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Rodents and primates split off from each other tens of millions of years ago, but they still have a lot in common: Both animal lineages belong to the superorder of placental mammals known as Euarchontoglires, which give live birth, have big brains relative to body size, and develop complex social structures and relationships. But one of the strangest things that unites them—and separates them from most other animals—comes down to the thumbnail.
Rodent thumbnails are not something most of us think about, or maybe even know about. Even Rafaela Missagia, an assistant professor at the University of São Paulo in Brazil and research associate at the Field Museum who studies the evolution of predatory rodents, admitted that she didn’t know anything about these tiny appendages until fairly recently.
Missagia is co-author of a new study published in Science that tracks the evolution of thumbnails across the entire order of Rodentia, which includes everything from capybaras to house mice. Thumbnails, she finds, are short and do not cover the tip of the digit, “potentially enhancing tactile sensitivity and manual dexterity.” Claws, on the other hand, are long, sharp, and narrow. So why do some rodents have thumbnails, while others evolved to live without?
Did these special appendages help rodents to spread across the globe?
To try to answer this question, Missagia and her colleagues analyzed preserved skins of rodents found in the Field Museum’s collections across 433 genuses in the rodent family tree. They discovered that 86 percent of the genera indeed included species with thumbnails. The team then analyzed iNaturalist photos of the species digging into their food to test their hypothesis that thumbnail evolution had something to do with the way they ate. Their thinking was this: It’s a lot easier to snack on an acorn or other high-energy nut snack if you can grip it with your thumb than if you have to rely on a claw or just a mouth.
What they discovered is that thumbnails in rodents do correlate with lifestyles and patterns of snacking. Rodents that live in trees and eat nuts generally have thumbnails, whereas rodents that spend more time on or under the ground digging for their food have claws.
“Nuts are a very high-energy resource, but opening and eating them requires good manual dexterity that a lot of other animals don’t have—maybe rodents’ thumbnails allowed them to exploit this unique resource and then diversify broadly, because they were not competing with other animals for this food,” said Anderson Feijó, assistant curator of mammals at the Field Museum and a study author, in a press release.
Thumbs and thumbnails aren’t new, evolutionarily speaking, for rodents. The authors suggest that a common ancestor of all modern rodents, dating back around 50 million years ago, had both. That means that somewhere along the line, some ground-bound rodents like guinea pigs and common mole rats lost these thumbnails and developed claws instead.
Primate thumbnails evolved separately, after primates and rodents diverged, an example of so-called convergent evolution. Did these special appendages help rodents to spread across the globe? The researchers think so.
“Rodents make up almost half of the mammal species on Earth, and they’re found on every continent except Antarctica,” Feijó says. “Their thumbnails might help explain why rodents became so successful.” 
Lead image: Konstantin Aksenov / Shutterstock
