Humans and many rodents share one feature in common that may come as a shock: thumbnails. Most people would think that rodents only have sharp claws, but the majority of these mammals actually have nails on their thumbs that look human in an uncanny way.
A new study published in Science went hands-on to find out how many rodents possess thumbnails and why they developed this trait in the first place. Combing through preserved rodent skins at Chicago’s Field Museum, researchers learned that most rodents possess thumbnails that have been helping them eat one of their favorite foods — nuts — for millions of years.
Nailing Down a Pattern in Rodent Thumbs
The thumbnail and hand of a kangaroo rat, an example of a rodent with thumbnails that uses its hands to manipulate its food.
(Image Credit: Missagia et al)
Throughout the years, scientists have noticed short, flat nails instead of thin, curved claws on the thumbs of many rodents. Take a squirrel, for example. As you watch one while it munches on an acorn, you likely wouldn’t ever notice that the nail on its thumb looks different from the claws on its other fingers. It seems that both scientists and non-scientists alike are taken aback by rodent thumbnails.
“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,” said author Rafaela Missagia, a research associate at the Field Museum and an evolutionary biologist at the University of São Paulo, in a statement.
To discern a larger pattern of this feature in the whole rodent family tree, researchers had their work cut out for them — that’s because rodents make up about 40 percent of all known mammal species. Luckily, the abundance of rodent specimens at the Field Museum came in handy.
Read More: Why Are We So Afraid of Mice and Other Rodents?
The Perfect Tool for Eating
The researchers looked through the preserved skins of rodents to find answers, focusing on their thumbs.
“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,” said author Anderson Feijó, the Field Museum’s curator of mammals.
From the genus groups they examined, they determined that 86 percent of them included species with thumbnails. They then compared this information with data on rodent feeding habits from various sources, including photos from the iNaturalist app, textbooks, and journal articles.
The researchers went on to reconstruct the rodent family tree based on rodents that handled food with their hands versus ones that only used their mouth. They found that rodents with no thumbs at all don’t usually handle food with their hands.
Rodents with thumbnails, meanwhile, have a long history that goes way back to prehistoric times. The researchers say that a common ancestor gave many modern rodents their thumbnails, and that there’s good reason for this feature to have survived all these years. They hypothesize that slim, flat thumbnails are better than long, sharp claws for handling and eating a nut.
“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 Feijó.
A Link Between Primates and Rodents
Thumbnails aren’t just biological cutlery for rodents, though. Since primates also have thumbnails and tend to dwell in trees, the researchers wanted to see if there was some correlation with thumbnails and habitats.
They ended up finding that rodents with nails were more likely to live aboveground or in trees, while rodents that dig underground were more likely to have claws and no thumbnails. This means that thumbnails may play a larger role for rodents beyond eating.
Rodents and primates are the only mammals that have evolved nails on their thumbs rather then claws, although the two lineages gained this feature independently of each other. Regardless, next time you see a squirrel, give one a thumbs-up in solidarity!
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