Image of the Day: Big Body, Little Brain

An extinct rodent relative of the capybara appears to have had a smaller brain-to-body ratio than similar species.

Amy Schleunes
Amy Schleunes
Feb 20, 2020
Depictions of the skulls and brains (colored portions) of extinct and living caviomorph rodents. Neoeplibema acreensis (top) had a lower estimated brain mass than expected for its body size compared to the four other main lineages of caviomorphs (below).
J.D. Ferreira et al., 2020

The extinct Neoeplibema acreensis, one of the largest rodents to have lived in South America,  weighed up to 180 pounds and had a remarkably small brain, according to a study published on February 12 in Biology Letters

Researchers in Brazil estimated the size of N. acreensis’s brain using CT scans of fossilized skulls and a number of equations for calculating brain mass relative to body mass.

“The first method predicted a brain weighing about 4 ounces, but the volume suggested a dinky 1.7 ounces,” reports The New York Times. “Other calculations, used to compare the expected ratio of the rodent’s brain and body size with the actual fossil, suggested that N. acreensis’s brain was three to five times smaller than one would expect.”

Big brains come with big energetic costs, and because the N. acreensis’s environment lacked active predators, the authors write in their paper, the rodents were able to survive despite their tiny brains.

J.D. Ferreira et al., “Small within the largest: brain size and anatomy of the extinct Neoeplibema acreensis, a giant rodent from the neotropics,” Biology Letters, doi:10.1098/rsbl.2019.0914, 2020.

Amy Schleunes is an intern at The Scientist. Email her at aschleunes@the-scientist.com.