The Science of Snakeskin: Black Velvety Viper Scales May be Self-Cleaning
Whether you love them or hate them, snakes have long captivated our interest and imagination. They’ve spurred countless stories and fears, some of which may have even affected the course of human evolutionary history. We must admit, there is something a little other-worldly about their legless bodies, willingness to swallow and digest animals much bigger than them, and fangs and potentially fatal (or therapeutic?) venomous bites.
Not least of all, their scaly skin is quite mesmerizing and often laden with intricate and beautifully geometric patterns just perfect for camouflaging, regardless of whether they live high up in a tree, deep in murky waters, or on the forest floor. Snakeskin was the focus of recent research by the authors of this PLOS ONE study who sought to determine whether it has any special properties less obvious to the naked eye.
Please meet the West African Gaboon viper, Bitis gabonica rhinoceros (pictured above). Native to the rainforests and woodlands of West Africa, these large, white-brown-and-black snakes can be identified by large nasal horns and a single black triangle beneath each eye—nevermind that, because they also lay claim to titles for the longest fangs and most venom volume produced per bite. The pattern of their skin is intricate and excellent for camouflage, and the black sections have a particularly velvety appearance. These eye-catching characteristics intrigued zoology and biomechanics researchers from Germany, who decided to take a closer look.
In a previously published paper, the authors analyzed the Gaboon viper’s skin surface texture by using scanning electron microscopy (SEM), as well as its optical abilities by shining light on the snakeskin in different ways to see how it’s reflected, scattered, or transmitted. They found that only the black sections contained leaf-like microstructures streaked with what they call “nanoridges” on the snake scales, a pattern that has not been observed before on snakeskin. What’s more, the black skin reflects less than 11% of light shone on it—a lot less than other snakes—regardless of the angle of light applied. The authors concluded from the previous study that both of these factors may contribute to the viper’s velvet-like, ultra-black skin appearance.
In their most recent PLOS ONE paper titled “Non-Contaminating Camouflage: Multifunctional Skin Microornamentation in the West African Gaboon Viper (Bitis rhinoceros),” the authors conducted wettability and contamination tests in hopes of further characterizing the viper skin’s properties, particularly when comparing the pale and black regions.
To test the wettability of the viper scales, the authors sprayed droplets of water, an iodide-containing compound (diiodomethane), and ethylene glycol on the different scale types shown above, on both a live and dead snake, and then measured the contact angle—the angle at which a liquid droplet meets a solid surface. This angle lets us know how water-friendly a surface is; in other words, the higher the contact angle, the less water-friendly the surface.
As you can see in the graph above, the contact angle was different depending on the liquid applied and the type of scale; in particular, the contact angle on the black scales was significantly higher than the others, in a category that the authors refer to as “outstanding superhydrophobicity,” or really, really, really water-repelling. This type of water-repelling has been seen in geckos, but not snakes.
The authors then took some of the snake carcass and dusted it with a sticky powder in a contamination chamber, after which they generated a fog for 30 minutes and took pictures.
After 30 minutes of fogging, the black areas were mostly free of the dusting powder, while the pale areas were still completely covered with dust. The powder itself was also water-repelling, and so the authors showed that despite this, the powder rolled off with the water rather than sticking to the black areas of snake skin. Therefore, as suggested by the authors, this could be a rather remarkable self-cleaning ability. The authors suspect that the “nanoridges,” or ridges arranged in parallel in the black regions, may allow liquid runoff better than on the paler areas of the snake.
How does this texture variation help the snake, you ask? The authors posit that all these properties basically contribute to a better form of camouflage. If the snake were completely covered in one color, it may stand out against a background of mixed colors (or “disruptive coloration”), like that of a forest floor. If the black regions have fairly different properties from the paler regions, mud, water, or other substances would rub off in these areas and continue to provide the light-dark color contrast and variation in light reflectivity that helps the snake do what it does best: slither around and blend in unnoticed.
Spinner M, Kovalev A, Gorb SN, Westhoff G (2013) Snake velvet black: Hierarchical micro- and nanostructure enhances dark colouration in Bitis rhinoceros. Scientific Reports 3: 1846. doi:10.1038/srep01846
Spinner M, Gorb SN, Balmert A, Bleckmann H, Westhoff G (2014) Non-Contaminating Camouflage: Multifunctional Skin Microornamentation in the West African Gaboon Viper (Bitis rhinoceros). PLoS ONE 9(3): e91087. doi:10.1371/journal.pone.0091087
First image, public domain with credit to TimVickers
Remaining images from the PLOS ONE paper