How the African elephant has cracked its skin to refresh itself

A complex network of tiny crevasses adorns the skin surface of the African bush elephant. By retaining water and mud, these micrometer channels greatly help elephants regulate their body temperature and protect their skin from parasites and intense sunlight. Today, researchers from the University of Geneva (UNIGE) in Switzerland and from the Swiss Institute of Bioinformatics (SIB) report in the journal Nature Communications that the cutaneous channels of African elephants are true fractures of the outer cutaneous layer fragile and deficient in desquamation. Scientists have shown that the hyperkeratinized skin of the elephant grows on a network of millimeter elevations, causing fractures due to local mechanical stresses due to bending.

African elephants love to bathe, spray and wallow in the mud. These behaviors are not just for fun. Indeed, African elephants do not have the sweat and sebum glands that allow many other mammals to keep their skin moist and supple. In addition, due to their large size and their warm and dry habitat, African elephants can avoid overheating only by evaporation from the water that they collect in and from. on their skin. By covering themselves with mud, African elephants also avoid relentless parasitic attacks and excessive exposure of their skin to solar radiation. A very careful inspection of the African elephant skin indicates that in addition to its characteristic wrinkles, the integument is deeply sculpted by a complex network of tiny, interconnected crevices. This thin pattern of millions of channels prevents the formation of applied sludge and allows to extend and retain five to ten times more water than a flat surface.

Through skin samples provided by Swiss, French and South African scientists and museums, a multidisciplinary team led by Michel Milinkovitch, Professor in the Department of Genetics and Evolution of the UNIGE Faculty of Science and Head of group at SIB Swiss Institute of Bioinformatics, shows today that the skin channels of African elephants are real cracks caused by local bending stress due to hyper keratinization of the epidermis , loss of loss formation and growth on a network of millimeter elevations of the skin. Using a custom computer model, Antonio Martins, Ph.D. student of Milinkovitch's lab, showed that the combination of these three parameters is enough to cause the accumulation of bending stresses. mechanical between elevations of the skin during the gradual thickening of the skin until the formation of cracks.

Folding rather than tensile cracking

The visual appearance of the cracking pattern on African elephant skin bears a striking resemblance to the cracks caused by tensile stress in desiccant mud, damaged asphalt, polar landscapes of the Earth and of Mars, as well as in spectacular geological formations such as basalt columns of the Giant's Causeway in Northern Ireland and the Devil's Stack in Eastern California. It was therefore tempting to hypothesize that the skin cracks of the African elephant are generated by the narrowing of the skin. However, the computer simulations performed by the Swiss team indicate that the withdrawal generates heterogeneous distributions of cracks that propagate on the taste buds, whereas cracks on the actual skin occur almost exclusively along the hollows, rejecting the cracks. 39, hypothesis of "shrinkage cracking". On the other hand, when researchers have implemented in their computer simulations the thickening of the skin (new layers are constantly added to the base of the epidermis), a mechanical bending stress occurs. is accumulated in the valleys, causing the keratinized epidermis-only cracks -valleys, reinforcing the new results "flexural cracking".

One of the goals of developmental biology is to describe how forms and forms are generated by the differentiation and growth of heterogeneous tissues, controlled by differential gene expression across the body. However, complex morphologies can also emerge as mechanical instabilities (folds, wrinkles) caused by inadequate growth between adherent tissue layers. Famous examples are the loop of the intestine and the folding of the cerebral cortex. Although cracking, another type of mechanical instability, is common in non-living materials, it is much less ubiquitous in biological systems. Milinkovitch's laboratory showed a few years ago that the pattern of scales of the crocodile's head formed during the folding of the skin, and that their dynamic of propagation and the formation of a polygonal pattern were similar to those cracking in the mud drying. Here, the Swiss team shows for the first time that African elephant skin presents a multitude of channels that are real physical cracks of its keratinized epidermis.

Elephants helping to understand a human pathology

The thickening of the keratinized epidermis of the African elephant is due to an imbalance between its formation at the base and its removal on the surface of the skin. Strikingly, the Swiss team shows strong similarities between the normal cutaneous morphology of African elephants and that of humans with ichthyosis vulgaris, a common genetic disorder (affecting approximately 1 in 250 people), known to have an impact on causes it to dry and flaky skin. If it were validated by detailed comparisons of molecular and cellular biology, then this equivalence would make a remarkable link between a human pathological condition and the skin of an emblematic species of pachyderm. "This correspondence also shows that similar mutations occurred independently in evolutionary lineages of humans and elephants were unfavorable in the first and adaptable in the second," says Michel Milinkovitch.

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More information:
António F. Martins et al, The locally curved geometry generates bending cracks in African elephant skin. Nature Communications (2018). DOI: 10.1038 / s41467-018-06257-3