Fossils of Human-Sized Rat With Tiny Brain Discovered in South America

Paleontologists have discovered the well-preserved skulls of two mega-rats the size of humans. No, not riding the subway in New York but living 10 million years ago in what is now western Brazil. While today’s rats continue to prove to be surprisingly intelligent, these man-sized rodents appear to have had extremely tiny brains. We’ll pause here for a moment so you can make you own politician jokes.

Finished?

“Neoepiblema acreensis is one of the largest rodents that inhabited South America. N. acreensis belonged to a diverse group of rodents known as caviomorphs that derived from African forms that rafted to the continent around 50 million years ago. Today’s caviomorphs include the capybaras, porcupines, guinea pigs and chinchillas, among several others.”

José D. Ferreira writes about the discovery of the two skull fossils in the journal Biology Letters and summarizes it in a recent interview with the British news agency SWNS. He helped identify two large skulls with giant incisors as belonging to a rare species of giant rats that outweighed modern 8-ounce brown rats by 179.5 pounds.

One of the well-preserved skulls yielded another shocking find – the brain of this rodent monstrosity weighed only 4 ounces (and possibly as little as 1.7 ounces), giving it a "very low encephalization quotient compared to other rodents." The encephalization quotient is the ratio between the expected size of an animal and the actual size of its brain. For comparison, humans have an average EQ of approximately 6, while most rodents in South America have an EQ of 1.05. If you haven’t yet done the math, N. acreensis had an EQ of 0.3, making it big and (fill in your politically-correct word for ‘stupid’).

"When Neoepiblema inhabited South America, carnivorous placental mammals such as felids, canids and ursids had not yet arrived on the continent, since the Isthmus of Panama was not yet formed and there was no terrestrial connection with Antarctica."

N. acreensis didn’t need a big brain because it had no real enemies (a lesson for modern times?) like big cats, wolves, bears or humans. Its only predators were crocodiles and it could easily react to and outrun them without too much thinking. That all changed when the Isthmus of Panama formed around 2.8 million years ago and the beasts from up north came south in the Great American Biotic Interchange. Like American tourists today, the primary thing they did was eat, and the primary things they ate were the meatiest, tastiest and easiest to catch creatures – like giant rats. As a result, N. acreensis quickly became extinct before it could evolve a larger brain to outwit, outplay and outlast the competition.

But wait a minute, you say. What about the capybara? Yes, everyone’s favorite modern giant rodent (Hydrochoerus hydrochaeris) lives in South America and weighs in at up to 150 pounds. Fortunately for them, they have larger brains that allow them to swim and live in groups in order to survive its modern enemies – a list that includes the deadliest of all … humans. A better example is the Cuban hutia (Capromys pilorides), which is smaller (up to 19 pounds) but has a similar tiny brain and a similar tiny list of predators in the isolated area where it lives. The Cuban hutia is distinct from the giant hutia, an extinct bear-sized rodent of the West Indies that weighed up to 440 pounds (200 kg) and was eaten out of existence by humans.

Big body, tiny brain. Did we really need the discovery of Neoepiblema acreensis to know this is a bad combination? Stay in school, kids, and eat your vegetables!