When squirrels nibble on a nut, hold them with their front paws. Your thumb will help you, who is the only finger to be equipped with a fingernail instead of a claw. A study now shows that this anatomical peculiarity in the entire family tree of rodents is widespread and probably developed at the same time as the rodents. This could open up the thumbnail of this group of animals new ecological niches and thus contributed to their evolutionary success.
Whether squirrels, hamsters or dormouse – many rodents have a special feature of their front paws that only occurs with this group of animals: four fingers are equipped with claws that help them climb or ditch. The thumb, however, has a smooth, flat fingernail. “The prevailing view has so far been that it is a non-functional remains,” explains a team about Rafaela Missagia from the Field Museum of Natural History in Chicago. After all, the short thumb for climbing is less important than the four other fingers. It would therefore be conceivable that the unused claw of this finger has receded over time.
Searching for traces in museum collections
But Missagia and her team come to the conclusion in a new study that the thumbnail of all people played an important role in the evolution of the rodents. He could even explain why this group of animals is now almost half of all mammals on earth. “Before we carried out the examination, we knew that some rodents have nails, others claws and others have no thumb at all,” says co-author Gordon Shepherd from Northwestern University in Chicago. “There were indications that the rodents who have thumb nails also use their thumbs to hold their food.”
In order to investigate this indications, the researchers examined hundreds of rodents from museum collections and catalogized in which types of thumb nails. “There are more than 530 different genres of rodents with over 2500 species. We examined 433 of these genera from the entire family tree of rodents,” says Missagia’s colleague Anderson Feijó. The team found that at least some of the associated species have thumb nails in 86 percent of the rodents examined.
Food between the paws
In the next step, Missagia and her colleagues resolved this data with information about the nutritional habits of the animals. To do this, they rolled textbooks, specialist journals and photo collections and looked at pictures of different rodents while eating. “Based on this information, we reconstructed the family tree of the rodents, in relation to rodents who touch their food with their hands, and those that they only absorb with their mouths,” reports Shepherd.
On the one hand, it became apparent that actually those animals have thumb nails that keep their grains, nuts and seeds between the front paws when eating. An example of this are squirrels. In the case of animals such as guinea pigs, on the other hand, which absorb their food directly with the mouth, the thumb nail and in many cases also lack the thumb. On the other hand, the analysis showed that such a thumbnail occurred early in the evolutionary history of the rodents. Fossil indications indicate that Paramys, one of the earliest known rodents that lived 55 million years ago, already had thumb nails.
With hands and teeth
According to Missagia and her team, the thumb nails probably developed at the same time as the incisors. Only the combination of thumbnails and gnawing teeth enabled the rodents to open up new sources of food such as nuts. Because even the strongest teeth are not very helpful when cracking the shell when the nut rolls away again and again. “Nuts are a very energetic resource, but to open and consume it requires good manual skill that many other animals do not have,” says Feijó. “Perhaps the enclosure of the rodents have made it possible for them to use this unique resource and then diversify broadly because they did not compete with other animals for this food.”
Only in the further course of the diversification did the thumbnail form back in some types or became a claw. As Missagia and her team found, this was particularly true for the species living underground who use their paws to dig. From the research of the researchers, these results throw a new light on the rodents’ evolution and suggest that the thumbnail caused new functions that increased the adaptability of the rodents.
Source: Rafaela Missagia (Field Museum of Natural History, Chicago, USA) et al., Science, doi: 10.1126/science.ads7926
