The thumbnail and hand of a kangaroo rat, an example of a rodent with thumbnails that uses its hands to manipulate its food. Credit: Missagia et al 2025
A deep dive into the world of rodent thumbs could help explain why the creatures spread and thrived all over the world.
The research examined hundreds of rodents in museum collections to track the evolution of their thumbnails. Many rodents have evolved smooth, flat nails rather than curved claws on their thumbs. But until now, no one had documented this trend across the evolutionary tree.
“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,” says study co-author Rafaela Missagia, an assistant professor at the University of São Paulo, Brazil.
“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.”
Rodents are an order of mammals characterised by a pair of continuously growing front teeth which they use to gnaw on their food. Rodents include animals like rats, mice and guinea pigs.
“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 co-author Anderson Feijó, curator of mammals at the Field Museum of Natural History in the US.
The research team studied the preserved skins of rodents across the museum’s collections.
“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 co-author Gordon Shepherd, a professor of neuroscience at Northwestern University in the US.
They found that 86% of the genera had thumbnails, with claws arising independently multiple times across evolutionary history. This suggests all modern rodents descend from a common ancestor that had thumbnails.
They then compared this data to information on how the different rodents went about eating and collecting their food.
(Left to right) Gordon Shepherd, Anderson Feijó, Lauren Johnson and Rafaela Missagia working in the Field Museum’s mammal collections. Credit: Field Museum
“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.”
For example, guinea pigs, which do not have thumbnails, don’t usually eat food with their hands. However, rodents that have thumbnails, like squirrels, often use their hands to nibble away on nuts and fruits.
The thumbnail-having rodent ancestor may help explain why modern rodents can be found on every continent except for Antarctica.
“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,” says Feijó.
“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.”
Primates like humans are the only mammals other than rodents to have evolved nails rather than claws on their thumbs.
“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 also were more likely to live above ground or in trees, while fossorial rodents, the ones that dig, were more likely to have claws and not nails on their thumbs.”
Thumbnails likely developed in primates independently from rodents in a process called convergent evolution.
The study still highlights how even something as small as a thumbnail can provide insightful windows into evolutionary history.
“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,” says Feijó.
“Museum collections are an endless source of discoveries.”
The results from this study have been published in the journal Science.