Chicago City Wire

Scientists have long observed that some rodents, such as squirrels, have thumbnails instead of claws on their thumbs. However, a comprehensive survey across the rodent family tree had not been conducted until recently. A new study published in Science examines hundreds of rodent specimens from museum collections to determine which species possess thumbnails and which do not.

Rafaela Missagia, a research associate at the Field Museum and assistant professor at the University of São Paulo, described her initial surprise about this trait: “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.”

The Field Museum in Chicago houses one of the largest mammal collections globally. Rodents account for about 40% of all known mammal species, making them well-represented in the collection. Gordon Shepherd, a neuroscience professor at Northwestern University and co-author of the study, decided to use these resources to identify broader patterns regarding rodent thumb anatomy.

“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,” said Shepherd.

The research team examined preserved skins from 433 out of more than 530 rodent genera. Anderson Feijó, curator of mammals at the Field Museum and another author of the study, explained: “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.”

Their findings revealed that 86% of surveyed genera included species with thumbnails. The team compared these results with data on feeding habits using sources like iNaturalist photos and scientific literature. “We used the app iNaturalist to look at photos of different kinds of rodents eating, as well as textbooks and journal articles,” said 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,” he added.

Rodents lacking thumbs or thumbnails—such as guinea pigs—typically do not handle food with their hands. The researchers’ phylogenetic analysis suggests all modern rodents descend from an ancestor with thumbnails. This adaptation may have contributed to their evolutionary success by providing greater manual dexterity for tasks like opening nuts.

“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,” said 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.”

Missagia noted additional implications beyond feeding behavior: “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,” she said. The study found that arboreal or aboveground rodents were more likely to have thumbnail-like nails while burrowing (fossorial) species tended toward claws.

Apart from rodents, primates—including humans—are unique among mammals in having evolved thumb nails rather than claws; however, this trait appears to have developed independently in both lineages through convergent evolution.

Feijó emphasized the importance of museum collections for discoveries like these: “Museum collections are an endless source of discoveries,” he said. “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.”

The research team included contributors from several institutions: Rafaela Missagia (University of São Paulo/Field Museum), Anderson Feijó (Field Museum/Chinese Academy of Sciences), Lauren Johnson (Field Museum), Maximilian Allen (Illinois Natural History Survey/University of Illinois), Bruce D. Patterson (Field Museum), Paulina Jenkins (Natural History Museum London), and Gordon Shepherd (Northwestern University).