The humble rodent "thumb" may not seem like an obvious window into evolution, but its keratinized tip – the unguis (hoof, claw, or nail) – turns out to reveal striking insights into rodent history and adaptation, according to a new study. The findings suggest that rodents owe much of their evolutionary success to their thumb-nail (the first digit, D1), an adaptation that gave them dexterous hands for cracking seeds and nuts. The tetrapod (four-limbed vertebrate) hand is a crucial structure for interacting with the environment, and its digits show great evolutionary diversity in both form and function. Among them, the first digit – D1 – is especially intriguing: it is the last to appear during development, the first to be reduced or lost in evolution, and in some lineages, like primates, it has enabled dexterous behaviors, such as grasping or climbing. Yet, the unguis has rarely been studied in detail. In rodents, the most speciose group of mammals, the D1 may bear a nail, a claw, or no unguis at all, but the evolutionary patterns and functional significance of this variation remain poorly understood.
Here, Rafaela Missagia and colleagues use advanced phylogenetic comparative methods to systematically examine the diversity, evolutionary history, and behavioral correlates of the D1 ungis types across Rodentia. Missagia et al. found that a nail, rather than a claw, is both the most common and the likely ancestral condition. Fossil evidence suggests that rodents have borne nail-like D1s since at least the Oligocene, marking this feature as a longstanding hallmark of the group and a unique trait among related mammalian orders. According to the authors, the D1 nail may have coevolved with the rodent's distinctive gnawing incisors, supporting dexterous manipulation of hard foods like nuts, which would have been a key ecological advantage during the group's early diversification. Rodent species with D1 claws and those without a D1 unguis arose later in specialized linages, likely to support specific behaviors; claws to subterranean or burrowing lineages, and the loss of the unguis to lineages that rely more heavily on oral feeding than hand use.
