Repost: Life in the Trees Shaped the Panda’s Thumb

[Author’s Note: A funny thing happened on the way to the floor the other day. I blacked out at the gym and, when I collided with the floormat, the temple of my glasses punctured my face. As the gym’s lifeguards told my wife when they ushered her in to see me, though, the damage looked worse than it actually was. I’m fine – just a few stitches, and all the high-tech investigations of my circulatory system have not turned up anything sinister – but I have taken a less frantic writing pace this week. Today I have elected to repost a short piece I wrote in April of last year about that most famous of carnivoran quirks – the panda’s “thumb.” Expect new posts to resume on Friday. Thank you for your patience.]

As the paleontologist Stephen Jay Gould observed in one of his most famous essays, the so-called thumbs of giant pandas (Ailuropoda melanoleuca) are nothing at all like the large digits on our own hands. The bear’s 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.

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 sesamoids which they use to manipulate bamboo. The same feature evolved twice and performs the same function today, but, according to a recent analysis of a fossil red panda, bamboo-eating may have had little to do with the origins of panda thumbs.

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 the carnivoran 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 comprised the skulls, most of the vertebral columns, and much of the limbs of both animals, but what was most peculiar about Simocyon was its wrists.

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

Among the preserved wrist bones of Simocyon were enlarged sesamoids. These bones were not as prominent as those seen in giant pandas, but were still relatively large and would have created an extra gripping surface. And, functionally speaking, the false thumb of Simocyon would have worked similar to the way giant pandas use them. When flexed, the finger-like bones 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 chasing prey through the trees. This difference in natural history may hold the key to the evolution of panda thumbs.

The gripping ability afforded to Simocyon by its sesamoid thumbs, the authors of the PNAS study hypothesize, would have allowed the carnivore to climb beyond the reach of the larger predators which it lived alongside. The saber-toothed cats Machairodus and Paramachairodus, as well as the “bear dog” Amphicyon, were the primary competition. If Simocyon could navigate thinner branches, the carnivore would have been able to pursue prey beyond the range of its more terrestrial competitors and, if needed, escape into the safety of the canopy. Taking refuge in the trees could have even been part of the predator’s strategy – Simocyon could have stolen a kill from another predator and hauled the meal up into the high branches, as leopards do today.

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.

Despite what we might expect based upon the habits of living giant pandas and red pandas, the origins of their specialized thumbs may have little to do with a bamboo diet. Simocyon was a close relative of the lineage which contains modern red pandas, and this affinity indicates that an enlarged sesamoid may have been a feature both lineages inherited from a common ancestor within a group called the Ailuridae. What started off as a specialization for gripping in the trees was later co-opted for manipulating bamboo. Giant pandas, on the other hand, are more distant relatives of red pandas, and are unique among bears in possessing a similar wrist bone. Their thumbs independently evolved by about 18 million years ago. Perhaps the thumbs of modern giant pandas went through a similar evolutionary history. For them, as well as red pandas, life in the trees could have led to adaptations which were later co-opted for trundling along on the ground. Future fossil discoveries will test this idea. For now, though, the panda’s thumb remains a wonderful and iconic example of evolutionary change – a neat story of contingency, convergence, and co-option in one, small osteological package.

Top Image: A red panda – Ailurus fulgens – at the Bronx Zoo. Photo by the author.

References:

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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