Rodents Conquered the World With the Help of Their Thumbnails, Study Suggests

The trait might have given rodents greater manual dexterity, allowing them to access new foods, such as nuts

September 10, 2025

rodent hand with claws and a thumbnail — Many rodents have nails on their thumbs and claws on the rest of their fingers. A new study suggests this trait might have been key to their world domination. Missagia et al.

Rodents are the largest group of mammals on Earth. They exist—and often thrive—on every continent except Antarctica. Now, new research is shedding light on an underappreciated rodent feature that might have helped them achieve this impressive spread. Here’s a hint: It’s something we also have on our hands.

In a study published last week in the journal Science, researchers suggest the evolution of thumbnails might have helped some rodents handle their food with more dexterity, giving them access to crucial resources such as nuts. Thumbnails could have also supported rodents as they spread to new environments by climbing.

The finding challenges previous assumptions that rodent thumbs are “an evolutionary leftover” that now serves no purpose, per a statement from London’s Natural History Museum.

“This is a nice way to rethink what we consider a human characteristic,” study co-author Anderson Feijó, a mammalogist at Chicago’s Field Museum, tells the Washington Post’s Vivian Ho. “We are showing with this paper that rodents actually have very good handling behavior, and we believe that their nail and thumb has played a very critical role in this.”

If the thumbnail seems like a strangely human trait, that might be because primates and rodents are the only mammals that have it—while others might have a claw or no thumb at all. However, the two groups seem to have developed the feature independently, as opposed to inheriting it from a common ancestor.

Fun fact: Rodents’ world domination

Rodents are the largest group of mammals, making up roughly 40 percent of all known mammal species.

Feijó and his colleagues traced thumbnails (and the lack of them) across hundreds of rodent specimens in museum collections. Specifically, they examined 433 rodent genus groups out of the more than 530 known to exist. Eighty-six percent of the groups they investigated contained species with thumbnails.

The team put this data together into a rodent family tree that mapped which species manipulate food with their hands and which are limited to using their mouths. To inform this diagram, they also investigated journal articles, textbooks and photos from the citizen science app iNaturalist. This approach revealed that thumbless (and, thus, thumbnail-less) rodents, such as guinea pigs and capybaras, usually don’t use their hands to manipulate food.

Their family tree also suggests the common ancestor of all modern rodents had thumbnails.

“Some of the earliest known rodents, dating to about 50 million years ago, have thumb bones that are short and wide—a shape more consistent with bearing nails rather than claws, since nails tend to be broader while claws are typically narrow and curved,” says Rafaela Missagia, an evolutionary biologist at the University of São Paulo in Brazil and lead author of the study, to Popular Science’s Laura Baisas.

The development of thumbnails could have played a role in rodents’ world dominance by enabling some of them to handle and eat food—namely, nuts—more nimbly than if they had a fully clawed hand.

“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,” Feijó explains in a Field Museum statement. “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.”

An Eastern Gray Squirrel handling acorns — An eastern gray squirrel handles acorns. Dkwikiedt via Wikimedia Commons under CC0 1.0

What’s more, the combination of both nails and claws indicates that these rodents can use their hands in two different ways, Feijó tells Science News’ Erin Garcia de Jesús. With their thumbs, they can maintain a good hold without the claws interfering, which instead come in handy when it’s time to find food, dig and grasp objects, the mammalogist adds. The team found that rodents with thumbnails were more likely to live in trees.

Rodents “have such fantastic traits that are sometimes similar to humans,” David Thybert, a computational biologist at the University of East Anglia in England who did not participate in the study, tells the Washington Post. The traits are occasionally “almost extraterrestrial.”

Moving forward, Feijó wants to use high-speed cameras to continue investigating whether short thumbnails really do lend rodents greater manual dexterity, per Science News.

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