The Luzon giant cloud rat (Phloeomys cumingi), native to the Philippines, uses its thumb nails to grip food. Photo: Forest Botial-Jarivis.
Nail, claw or nothing: the thumb of rodents comes in five variants that correlate with lifestyle and feeding behaviour. These modest differences lie at the root of their evolutionary success.
He humble rodent thumb turns out to be a crucial innovation in evolutionary history. A phylogenetic comparison of 522 genera, spanning virtually all rodent families, shows that most species bear or bore a nail or claw on their first digit (D1) – a trait that is at least thirty million years old (Science, 5 September). Using an extensive morphological dataset and models for ancestral character reconstruction, the researchers were able to determine how many times nails, claws and nail-less thumbs arose independently of one another.
Dexterity
That small nail increased the dexterity of the rodent forepaw, allowing animals to hold food in place whilst using their characteristic incisors to crack open nuts and seeds. The interplay of grip and biting force gave rodents an ecological advantage following the extinction of the multituberculates, a group that had previously occupied the same niches. The nail on D1 proves to be the ancestral character state of the order. Variants arose later and independently in diverse clades. Burrowing species, such as pocket gophers, repeatedly evolved a claw, whilst other groups – including guinea pigs and porcupines – lost the nail structure entirely. The researchers distinguish five states: nail, claw, no nail, intermediate forms, and complete loss of the phalanx. This diversity is strongly associated with behaviour. Species that handle their food with both mouth and hands almost invariably have a nail. Species that feed exclusively orally have often lost the nail. Lifestyle also plays a role: arboreal and climbing species generally have a nail, whilst burrowers more frequently display a claw. Terrestrial species show all variants, underscoring the flexibility of their adaptations. The researchers emphasise that the thumb nail is therefore not a rudimentary vestige but a functional adaptation that enhanced the versatility of the rodent forepaw. Just as the opposable thumb became a key innovation in primates, this modest variant too appears to have contributed to the success of the largest mammalian order.
Photographic overview of the occurrence of nails in rodents in relation to food handling. Source: Missagia et al., Science, 5 September 2025
‘It is a detailed study that suggests the evolution of the thumb has made an important contribution to the diversification of rodents’, says Rampal Etienne, an evolutionary biologist at the University of Groningen not involved in the study. ‘They show that variation in the thumb and thumb nail is linked to variation in feeding behaviour and lifestyle. They distinguish five forms, but the question remains how much each of those forms has contributed to the enormous species richness. An analysis of whether a specific form increased the rate of speciation would also have been welcome, but it is an inspiring study with a wealth of new data.’