Cracking the reason for channels on elephant’s skin

October 2nd, 2018

An intricate network of minuscule crevices adorns the skin surface of the African bush elephant. Today, we report in the journal Nature Communications that African elephant skin channels are true fractures of the animal brittle and desquamation-deffcient skin outermost layer. We show that the elephant hyperkeratinised skin grows on a lattice of millimetric elevations, causing its fracture due to local bending mechanical stress.

African elephants are well-known to love bathing, spraying, and mud-wallowing. These behaviours are not just for  fun. Indeed, African elephants lack the sweat and sebum glands that  allow many other mammals to keep their skin moist and flexible. Furthermore, because of their huge body size, and their warm and dry habitat, African elephants can avoid over-heating only by loosing  calories through evaporation of the water they collect in and on their  skin. By covering themselves with mud, African elephants  also avoid the attacks of relentless parasites and the  excessive exposure of their skin to solar radiations. Very close  inspection of the African elephant skin indicates that, in addition to  its characteristic well-visible wrinkling, the integument is deeply  sculptured by an intricate network of minuscule interconnected  crevices. This beautiful fine pattern of millions of channels prevents  shedding of applied mud and allows spreading and retention of 5 to 10  times more water than on a flat surface.

Cracking the skin
Thanks to skin samples provided by scientists and museums in  Switzerland, France and South Africa, a multidisciplinary team lead by  Michel Milinkovitch, professor at the University of Geneva  (UNIGE), Switzerland and Group Leader at the SIB Swiss Institute of Bioinformatics,  shows today that African elephant skin channels are true cracks due to  local bending stress caused by the epidermis being simultaneously hyper-keratinised, shedding-deficient, and growing  on a lattice of millimetric elevations of the skin. Using a custom  computer model, Antonio Martins, PhD student in  the Milinkovitch’s laboratory, showed that the combination of these  three parameters is sufficient to cause mechanical bending stress to  accumulate  in between the skin elevations during the progressive thickening of  the skin until cracks are formed.


Bending rather than tensile cracking
The visual aspect of the cracking pattern on the skin of the African  elephant bears a striking resemblance, albeit at very different spatial  scales, to cracks caused by tensile stress in drying mud, in damaged  asphalt, on Earth’s and Mars’ polar landscapes,  as well as in spectacular geological formations such as the basalt  columns of the Giant’s Causeway in Northern Ireland and of the Devil’s Postpile in eastern California. It was therefore tempting to hypothesise  that the African elephant skin cracks are generated  by shrinkage of the skin. However, the computer simulations performed  by the Swiss team indicates that shrinkage generates  heterogeneous distributions of cracks that propagate over papillae,  whereas cracks on the real skin occur near-exclusively along the troughs, rejecting the ‘cracking by shrinking’ hypothesis. On the other  hand, when the researchers implemented in their computer simulations  the thickening of the skin (new layers are constantly added at the basis  of the epidermis), mechanical bending stress accumulated  in the  valleys causing the keratinised epidermis to crack only in the  micro-valleys, reinforcing the new ‘cracking by bending’ results.

Physical processes in biology
One focus of developmental biology is to describe how forms and shapes  are generated by heterogeneous tissue differentiation and growth,  controlled by differential gene expression across the body. However,  complex morphologies can also emerge as mechanical instabilities  (folds, wrinkles) caused by mismatched growth between adherent tissue  layers. Famous examples are the looping of the gut and folding of  the cerebral cortex. While cracking, another type of mechanical  instability, is commonplace in non-living materials, it  is far less ubiquitous in biological systems. The Milinkovitch’s lab  showed a few years ago that crocodile head scales form through skin folding. The dynamic of fold propagation and  polygonal scale pattern formation are analogous to those of cracking in  drying mud. Here, the Swiss team shows  for the first time that the African elephant skin exhibits a myriad of  channels that are not only looking like cracks, but are true physical  cracks of its keratinised epidermis.


Elephants helping understand a human pathology ?
Thickening of the African elephant keratinised epidermis is due to an  imbalance between its formation at its base and its shedding at the skin  surface. Strikingly, the Swiss team shows strong similarities between  the normal skin morphology of African elephants and that of humans affected by ‘ichthyosis vulgaris’, a common genetic  disorder (affecting about 1 in 250 people) that is known to impact  desquamation (shedding of the skin) and cause dry, scaly skin. If  validated by future detailed molecular and cell biology comparisons, this equivalence would then make a remarkable link between  a human pathological condition and the skin of an iconic species of  pachyderm. “This correspondence would also demonstrate that similar mutations that occurred independently  in the evolutionary lineages of humans and elephants turned out to be unfavourable in the former and adaptive in the latter” says Michel Milinkovitch.

For additional informations, please download the original article here:

Locally-Curved Geometry Generates Bending Cracks in The African Elephant Skin
Martins, Bennett, Clavel, Groenewald, Hensman, Hoby, Joris, Manger & Milinkovitch*
Nature Communications (2018)

*Corresponding author: michel.milinkovitch@unige.ch

A video illustrating our findings is available here.