Ancient Rodents With Large Brains Were Not Necessarily Smart

A new study from the University of Toronto (U of T) suggests an animal's brain size is not necessarily a good indicator of how smart it was. In fact, for some ancient rodents, bigger may not have been better.

For their study, researchers reconstructed two endocasts - or internal casts - of Paramys, the oldest and best-preserved rodent skulls on record.

"The brain was certainly larger than we expected considering the time period," Ornella Bertrand, one of the study researchers and a U of T Scarborough Ph.D. candidate, said in a news release.

Paramys is an extinct genus of rodents that once roamed throughout North America, Europe and Asia. It is perhaps even one of the first genera of rodents to live in trees.

"Even more surprising is that it was almost as large, and in some cases larger, than primitive primates of the same time period," Bertrand continued.

However, the key difference is that Paramys' neocortex region - the area involved with "higher" brain functions like sight and hearing - is smaller than most primitive primates.

"This tells us that something is going on in the neocortex of early primates that is not observable in early rodents," Bertrand added. "The changes in the neocortex of rodents occurred later in time and with less intensity than in primates."

One of the ancient rodent samples was about three kilograms, which is about the size of a small cat. This specimen was alive during the mid-Eocene, between 47 and 49 million years ago. An even smaller sample measured in at a mere one kilogram, and was alive between 50 and 52 million years ago.

"It also sheds some light on what's unique about primate brains - they were not always exceptionally large, but they were certainly 'smart'," Associate Professor Mary Silcox said in the university's release. 

Their findings, recently published in the journal Royal Society B, provide a better understand of how early rodents' brains evolved. 

Surprisingly, Bertrand and Silcox also discovered Paramys's brain was larger than those of some later occurring rodents, contradicting the idea that brains grew in size over time.

"It's been assumed for a while that mammal brain size increases over time. The idea is that it's probably an evolutionary arms race because if prey becomes smarter predators have to adapt. But these animals were already pretty smart prey items to begin with," Silcox explained in the release. "Size is certainly important, but we're starting to look at different measures that give us a more nuanced understanding of how brains, especially in primates, evolved over time."

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