Why elephants do not lose their skin

The African elephant is known for its thick, wrinkled skin. But take a closer look and you'll see a complex network of tiny crevasses that make this powerful mammal's skin look like cracked mud or damaged asphalt.

The purpose of these cracks is not a mystery. An elephant has no glands of sweat or sebum; it is covered with skin in the water or mud to stay cool. The micrometric cracks in the skin retain 10 times more moisture than a flat surface, thus helping the animal to regulate its body temperature. They also help the mud to adhere to the skin, which protects against pests and sunlight.

Now, a team of researchers think they have discovered that these cracks are formed under the effect of the stress due to the flexion of the skin and not to its contraction. Their explanation, released Tuesday in Nature Communications, could even treat a common human genetic skin disorder.

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Michel Milinkovitch, an evolutionary biologist at the University of Geneva in Switzerland, knows a little about cracked beast skin. In 2013, he was part of a team that discovered that the scales of a crocodile's face and jaw were actually skin folded and healed. When he went on to investigate the cracks in the elephant skin, he was expecting to see a process similar to the work.

This insight was stimulated when Dr. Milinkovitch's team examined elephant skin samples under a microscope. They discovered that just under the thick top layer was a lower layer of papillae, the same kind of small protuberances found on the human tongue. These projections form a vast web of peaks and thorny valleys. This seemed to support their hypothesis on the formation of cracks.

"We thought," Oh, it's a simple story, "he said. "If we reduce something that is attached to a geometry like this, the cracks will appear in the valleys below."

But when they computer simulated the growth of elephant skin, they found that the skin did not crack as expected. Reducing alone would not take into account the skin patterns of an elephant.

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Instead, the simulations suggested that the skin broke due to the stress of bending. Obviously, the skin thickened as the elephants aged, to the point of collapsing under their own weight. But why was not clear yet.

Further study of cutaneous tissue revealed that dead elephant skin cells resembled those of humans suffering from ichthyosis vulgaris, a disease found in one in 250 people and preventing skin loss. In humans, the condition for which there is no curative treatment causes dry, thick scales on the surface of the skin that are usually treated with moisturizers.

But among African elephants, hanging on to all those dead skin seems to be beneficial: the lack of loss results in skin formation as the elephant ages, eventually bending and shattering. on the papillae layer, which allows the animal to stay cool.

"If the skin tapered, it would never be thick enough to generate stress inside the small valleys of this elevation network, and cracks would not appear," he said. said Dr. Milinkovitch. "In humans, this is not a useful problem."

Further research is needed to determine whether the lack of skin that comes off in elephants shares a genetic basis with ichthyosis vulgaris in humans. But the confluence of conditions might suggest a way forward to treat the disease.