How thumbnails helped rodents to spread all over the world

The humble thumbnail might have contributed to the rise of rodents.

On their evolutionary journey to becoming the largest group of mammals on Earth, rodents have adapted to a variety of lifestyles. From burrowing mole rats to well-defended porcupines, these animals have become ubiquitous across the world.

While there are many different reasons for this, new research suggests that the role of their thumbs, and their nails in particular, has been overlooked. Though thumbnails might seem common enough to us, primates and rodents are the only two groups of mammals that have them.

Together, the evolution of thumbnails and claws might have helped rodents to eat different foods and enter new environments, from climbing trees to digging underground. This adaptability has allowed them to spread around the world, reaching every continent but Antarctica.

Paula Jenkins, our Senior Curator of Mammals, was a co-author on the paper. She says that it was “quite a surprise” to find out how important thumbs were to rodents.

“Everyone thinks that thumbs in rodents, which are really tiny in most cases, were an evolutionary leftover that no longer had a role,” explains Paula. “However, we’ve shown that they’re an important part of the behaviour and ecology of the vast majority of rodents.”

“This offers new avenues to understand the evolution of rodents and their lifestyle. Investigating other overlooked parts of their anatomy could also reveal new insights into these mammals.”

The findings of the study were published in the journal Science.

Rodents – Earth’s most successful mammal group

There are over 2,500 species of rodent, representing over a third of all mammals. While you might imagine the typical rodent looks like a mouse or a rat, they can have many different forms – meaning squirrels, woodchucks and beavers are all rodents too.

On the other hand, some animals that initially seem rodent-like aren’t actually part of this group at all. Raccoons and rabbits are not rodents, for example, and are instead on different branches of the tree of life.

The key features which set rodents apart from other mammals are found in their mouth. They have a single pair of incisor teeth in the upper and lower jaws, no canines and a broad space between the incisors and cheek teeth. This specialisation allows their incisors to gnaw through the tough parts of foods, such as the outer husks of nuts, while the molars grind down on the edible contents.

But while there has been plenty of research on the rodent skull, less has focused on other parts of their body. The thumb has been particularly understudied because it’s generally much smaller than the fingers, being the digit that is normally last to appear and first to be lost as mammals evolve.

However, research on other mammals show that the thumb can have an important role to play. It helps primates grasp objects and surfaces, while animals such as stoats use their thumbs to help move around.

In rodents, the specialisation tends to take place in the unguis – the keratinous structure found at its end. It’s normally either a thumbnail, a claw or missing entirely. While a nail is flat and has a curved edge, a claw is generally hook-shaped and has a pointed end.

To study these features in more detail, the team made use of rodent specimens from multiple collections, including ours at the Natural History Museum. Their investigations revealed how these key parts of the thumb have evolved across the rodent family tree for the first time.

“Museum specimens were a crucial part of this study, allowing us to investigate hundreds of different rodent genera,” Paula adds. “The mammal collections I help to look after at the Natural History Museum are particularly useful as they contain historically important specimens that can help to uncover otherwise hidden evolutionary relationships.”

“By combining details of these specimens with photos and videos of living rodents, this research was able to get a much better idea of how these animals use their hands for feeding.”

What do rodents use their thumbs for?

The study revealed that the vast majority of rodents, more than 85% of the sample, had thumbnails. It suggests that the common ancestor of all rodents, which lived over 30 million years ago, probably had thumbnails as well.

Dr Anderson Feijó, a co-author of the study and Curator of Mammals at the Field Museum, says that the rodent thumbnail might have helped them to hold and eat certain foods, such as nuts, more easily than other animals could.

“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,” Anderson explains. “Rodents’ thumbnails maybe allowed them to exploit this unique resource and then diversify broadly, because they were not competing with other animals for this food.”

Just as thumbnails helped primates to climb trees, they’ve also helped rodents such as squirrels and the distantly related flying squirrels do the same. Dr Rafaela Missagia, the lead author of the study, says that the unguis reveals more about how rodents live beyond the way they eat.

“I knew that primates, which mostly have nails, usually live in trees,” explains Rafaela. “We tested that correlation as well, and we found that rodents with nails also were likelier to live above ground or in trees, while fossorial rodents, the ones that dig, were more likely to have claws on their thumbs.”

The team found that claws have evolved from nails many different times by convergent evolution. It’s likely that this is a multi-step process, with some rodents such as the naked mole rat having an intermediate structure that’s not quite a claw or a thumbnail.

Other rodents, like the guinea pig and capybara, have lost their thumbs entirely. This is probably because they are grass-eating animals which handle food with their mouths so the thumb isn’t as essential as it is in other rodents.

While the research has focused on living rodents so far, comparisons with an extinct group of rodent-like mammals known as the multituberculates suggest that these animals could have handled food in a similar way.

Finding preserved hand bones from these animals would help scientists decide whether to give the thumbs up to this idea.