Buried beneath the leathery skin of an elephant's foot lies one of anatomy's unappreciated mysteries. Three hundred years ago, a surgeon claimed elephants had six toes instead of the usual five, setting off a debate about whether an extra digit was really possible. Modern anatomists scoffed at the idea, insisting instead that the extra toe was really just a big lump of cartilage. Now a study of scores of elephant feet shows that the lump really does turn into bone. The digit is not a true toe—it's more like a panda's faux thumb. But it nonetheless helps support the pachyderm's mighty girth.
In elephants, "the unique structure of the foot must clearly be considered a key innovation," says Matthew Vickaryous, a vertebrate morphologist at the University of Guelph in Canada who was not involved in the study. "The elephant foot is deceptively complex."
The giant panda's extra thumb is a famous example of evolution's inventiveness. The animal's real thumb looks just the rest of its fingers, and together they form a paw with five claws. But in addition, pandas have a somewhat opposable digit low on the inside edge of the paw that helps them grasp bamboo. This "thumb" is really just a sesamoid, a bit of bone that typically forms inside tendons and ligaments where they cross joints. The knee cap is one example of a sesamoid. But in the panda, the sesamoid on the outside base of the true thumb became enlarged, taking on a digit-like identity that helps the animal eat more efficiently.
John Hutchinson, an evolutionary biomechanist at the Royal Veterinary College in the United Kingdom, wondered if something similar was going on with the elephant's toe. An expert in elephant locomotion, he had for years collected and preserved elephant feet—flesh and all—from animals that died in zoos. The animals ranged in age from newborns to those in their 50s. He had been performing computed tomography (CT) scans, which use x-rays to image tissues in slices to get 3D pictures of them, and other studies to understand how the feet worked, when he noticed that the cartilaginous lump often became denser, like bone, as each elephant aged. The lump could be up to 15 centimeters long and 6 centimeters wide, and it really did seem like it could work like a toe, he says. It's in the same position as the panda's thumb, but it's embedded in cushiony tissue called a fat pad.
Though they are not visible, an elephant's real toes are oriented somewhat vertically, so that the animal is actually walking on tiptoe, with the wrist and heel off the ground. At first glance, the extra toe seems to be too high off the ground to bear weight or do much good. But by putting some of the collected elephant feet in a device that made it seem as if the foot was supporting the elephant's weight and imaging them with additional CT scans, Hutchinson and his colleagues showed that the faux toe also acts to support weight, as they report online today in Science. "The extra digits do change position and come into contact with the ground," says Elizabeth Brainerd, a functional morphologist at Brown University who was not involved in the study.
To trace the evolution of the extra toe, Hutchinson and colleagues performed CT scans on feet of tapir-like species representing the earliest elephant-like mammals and on more recent elephant fossils. They found no evidence of the extra toe in 50-million-year-old fossils, which appeared to walk flat-footed, leaving no room for the sixth toe. Those animals likely spent most of their time in the water. But by 40 million years ago, the more recent fossils had telltale signs of this sixth toe. At that time, elephants were getting larger and becoming more land-based. Their feet were changing to better support their weight, with an expansion of the fat pads.
Although extra fingers and toes sometimes arise as genetic anomalies, and are even common in certain cat breeds (a condition known as polydactyly), Hutchinson thinks it was easier for the sesamoid bone to be recruited for extra support than for a true sixth toe to evolve in elephants. Making a sixth toe would have required a revamping of the complex developmental program that leads to the formation of the foot, he explains.
The sesamoid bone came in handy for the elephant, notes Vickaryous. "Gigantic body forms require innovative adaptations to cope with large increases in body mass." The researchers are investigating whether other very large animals, such as sauropod dinosaurs, had similar innovations.
Many scientists think that the study of anatomy is past its prime. But that's not true, says Marcelo Sánchez, an evolutionary morphologist at the University of Zurich in Switzerland. "Even animals as 'well known' as elephants can be subject of exciting, new discoveries, the study of which provides major insights into evolution."