like rodents; however their different teeth puts them in the Lagomorpha order. Rabbits were classified as rodents until 1912. It took decades for textbooks to catch up with the evolving taxonomy, and even longer for lagomorphs to be divorced from rodents in popular culture.
You can see that it is hard to tell the players without a program. Like other taxonomy classifications, rodent species are a diverse lot that don't particularly look alike. And many species that resemble some rodents belong to other orders. Also, classifications do change as scientist uncover more information. If you aren't sure how to classify an animal, it's better to look it up than to guess.
PS: Now that you've read this, take the quiz based on this post at Buzzfeed.