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.
So 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.
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.
panda's, thumb comes