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This Rodent Was Giant. Its Brain Was Tiny.

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Science|This Rodent Was Giant. Its Brain Was Tiny.


Reconstruction of a fossil of an extinct South American relative of the capybara shows it didn’t have much space in its skull for a brain.

Credit…Márcio L. Castro

When you look at a reconstruction of the skull and brain of Neoepiblema acreensis, an extinct rodent, it’s hard to shake the feeling that something’s not quite right.

Huddled at the back of the cavernous skull, the brain of the South American giant rodent looks really, really small. By some estimates, it was around three to five times smaller than scientists would expect from the animal’s estimated body weight of about 180 pounds, and from comparisons to modern rodents. In fact, 10 million years ago the animal may have been running around with a brain weighing half as much as a mandarin orange, according to a paper published Wednesday in Biology Letters.

The glory days of rodents, in terms of the animals’ size, were quite a long time ago, said Leonardo Kerber, a paleontologist at Universidade Federal de Santa Maria in Brazil and an author of the new study. Today rodents are generally dainty, with the exception of larger creatures like the capybara that can weigh as much as 150 pounds. But when it comes to relative brain size, N. acreensis, represented in this study by a fossil skull unearthed in the 1990s in the Brazilian Amazon, seems to be an extreme.

The researchers used an equation that relates the body and brain weight of modern South American rodents to get a ballpark estimate for N. acreensis, then compared that with the brain weight implied by the volume of the cavity in the skull. The first method predicted a brain weighing about 4 ounces, but the volume suggested a dinky 1.7 ounces. Other calculations, used to compare the expected ratio of the rodent’s brain and body size with the actual fossil, suggested that N. acreensis’ brain was three to five times smaller than one would expect.

There is some uncertainty about whether the process of fossilization might have compressed the brain cavity, Dr. Kerber cautioned. But making reasonable estimates about that effect did not significantly affect the overall picture: This brain was small.

Big brains are useful but can take a lot of calories to maintain, so it makes sense to have one no larger than needed. Perhaps one reason N. acreensis had such a small brain is that it lived relatively isolated in South America before the rise of swift, stalking hunters like felines, Dr. Kerber said. Its primary predators may have been giant, crocodile-like animals that preferred to lie in wait. That particular environment may have resulted in the ancient rodents evolving a smaller brain — easier to maintain but just big enough to help avoid such sedentary predators.

Looking at the range of brain and body sizes in other rodents, the researchers noticed one other outlier, a creature that is still alive today: the Cuban hutia, which weighs no more than about 19 pounds. Its brain is similarly diminutive, which fits with the idea that brain size may be related to pressure from predation. Like N. acreensis, the island-dwelling hutia may not have had contact with large, aggressive predators as it evolved.

“This is the hypothesis, that there is some ecological influence to maintain this small brain,” Dr. Kerber said. As the researchers analyze the brain sizes of more rodents, particularly giant varieties, they are curious to see whether this pattern holds.

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