A South American rodent had the heft of a Saint Bernard dog — and a brain the weight of a golf ball.

Behold Neoepiblema acreensis, an 80-kilogram rodent related to chinchillas that lived 10 million years ago in what is now Brazil. José Ferreira and Leonardo Kerber at the Federal University of Santa Maria in Brazil and their colleagues were curious about the brains of these beasts, so they used computerized tomography to peer inside two fossil skulls.

To compare brain sizes between creatures of varying weights, scientists can calculate a species’ ‘encephalization quotient’, a measurement of the difference between the expected brain size and actual brain size for an animal of a certain weight. Any value under 1 means an animal’s brain is smaller than expected.

The team estimates that the brain of N. acreensis weighed just 47 grams. The encephalization quotient of one individual studied was 0.20; that of the other individual was 0.33. In other words, N. acreensis’ brain was unusually puny in comparison to its body. By contrast, modern South American rodents have an average encephalization quotient above 1.05.

The researchers suggest that because N. acreensis had few predators to outwit, a large brain simply wasn’t worth the maintenance costs.