The Elephant’s Neglected Sixth Toe

One of the elephant’s most notable features is its legs — thick, sturdy pillars that give way to broad, flat feet. Indeed, elephants have large pads of fat on the bottom of their feet, which is what gives them their flat-footed appearance. A new study published in Science, however, reports that these pads may actually hide a sixth toe that has long been overlooked by zoologists, veterinarians, and museum curators alike — meaning that elephants actually walk on tip-toe.

The study was led by John Hutchinson, an evolutionary biochemist at the Royal Veterinary College in London. He and his team posit that the extra digit, which is 5-10 cm long, helps support elephants’ weight by stiffening the heel of their cushiony foot pads.

The kneecap, or patella, is an example of a sesamoid bone.

This sixth toe is a backward-pointing bone that has historically been dismissed as a superfluous piece of cartilage.

Hutchinson and his colleagues believe that the extra phalange is an example of a sesamoid bone, a usually short or irregular bone that is embedded within a tendon. Sesamoids serve the function of protecting and enhancing the movement of tendons as they pass over joints.

Some creatures, however, have adapted their sesamoids for other purposes. The giant panda, for example, uses a modified sesamoid to eat more efficiently. In addition to their five claws, pandas have an extra opposable digit on the base of their true thumbs. This second “thumb” is actually an enlarged sesamoid that allows the panda to more firmly grasp the bamboo it eats.

Pandas have a modified sesamoid bone that serves as an extra “thumb” for gripping bamboo

The elephant’s sixth toe may be a similar evolutionary phenomenon. Hutchinson and his team studied fossils, and found that the elephant’s earliest ancestors walked flat-footed and showed no signs of a sixth toe. These tapir-like creatures were smaller than present-day elephants, and likely spent most of their time in the water, like hippopotamuses. As they evolved into colossal beasts and adopted a more terrestrial lifestyle, elephants began to walk on tip-toe, which allowed them to straighten their legs and better support their swelling body mass. Fossils dating as far back as 40 million years ago demonstrate evidence for an emerging sixth toe, and the expansion of the fat pads on the elephant’s foot.

An artist’s depiction of Moeritherium, a prehistoric ancestor of elephants that likely led a semi-aquatic lifestyle

Hutchinson believes that, as they became larger, it was easier for elephants to co-opt the sesamoid bone for extra support rather than evolve a whole new toe. After all, the sesamoid bone was already conveniently there, whereas the evolution of a sixth toe would have required an entirely new pattern of foot formation.

This change in sesamoid function illustrates a broader evolutionary trend. As organisms become larger, they must acquire new adaptions to compensate for their increase in body mass. The researchers of this study are now looking for similar adaptions in other gargantuan animals, such as sauropod dinosaurs.

Researchers are now searching for similar features in other enormous animals, such as sauropods.

The elephant’s sixth toe reminds us that animals have all sorts of hidden complexities. Elephants are familiar creatures; we read about them in storybooks, view them as symbols of wisdom, and even train them to perform tricks. Now, just when we thought we had elephants all figured out, we discover that, contained within the soft pads of their large, wrinkly — and not-so-flat — feet, is an evolutionary marvel that makes possible their towering, formidable size.


*Note: This piece borrowed from Ed Yong’s original article for Nature News.

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