86% of Rodents Possess a ‘Nail’ Instead of a Thumb: How This Evolutionary Trait Transformed Their Survival

Present on all continents except Antarctica, rodents represent almost half of the species of mammals identified to date. Their planetary success has long intrigued biologists, who have often highlighted their varied diet, their rapid reproduction or their adaptability. But a study published in the Revue Science adds an unexpected element to this equation: a tiny nail, discreet, but potentially decisive.

A large -scale morphological survey

The study relied on the direct examination of specimens kept in museum collection. The Field Museum, which has one of the largest collections of mammals in the world, has served as the basis for this large -scale morphological survey. The objective: to identify the presence or absence of these small flat nails located on the thumb, on different rodents. The researchers studied 433 kinds of rodents, which represents exceptional coverage. Indeed, the Radentia order has just over 530 genres in total, for around 2,500 species identified.

The work has been completed by a large collection of information from scientific literature, field observations, videos and photos, in particular via the inaturalist platform. This database made it possible to correlate the morphology of the thumbs to observable eating behaviors.

© Missagia et al., 2025

This crossed approach between morphology, phylogeny and behavior represents a novelty in the study of the evolution of rodents. “” We wanted to go beyond the simple anatomical description to connect the form to the function “Explains Anderson Feijó to Phys.org. The choice to use old collections also reinforces the interest of this research. Specimens collected over a century ago deliver crucial data today to understand the evolutionary history of mammals.

Thumbs up to grasp and manipulate

One of the major lessons in the study concerns the functional role of this nail in the handling of food. In many species of rodents, this small flat nail located on the go contrasts with the curved claws present on the other fingers. This hybrid configuration is not trivial. It allows a precise and stable seizure of objects, especially food.

“” What we observe is a clear association between the presence of nails and the use of hands to eat “Explains Gordon Shepherd. Rodents with this type of thumb are regularly observed holding seeds, fruit or nuts between their front legs. They use their thumb as a point of support. This is particularly striking in squirrels, arboreal rats or even rats-kangaroos. They show great skill in peeling food.

Conversely, species devoid of thumb or flat nail on this finger, like guinea pigs, are content to use their mouths to consume. This behavioral distinction is important, because it suggests that the nail plays an active role in the ecological diversification of rodents. Access to food resources requiring dexterity, such as nuts, has been able to constitute a major evolving opportunity.

This level of manual grip remains rare in mammals outside the primates. The appearance of a flat nail, close to a human nail, allows an optimized contact zone between the pulp of the finger and the manipulated object. This facilitates the maintenance of small objects without interference of the claws. Because although useful to dig or climb, they hinder the delicacy of the gestures. A simple, but crucial anatomical adaptation.

An ancient origin and an uneven distribution

The study highlights the ancient origin of this nail. The phylogenetic analysis carried out by the researchers suggests that it would date from around 60 million years ago. It would have appeared in an ancestor common to a large part of modern rodents. This trait would therefore have emerged very soon after the diversification of placental mammals, in a post-critical context marked by vast vacant ecological niches.

This hypothesis is supported by paleontological data. Some rodent fossils dated 50 million years have short and wide inch bones. They are typical of a finger wearing a nail rather than a claw. However, this morphology is not frequent in fossil mammals, where claws dominate. This strengthens the idea of ​​an old, non -anecdotal structural adaptation.

However, the distribution of the nail in current species is not uniform. The study reveals that 86% of the genres studied include at least one species with a nail. But this presence varies according to the lifestyle. Arboret or terrestrial rodents, actively using their hands to eat or climb, more frequently have nails. Conversely, foul species, like some gerboises, tend to keep claws on all fingers, including the thumb.

“” We have highlighted a correlation between the form of the thumb and the living environment “, Specifies Rafaela Missagia. This morphological diversity is part of an adaptive evolutionary process. The members' anatomy is shaped by functional constraints: input, digging, locomotion. The same structure – the thumb – can thus be modified according to the selective pressures specific to each ecological niche.

An evolutionary lever for a global conquest

The evolutionary impact of the nail goes beyond the simple food grip. This adaptation could have acted as a real lever for diversification, allowing rodents to access under-exploited energy resources. Nuts, for example, are rich in lipids and proteins. However, their consumption requires fine handling capacities and a significant jaw force. By combining effective robust teeth, some rodents have been able to monopolize these resources.

Anderson Feijó explains: ” This ability to grasp, keep, orient and work a food with your hands has probably reduced food competition between rodents and other less skilful mammals ». By facilitating access to specific food niches, the nail would have contributed to the rapid expansion of rodents on almost all continents.

This hypothesis joins broader theories on the co-evolution between anatomical features and survival strategy. A small morphological change, apparently harmless, can have deep repercussions on a macroevolutive scale. Here, a simple nail would have enabled the emergence of new behaviors – like manipulating objects or feeding in a sitting position. In turn, they opened ecological doors hitherto closed.

In addition, this adaptation may have favored arboreal life in certain lines. Fine grip is useful for climbing, handling branches, or moving to a three -dimensional environment. Thus, the nail could represent a key line in the conquest of forests by many species.

This observation also underlines the importance of integrated approaches in evolutionary biology. Connect form, function, behavior and ecology remains essential to understand the planetary success of a group such as rodents.

Source: Rafaela V. Missagia et al., “Evolution of Thumbnails across Rodentia”, Science (2025).