Implications of the functional anatomy of the hand and forearm of Ailurus fulgens (Carnivora, Ailuridae) for the evolution of the ‘false‐thumb’ in pandas

Abstract

Both the giant panda ( Ailuropoda melanoleuca ) and the red panda ( Ailurus fulgens ) possess a 'false-thumb', actually an enlarged radial sesamoid bone, which contributes to the gripping action of the hand. These species are not closely related, however, as one is an ursid and the other an ailurid, so the fact that they share this adaptation implies a remarkable convergence. We studied the functional anatomy of this structure in the red panda, comparing it with existing descriptions of the grasping mechanism in both pandas. Previous interpretations of the radial sesamoid in Ailurus as a rod-like structure without direct articulation to the wrist bones are inaccurate. There are various important differences between the red panda and the giant panda. In the former, the lesser development of the radial sesamoid, its connection with the flexor retinaculum, the presence of an insertion of the muscle abductor pollicis longus in the first metacarpal, which enhances its supinatory action, and the presence of a muscle flexor brevis digitorum manus point to thin-branch climbing features serving as an exaptation to the more recent role of the red panda hand in the manipulation of bamboo.

Comparative genomics reveals convergent evolution between the bamboo-eating giant and red pandas

“…1) that significantly facilitates feeding dexterity by grasping bamboo (11)(12)(13)(14), a phenotype of long-standing interest to evolutionary biologists. D. Dwight Davis, for example, noted that in the giant panda, "the highly specialized and obviously functional radial sesamoid has a specific, but probably very simple, genetic base" (13), and Stephen J. Gould later featured the new digit in the title of his popular 1980 book, "The Panda's Thumb" (14).…”

Tracing the origin of the panda’s thumb

“…In most instances, this bone is of similar size to other sesamoids, or even vestigial, but in some mammals, such as talpids (Krause and Jenkins 1983;Sánchez-Villagra and Menke 2005), many tenrecids (Salton and Sargis 2008) and elephants (Hutchinson et al 2011), it constitutes a digit-like element that is variously called 'os falciforme', 'prepollex' or 'predigit'. Furthermore, a truly hypertrophied radial sesamoid, constituting a functional 'false thumb', is considered to be present in the giant panda, Ailuropoda melanoleuca (Lankester and Lydekker 1901;Wood-Jones 1939a, b;Davis 1964;Gould 1978;Chorn and Hoffmann 1978;Endo et al 1996Endo et al , 1999aEndo et al , b, 2001aAntón, et al 2006;Salesa et al 2006a, b) and, to a lesser extent, in the red panda, Ailurus fulgens (Roberts and Gittleman 1984;Endo et al 2001bEndo et al , 2007Antón et al 2006;Salesa et al 2006b). The functional role of the radial sesamoid as a grasping structure essentially depends on its size and degree of movement relative to the metapodials and other wrist bones, thus being only possible when the radial sesamoid is relatively long compared to other bones of the manus (Abella et al 2013a).…”

“…More recently, with the recognition that lesser pandas are only distantly related to giant pandas but also display an enlarged radial sesamoid involved in bamboo feeding activities, the (Torrejón de Velasco, Comunidad de Madrid, Spain), showing the location of the nine sites. The site of Batallones-3 is denoted with a star false thumb of both pandas has acquired further significance as one of the most remarkable examples of convergent evolution (Antón et al 2006;Salesa et al 2006b). Given their similar grasping function during bamboo feeding and based on extant taxa alone, it might seem warranted to infer that the giant and lesser pandas' hypertrophied radial sesamoids constitute a remarkable case of convergent adaptation.…”

“…However, the radial sesamoid is not employed as a truly opposable and freely movable thumb in either the giant or lesser panda (contra Davis 1964). In the former, the 'false thumb' constitutes a part of a double pincer-like, manipulative functional apparatus in which the hand flexes around the scapholunar and the unciform, so as to grasp objects between the true digits and both the radial sesamoid and the pisiform (Endo et al 1999a(Endo et al , b, 2001aAntón et al 2006). In contrast, the less marked hypertrophy of the radial sesamoid of the lesser panda, coupled with associated musculoskeletal differences, indicate the possession of a different grasping mechanism (Endo et al 2001b(Endo et al , 2007Antón et al 2006;Salesa et al 2006b;Abella et al 2013a), more similar to that displayed by other small-to medium-sized carnivorans.…”

Untitled

“…In A. melanoleuca, the muscle attaches only on the radial sesamoid, lacking any attachment on the first metacarpal (Davis, 1964). The function of the muscle abductor pollicis longus is to contribute to the supination of the palm and flexing the thumb on the palm (Davis, 1964;Antón et al, 2006).…”