How rodent thumbnails helped mammals rise to dominance

Many rodents do not have a claw on every finger. Quite a few possess a flat nail on the first digit – a small thumbnail that changes how their hands work.

A new study links that tiny feature to how rodents feed, where they live, and how they diversified into thousands of species worldwide.

The research draws on museum collections to map where thumbnails occur and which behaviors are associated with them.

A small trait, big impact

The study was led by Rafaela Missagia, a research associate at the Field Museum and an assistant professor at the University of São Paulo (USP).

Rodents make up about 40 percent of all mammals, so even small shifts in their hands can echo across the tree of life.

The researchers examined 433 rodent genera and reported that 86 percent included species with thumbnails on the first digit. That pattern is not a one off observation – it spans the rodent family tree.

The analyses identified a nail, not a claw, as the likely ancestral state for that digit. This points to a deep origin for thumbnail equipped hands among rodents.

Lineages that live underground or that feed mostly with the mouth show different paths. Some gained a claw on the first digit, while others lost the structure altogether.

Grip power in rodent hands

The unguis is the hardened tip of a digit, whether nail, claw, or hoof, and its shape on the first digit shifts what a hand can do. The team paired anatomy with behavior captured in textbooks and videos to test how rodents use their hands while they eat.

“Rodents, as we know today, use their resources very efficiently,” said Missagia. A flat thumbnail offers a broad, blunt surface that steadies small items and supports precise grips. 

That grip likely worked alongside specialized incisors and jaw muscles that excel at gnawing hard foods such as nuts. These foods are energy dense, so being able to hold and turn them quickly is a real edge when competition is high.

The study treats the first digit, often labeled D1, as a key part of that story. Changing the unguis on D1 changes the whole hand, which changes the menu an animal can handle.

Rodent thumbnails tied to lifestyles

Outside of primates, rodents are the only mammals that show a true thumbnail on the first digit. That makes this little structure especially useful for comparing how different groups solve similar problems.

Species that are fossorial, meaning adapted for digging or living underground, tend to carry claws on every digit. Those claws act like pointed tools that cut soil better than a nail would.

Species that are arboreal – meaning they live above ground or in trees – are more likely to keep thumbnails on the first digit. A nail is less likely to snag and more likely to steady food or a twig during a careful climb.

Rodents without thumbs, such as guinea pig relatives, usually do not handle food with their hands. They bring the mouth to the food and skip manual handling altogether.

Fossil clues in tiny thumbs

The dataset places thumbnails early in the rodent story, present at ancestral nodes across the family tree. Fossil clues stretch back many millions of years, which supports an origin deep in time for first-digit nails in this group.

Thumb bones vary across genera, and that variation aligns with unguis type. Short, wide bones tend to support nails, while narrow, curved bones tend to anchor claws.

That bony scaffolding matters because it influences how much pressure a digit can apply and in what direction. It also limits – or expands – the range of grips an animal can use on seeds, insects, or other foods.

The study used phylogenetic models to test how often thumbnails were gained or lost in different branches. Gains of claws and losses of nails reappeared in lineages that burrow or rely on oral feeding, pointing to repeated solutions to similar ecological tasks.

Rodent nails in museum drawers

The Field Museum houses one of the largest mammal collections on Earth, which made a broad test like this possible. Old skins and skeletons answered fresh questions because the parts that matter most remain intact.

Researchers could compare a kangaroo rat, a capybara, and a pygmy mouse in the same drawer, then connect those features to field notes and filmed behavior. Museum collections support datasets that would be nearly impossible to assemble in the wild on short timelines.

Large collections also anchor claims with physical evidence. When new tools come around, from micro-imaging to machine learning, those same specimens can be studied again from new angles.

Projects like this remind us that small structures can set big limits. A thumbnail is simple, but it can steer what an animal eats, how it climbs, and where it thrives.

The evolution of nails and claws

Simple anatomical traits can open or close whole ecological doors. A flat nail supports grips that a curved claw makes awkward, so the presence or absence of that small part sets a boundary for behavior.

The research shows how evolutionary changes are not one way streets. Nails can be lost when digging dominates a lifestyle, and claws can reappear when the environment rewards them.

The study also demonstrates how anatomy, behavior, and ecology fit together. Hands are not just for climbing or digging. They are for eating too, and what a hand holds can ripple through a lineage for millions of years.

The study is published in the journal Science.

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