Life in the trees, not bamboo, shaped the panda’s “thumb”

A red panda (Ailurus fulgens, left, photographed at the Bronx Zoo) and a giant panda (Ailuropoda melanoleuca, right, photographed at the National Zoo).

As the paleontologist Stephen Jay Gould observed in one of his most famous essays, the thumbs of giant pandas (Ailuropoda melanoleuca) are nothing at all like the large digits on our own hands. Their accessory “thumbs”, visible on the surface as a differentiated part of the pad on the “palm” of the hand, are modified sesamoid bones derived from the wrist. They are jury-rigged bits of anatomy which cast nature as an “excellent tinkerer, not a divine artificer.“

Surprisingly, however, these highly-modified wrist bones are not unique to the black-and-white bears. Red pandas (Ailurus fulgens), which are much more closely related to raccoons than bears, also have modified sesamoid “thumbs” which they use to manipulate bamboo. Hence, given the similar wrist anatomy and diet of these two carnivorans, it might be assumed that the false thumbs are adaptations related to bamboo-eating, but a recent analysis of a fossil relative of red pandas suggests that the peculiar structures evolved for an entirely different reason.

The skeleton of Simocyon as reconstructed by Mauricio Anton. From Salesa et al, 2006.

The fossil carnivore Simocyon has been known to paleontologists since the 1850’s, but for about a century and a half our understanding of it primarily came from teeth and a few bits of skull. It was not until 2006 that scientists Manuel Salesa, Mauricio Anton, Stephane Peigne, and Jorge Morales described the partially complete remains of two Simocyon individuals from a Late Miocene (~11.5-5 million years old) site Batallones-1 in Madrid Spain. Altogether the remains of the cougar-sized carnivores represented the heads, most of the spines, and much of the limbs of both animals, but what was most interesting about Simocyon was its wrists.

The right hands (palm down view) of the giant panda (left) and Simocyon (right), as reconstructed by Mauricio Anton. RS denotes the location of the radial sesamoid. From Salesa et al, 2006.

Among the preserved wrist bones of Simocyon were enlarged sesamoids very much like those of giant pandas, and, functionally speaking, the false thumb of Simocyon would have worked similar to the way giant pandas use them. When flexed they would have allowed for a tight grip on branches, but Simocyon does not appear to have been a terrestrial bamboo-chewer like giant pandas. Instead it was well adapted to a life spent hunting animal prey in the trees, and this may hold the key to the evolution of its false thumb.

As argued by the authors of the PNAS study, the gripping ability afforded to Simocyon by its sesamoid thumbs would have allowed it to climb beyond the reach of the larger predators which it lived alongside. Among the other carnivores living in the area at the time were the saber-toothed cats Machairodus and Paramachairodus and the “bear dog” Amphicyon, and if Simocyon could navigate thinner branches it could not only escape these predators, but also pursue prey outside of their range. Even though Paramachairodus itself may have been a climber, Simocyon would have been proficient enough to steal part of the cat’s kill and run away with it into the trees.

An evolutionary tree depicting the relationships of carnivores with false thumbs. The grey circles denote the hypothesized origin of sesamoid thumbs in the bear and red panda lineages. From Salesa et al, 2006.

This has interesting implications for how the false thumb of both the red and giant pandas evolved. Clearly it was a case of evolutionary convergence, but why? The fact that Simocyon lacks the dental adaptations related to herbivory seen in living pandas hints that the evolution of the “false thumb” was not necessarily related to plant-eating, and the close relationship between Simocyon and red pandas may indicate that they inherited their specialized sesamoids from a carnivorous common ancestor. If this was the case, the authors hypothesize, then the development of the thumbs in the Simocyon-red panda lineage (the Ailuridae) would be attributable to the advantage a stronger grip would afford during a life in the trees. It would only be much later that the modified bone would be co-opted into a bamboo-eating role by red pandas, and also raises the question if the same might be true of giant pandas.

It can be risky to infer the opinions of deceased authorities, but I think Gould would have been delighted by this idea. Not only would the specialized sesamoids of both living pandas be prime examples of how contingency shapes the way in which evolution modifies organisms, but it would also illustrate how an adaptation can be co-opted for a different purpose over time. More than ever, the panda’s thumb represents some of the core evolutionary ideas which Gould worked to bring to our attention.

Salesa, M. (2006). Evidence of a false thumb in a fossil carnivore clarifies the evolution of pandas Proceedings of the National Academy of Sciences, 103 (2), 379-382 DOI: 10.1073/pnas.0504899102