by Don Riefler

Anyone who has seen The Princess Bride probably remembers the scene where Westley and Princess Buttercup have to cross the Fire Swamp, home to the dreaded R.O.U.S. (Rodents Of Unusual Size). In the movie, the R.O.U.S. is as big as a person! Most of the rodents we think of on a day-to-day basis are pretty tiny, though. Think: mice, rats, squirrels, and hamsters. Those aren’t the only animals in the order Rodentia, however, and some of them are pretty big!

Full disclosure: My wife is a veterinarian, and she works with exotic animals and has an especially soft spot for rodents. We even had pet rats for a while, but I’m super allergic. Thanks to my wife I know more than the average person about rats. I know, for example, about the Gambian pouched rat. These dudes are pretty huge. They can be up to 3 feet long and weigh more than 3 pounds! 

But we can’t limit ourselves to familiar rodents. Sure, the Gambian pouched rat is a giant rat, but it’s far from the biggest rodent out there. Heck, half of its length is its tail. It’s not so much bulky as it is long. To really find an R.O.U.S., we need to get a little more exotic. 

If you're looking for a bulky rodent, you want to find a beaver. Many people don’t know that a beaver is a rodent but look at its teeth. They have big, flat incisors for gnawing, just like a rat or a squirrel. In fact, the word “rodent” comes from a Latin word meaning “to gnaw.” Beavers use their teeth to gnaw down trees to build their dams, and a big one can be more than 4 feet long and weight almost 60 pounds. They’re the largest rodents in North America.

If we leave our continent, we get even crazier types of R.O.U.S. Porcupines are also rodents, and the crested porcupine, which lives in Italy and Africa, can also get up to 60 pounds, and it packs that mass into a much smaller frame than a beaver.

The hands-down winner for the title of real R.O.U.S. is the capybara. It’s native to a large portion of South America, and it’s the largest rodent in the world. An adult can be almost 4.5 feet long, 2 feet tall and weight nearly 150 pounds! They’re related to guinea pigs, but you wouldn’t want to have one in a cage in your room.

Before I finish, though, I want to travel back in time to the Pleistocene, the last major ice age. During that epoch of prehistory the world was full of megafauna, giant animals like mastodons, wooly mammoths, and the giant ground sloth. As you might expect, there were also megarodents, the real Rodents of Unusual Size. Giant beavers, 6 feet tall, roamed North America, while, in South America, a rodent related to (of course) the capybara caused minor earthquakes as it moved its 10-foot, 2,000-pound body around!

Looking for more Never Stop Asking "Why?" questions? Catch up on all of the past "Why's" on the blog!