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

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That's quite fascinating. We've always known that both "pandas" must have had carnivorous ancestors, but we didn't know their ancestors had an adaptation later used in eating plants.

agreed with above - this is awesome. Also the first blog post I've read about my favourite animal, the Panda, concerning evolution. :-)

Hmm...fascinating stuff.

But, why would an arboreal creature make use of an elongated wrist bone in climbing trees, instead of simply employing an opposable digit? Wouldn't a "true" thumb allow a greater mobility and a better grip in the trees than a wrist bone?

By Practically Un… (not verified) on 14 Apr 2010 #permalink

exaptations happen!

By Occam's Razor (not verified) on 14 Apr 2010 #permalink

Practically; An true opposable digit would definitely come in handy (ha!) for pandas, but it seems that the shape of their ancestor's hands constrained the way in which a "gripping" structure could evolve. Given that this kind of sesamoid thumb have evolved at least twice, if not three times, among carnivores, it would suggest that there are anatomical and functional constraints which prevent any of the digits of the carnivore paw from diverging from the others to become a thumb. Instead, it seems that variations in the enlarged sesamoid provided better grip, and perhaps natural selection favored increasingly large radial sesamoids to form the panda's thumb. That is what is at the crux of Gould's argument - if evolution/nature was an intelligent engineer we might expect pandas to be equipped with opposable thumbs like ours, but the fact that their "thumbs" are jury-rigged bones which have changed purpose over time points to the fact that there is no intelligent design behind the form of organisms.

Well, the panda has five fingers and the wrist bone adaptation; one wonders how advantageous it was to have five fingers to grasp bamboo with; why not four like us or even three? What about two; do you think the bamboo could have been grasped with two fingers? Mutations survive if they don't deny the animal feeding or reproduction ability. The criteria that we assume when dealing with mutations is whether it is harmful: it is more correct to see mutations as being % harmful. Consider the hammerhead shark; is the mutation that put the eyes outside the outside the head on a slab of meat extending a foot harmful? What if the shark was only a foot long? Would it have survived in an ocean where it was prey and not big enough to be a predator? EVERYTHING IS EVERYWHERE, THE ENVIRONMENT DECIDES.

By katesisco (not verified) on 17 Apr 2010 #permalink