Thumbnails may give rodents the dexterity needed to crack open seeds and nuts. Here a southern giant slender-tailed cloud rat (Phloeomys cumingi), a thumbnail-bearing rodent native to the Philippines, holds food with its hands.
Forest Botial-Jarvis
Rodents may have a rule of thumbnails.
Many rodents have curved claws that help the animals climb or dig. But most also have flat nails on their thumbs, researchers report in the Sept. 4 Science. This feature may help the critters deftly break open nuts and seeds, a skill that set rodents up to take over the world.
Rodents make up nearly half of mammals on Earth, with myriad squirrels, mice, rats and more scurrying around the world. Studies have shown that having big teeth and strong chewing muscles to break open hard foods such as nuts — a food source that few ancient mammals could access, meaning less competition for early rodents — was key to the group’s evolutionary success.
Yet the new findings suggest that bite and brawn aren’t the only things that have helped rodents flourish around the world, says Rafaela Missagia, an evolutionary biologist at the University of São Paulo. “It’s also the ability to hold and manipulate food efficiently.”
Missagia and colleagues scrutinized the thumbs of museum specimens from more than 425 genera in the rodent family tree, with at least one species from each genus. The team also scoured sources such as textbooks and videos to find information on rodent feeding habits and where they live.
Rodents can have a nail, a claw or nothing at all on their first digit, the team found.
Nearly 90 percent of genera have species with thumbnails, and analyses of the rodent family tree suggest that the trait goes back millions of years. Species that spend time underground, such as gophers, tend to have claws on every digit; critters that don’t eat with their hands, like guinea pigs, typically don’t have thumbs, or nails, at all.
Having claw and nail combos suggests that rodent hands can serve two functions, says study coauthor Anderson Feijó, an evolutionary biologist at the Field Museum of Natural History in Chicago. Rodents might “use the thumb to have a very good grasp and not have the claws get in the way. But sometimes the claw is useful to dig, to grasp, to find food.”
Feijó plans to film rodents using their hands with high-speed cameras to test whether short thumbnails do give the animals more dexterity. Missagia also wants to look for differences in the bones that support the nails and claws.
“If they indeed differ,” she says, “we can infer the type of [nails or claws] for species that are extinct.”
