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Picture a squirrel eating an acorn. It's holding the nut in its front paws. More specifically, squirrels handle their food with their thumbs. And instead of the thin, curved claws on the rest of the squirrel's fingers, their thumbs have smooth, flat nails.

For hundreds of years, scientists have noticed that some rodents, including squirrels, have thumbnails instead of claws. But no one went through the whole rodent family tree and documented which ones have nails and which ones have claws.

In a paper published in Science, researchers examined hundreds of rodents in museum collections to see where thumbnails crop up. This study could help explain how rodents evolved into so many different species that thrive all over the world.

"When I talk with people about this research, I always start by asking, 'Did you know rodents have thumbnails?' Most people don't. I didn't. I had studied rodents for years, and I didn't know anything about their nails until I started working on this project at the Field Museum," says Field Museum's research associate, Rafaela Missagia, an assistant professor at the University of São Paulo in Brazil and an author of the study.

The Field Museum in Chicago is home to one of the world's largest collections of mammals, and as rodents make up about 40% of all known mammal species, there are lots and lots of rodent specimens at the Field. Gordon Shepherd, a professor of neuroscience at Northwestern University, heard about how some rodents have thumbnails and some have claws, and he decided to use the Field Museum's collections to find a bigger pattern.

"Before we did the research, we knew that some had nails, some had claws, and some had no thumbs at all. "There were hints that the rodents that have thumbnails also use their thumbs to hold their food," says Shepherd, one of the paper's authors.

The research team set about examining the preserved skins of rodents in the Field's collections, looking at the animals' thumbs (or lack thereof).

"There are more than 530 different genera of rodents, containing over 2,500 species. We looked at 433 of those genus groups from all across the rodent family tree," says Anderson Feijó, the Field Museum's curator of mammals and an author of the study.

The team found that of the genera they surveyed, 86% of them included species with thumbnails. They compared this information with data about the different rodents' feeding habits.

"We used the app iNaturalist to look at photos of different kinds of rodents eating, as well as textbooks and journal articles," says Shepherd. "Using that information, we reconstructed the rodent family tree in terms of rodents that handle food with their hands versus ones that only use their mouths," says Shepherd.

The researchers found that rodents like guinea pigs that don't have thumbs, let alone thumbnails, generally don't handle food with their hands. The family tree that the researchers assembled also indicated that rodents with thumbnails go way back.

The data suggest that all modern rodents descend from a common ancestor that had thumbnails too. This early rodent ancestor's thumbnails may help explain how rodents took over the world.

The researchers hypothesize that slim, flat thumbnails allow for more manual dexterity than long, sharp claws— thumbnails make it easier to handle and eat a nut.

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"Rodents make up almost half of the mammal species on Earth, and they're found on every continent except Antarctica. Their thumbnails might help explain why rodents became so successful," says Anderson Feijó.

"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."

Animals' nails or claws tell us about more than just how they handle their food, notes Missagia. "When I got involved with this project looking at rodents' nails or claws, I immediately thought about their life modes—where they live, how they use their hands in ways beyond just eating," says Missagia.

"I knew that primates, which mostly have nails, are usually arboreal, they live in trees. We tested that correlation as well, and we found that rodents with nails were also likelier to live aboveground or in trees, while fossorial rodents, the ones that dig, were more likely to have claws and not nails on their thumbs."

Aside from rodents, primates like humans are the only mammals that have evolved nails on their thumbs as opposed to claws. However, the two lineages appear to have evolved this feature on their own, in a process called convergent evolution.

This study, in addition to helping explain the evolution of rodents into the thousands of species that exist today, also underscores the value of museum collections.

"Museum collections are an endless source of discoveries," says Anderson Feijó. "For all of the rodents that were used in this study, I bet none of the collectors would have imagined that someone someday would be studying those rodents' thumbnails."