|Athro, Limited Biology Evolution The Panda's Thumb|
|In memoriam Stephen Jay Gould 1941-2002|
The Giant Panda, Ailuropoda melanoleuca, is a rare and endangered mammal from central China. The riddle of the panda's thumb is one of the classic stories of evolution.
|A Giant Panda. Source: Corel|
The Giant Panda is a a member of the large group of mostly
carnivorous mammals that includes dogs, bears, raccoons, weasels, skunks, seals, and cats. This group of related animals is known as the Carnivora. Most of these animals eat just meat or are
omnivores. The giant panda, however, subsists primarily on bamboo.
In 1964 the anatomist D. Dwight Davis published the classic monograph on the anatomy of the giant panda, based on a individual that died in a US Zoo. Davis argued the giant panda is a bear that adapted to eating bamboo. The relationship of the giant panda to other carnivorans has been somewhat controversial, but it is now widely accepted that the giant panda is a close relative of bears.
Look carefully at the paw. How many claws do you see?
|The panda's thumb pad. Modified from Davis, 1964.|
The giant panda also has an opposable thumb.
Giant pandas have a thumb pad on the hand. The panda can press this pad against the pad of its palm. This thumb pad is used by the panda to grasp bamboo shoots. It uses its thumb to hold and manipulate the bamboo while eating.
But, this thumb pad doesn't look like the rest of the fingers.
|The panda's "thumb" is not a true thumb. Modified from Davis, 1964.|
Look carefully at the bones in the giant panda's hand. Inside the thumb pad is the blue "thumb" bone. Does it look the same as the rest of the panda's fingers?
Look at the bones in the giant panda's hand. There are five digits with claws. Each of these has three or four bones (a metacarpal and 2 or 3 phalanges). Beneath the panda's thumb pad, however, is a single bone.
The panda's "thumb" is not a true thumb. The panda's true thumb is the adjacent
digit, marked I in the figure (because it is the first digit and digits are numbered with roman numerals). Like digit I of other carnivorans, digit I of the giant panda is made up of just 3 bones, while the other digits are made up of 4 bones each. The "thumb" that underlies the thumb pad of the giant panda consists of a single bone that is not one of its normal digits.
If we compare the hand of the Giant Panda to that of other carnivorans, such as bears, we see that the panda's "thumb" is in the same place as a tiny bone known as the radial
sesamoid. Sesamoid bones are strange little bones that form in the bits of connective tissue that cross joints. Connective tissues (tendons and things) tear easily when they are bent around a corner. Bones don't tear. A bone added to a tendon where it bends around a joint will thus reduce the chance of a small tear (a sprain) expanding and ripping the entire tendon in two.
The panda's "thumb" is a much enlarged sesamoid bone. Not only is it not a true thumb, but it can't move much. It is primarily a bony support for the pad above it, a support the panda's true thumb and fingers can squeeze against to hold bamboo (Endo et al 1996).
It is normal for mammals to have sesamoid bones in the hand. A sesamoid bone on the
ulnar side is nearly ubiquitous, and radial sesamoids are widespread.
All of the Canoidea (dogs, bears, raccoons, pandas, weasels) have a
radial sesamoid bone (Ewer, 1973 p.21).
Rather than making digit I opposable to increase grasping ability, as raccoons have done to some extent, the giant panda enlarged the radial sesamoid as a contraption. Putting this as an evolutionary story: early pandas had heritable variability in the size of their radial sesamoid, and larger sesamoids conferred a selective advantage through greater ease in manipulating food.
|The tibial sesamoid of the giant panda is also large. Modified from Davis, 1964|
Take a look at the bones of the feet of the giant panda. There is something very interesting here. There is a relatively large bone in the foot in the same location as the radial sesamoid is in the hand. This bone is the tibial sesamoid, and it is larger than the same bone in other bears. This complicates our evolutionary story. How could selection to build a thumb pad support in the hand also enlarge the same bone in the foot?
Two answers have been suggested to explain the enlarged sesamoid bones of the panda. These two evolutionary stories are: Story 1 - The radial sesamoid in the hand originally served to strengthen its tendon. It then got bigger to help the panda grip bamboo. The tibial sesamoid in the foot was also enlarged as an incidental side consequence of the way limb development works in mammals (with bones in the limbs tending to mimic each other). Story 2 - The radial and tibial sesamoids originally served to strengthen their tendons. They then enlarged to serve as tree climbing aids in the ancestors of the giant panda (Macdonald, 1992 p.178). The radial sesamoid was then co-opted to help the panda grip bamboo.
In either case, the panda's thumb is a contrivance that is an artifact of the history of the panda. It is not an ideal design fitted perfectly for holding bamboo, it is an evolved contraption. The original function of the radial sesamoid was to reduce the chance of tears in the tendon that runs to the thumb from a muscle in the arm (the sesamoid forms where this tendon bends around the edge of the wrist). The radial sesamoid was then co-opted to serve its present role in handling bamboo. Indeed, much of the panda's anatomy speaks of contrivance - it is a bear from a meat eating ancestry that has evolved to eat bamboo. Many details of its anatomy appear to be strange contrivances that reflect this history.