Gould, The Panda's Peculiar Thumb

D. L. Cramer

all the milk bottles actually tumbled at the county fair, I was delighted when the first fruits of our thaw with China went beyond Ping-Pong to the shipment of two pandas to the Washington zoo. I went and watched in appropriate awe. They yawned, stretched, and ambled a bit, but they spent nearly all their time feeding on their beloved bamboo. They sat upright and manipulated the stalks with their forepaws, shedding the leaves and consuming only the shoots.

I was
amazed by their dexterity and wondered how the scion of a stock adapted for running could use its hands so adroitly. They held the stalks of bamboo in their paws and stripped off the leaves by passing the stalks between an apparently flexible thumb and the remaining fingers. This puzzled me. I had always learned that a dexterous, opposable thumb stood among the hallmarks of human success. We had maintained, even exaggerated, this important flexibility of our primate forebears, while most mammals had sacrificed it in specializing their digits. Carnivores run, stab, and scratch. My cat manipulates me in the psychological sense, but he'll never type or play the piano.

I counted the panda's other digits and received an even greater surprise: there were five, not four. Was the "thumb" a separately evolved sixth finger? Fortunately, the giant panda has its bible, a monograph by D. Dwight Davis, late curator of vertebrate anatomy at Chicago's Field Museum of Natural History. Probably the greatest work of modern evolutionary comparative anatomy, it contains more than anyone would ever want to know about pandas (The Giant Panda, a Morphological Study of Evolutionary Mechanisms). Davis had the answer, of course.

panda's thumb is not, anatomically, a finger at all. It is made from a bone called the radial sesamoid, normally a very small component of the wrist. In pandas, the radial sesamoid is greatly enlarged and elongated until it almost equals the true digits in size. The radial sesamoid underlies a pad on the panda's forepaw; the five digits form the framework of another pad, the palmar. A shallow furrow separates the two pads and serves as a channelway for bamboo stalks.

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