When cotton rats grasp like pandas

Abstract

The panda’s false thumb is an iconic structure, described as one of the most shocking cases of anatomical convergence, and has been studied in many essays about evolution. However, in a recent paper in which we evaluated this feature within the Carnivora, we concluded that the developed radial sesamoid could be taken as a plesiomorphic character for at least the Arctoidea, rather than an anatomical convergence of the pandas (both red and giant). Following this argument, in this research we describe the action of a radial sesamoid as a real false thumb for the first time outside the mammalian carnivorans. The cricetid Sigmodon peruanus shows a very similar radial sesamoid compared to that of the giant panda (Ailuropoda melanoleuca) but somehow relatively even more developed, compared to the other bones of the manus than in the latter, showing that the use of this structure as a opposable pincer is much more expanded in mammals than thought previously.