The microornamentation in velvet black regions of the Gaboon viper skin is unique among snakes. Leaf-like microstructures with both nanoridges and hair-like nanoprotuberances that coincide with black skin colouration have never been described before. Close relatives of the West African Gaboon viper, the western many-horned adder, Bitis caudalis51 and the mountain adder, B. atropos57, feature only a less complex dorsal pattern of lower microscaled elevation with pits. This structuring is similar to that of the pale regions of B. rhinoceros. Location of the leaf-like microstructures in the West African Gaboon viper and their dimensions in the range of visible light wavelengths suggest the contribution of microornamentation to the velvet black appearance. Gold-palladium coating on relatively flat ventral scales turns it to the metallic appearance. In contrast to this, after coating of dorsal black and pale scales, the colouration contrast remains because coated black sites remain black. Consequently, the surface structures of the black scales must be responsible for the velvet black appearance or at least enhance it.

According to the conservation law, incident light is reflected, transmitted, or absorbed by a sample. Absorbance ability is important for surface′s dark appearance. The theoretical darkest surface, an ideal black body, is a total absorber, which absorbs incident light and does not reflect or transmit any radiation58. Since pale scales reflect and transmit more light than black scales, especially the wavelength range 400–700 nm, black scales must absorb more light than pale scales. According to Kirchhoff′s law of thermal radiation, at thermal equilibrium, the emissivity of a body or surface equals its absorptivity58. In the West African Gaboon viper, the resulting emitted heat of the absorbing black areas was detectable by IR imaging. As there is no solid evidence in the literature that the arrangement and material of adjacent scales are different in snakes, the faster heat up process of black scales in comparison to pale scales must be solely caused by their pigmentation and surface structure.

The hierarchical microornamentation of the West African Gaboon viper′s black scales gains the pigmental absorbance. Incident light is reflected multiply and scattered by surface irregularities in the nano- and micrometre range (light trapping). In any case, one part of the incident (or previously by the structures scattered or reflected) light could be absorbed by dark pigments that are deposited within the uppermost layers of skin59. However, our data show that absorbing pigments are not the main reason for the black appearance of the snake scales. The refractive index of a material and its relation to that of the surrounding medium is the decisive factor which defines whether electromagnetic radiation is reflected or led into the material where it can be absorbed. The refractive index of the snake′s epidermal material mainly consisting of α- and β-keratin60 can be estimated as that of the keratin of birds (refractive index 1.56)61,62, which is lower than that of metal. As known from the structures on the wings of butterflies63, the intrinsic refractive index of the epidermis of black scales of B. rhinoceros can be shifted by the nanostructures in wavelength dimension. Thereby surface structuring influences the rate of reflection and transmission which corresponds to the absorption. In our measurements, metal coated black snake scales maintained their black colouration and became even less reflective than the untreated scales. Considering the high refractive index of metal, the ultrablack appearance might not be due to the effect of pigment absorption. The dark colouration must be rather caused by a structure-based increase of the refractivity.

In order to confirm the nanostructures′ responsibility for the appearance of black snake scales, we compared the angle dependence of scattering of snake scales with calculated data from an established model for reflection of a surface with microscopic v-shaped cavities56. The model parameters were selected to obtain the best fit to the biological surface. Whereas the model provided an approximation for the scattering properties of pale scales, particularly with homogenous distribution of cavity angles, for the black surfaces it failed and even led to contrary results with regard to the scattering at small and large angles. These differences can be only explained by the effect of nanostructures, which is not considered in the model of Oren and Nayar56.

In both black and pale scales angle-dependent scattering was nearly similar in both orientations. This can be explained with the nearly isotropic arrangement of leaf-like structures, crests and nanoridges. Slightly differences can be explained by a low degree of orientation of micro- and nanostructures toward the axis of the snake's body.

The term “velvet-like” is used in many contexts, in scientific publications as well as in the everyday language. Velvet tissue fascinates in many respects. Its intrinsic gloss (velvet gloss) only occurs at certain angles64. At all other angles the low reflecting velvet shows high colour saturation and appears darker than other materials (velvet effect). Similar characteristics also occur in plants. At petals of Viola tricolor hortensis (pansy) and Primula sinensis papillate structures that are filled with a pigment containing cellular fluid and covered by a pellucid reflecting cuticle lead to a deep colour that change into a velvet-like gloss depending on illumination and viewing angle65,66. The West African Gaboon viper′s black scales feature a low-reflecting black colouration for a large range of incident wavelengths. Regarding this, black scales have a velvet effect. However, low reflectance is maintained for any viewing angle. According to this, the optical properties of Gaboon viper′s black scales differ from that of glossy velvet. In animal kingdom a hierarchically structured surface, like the microornamentation at black scales of the Gaboon viper causing low-reflectance, is unique and only comparable with the structures responsible for ultra-black in butterflies. Papilio ulysses features a hierarchical structured surface which enhances pigmental blackness63. A microscopic lattice of struts and walls between a periodic ridging of pitch about 2–3 μm and densely distributed lattice of cuticle underneath these structures scatter incident light towards the diffuse distributed pigmentation. Additionally, nanoscopic ridges on the ridging in subwavelength range are hypothesized to function as impedance matching elements63. In contrast to the butterfly, the West African Gaboon viper's microornamentation is isotropic at micro- and nanoscale and not arranged in several layers. This feature causes suppression of specular reflection. However, the physical principle of the guiding of incident light towards embedded absorbing pigments is fundamental for both the snake′s and butterfly′s system. This effects the low-reflectance of these surfaces.

The benefits of structures enhancing black colouration in snakes seem to be obvious. However, biological surfaces are generally multifunctional. Ecology and lifestyle of species may suggest the functions of their morphologic features but are always speculative. The West African Gaboon viper inhabits West Africa from Togo to Guinea67. It lives at margins of and within the tropical rainforest, in swamp areas and near water streams35,67. But also anthropogenic habitats like plantations and secondary forest are inhabited. B. rhinoceros is recognized as separate species only recently68,69,70 and has previously been a subspecies of Bitis gabonica. As its relative B. gabonica71, B. rhinoceros is a terrestrial nocturnal ambusher sitting for hours in the vegetation and waiting for birds and mammals, especially in nightfall30. Prey is captured with one final fast strike and killed by the injection of venom by the fangs72. If the snake feels threatened, it hisses loudly by the expiration of each breath35,72. The camouflage of the snake on the rainforest floor plays a key role for hunting as well as for the prevention of becoming food to other animals.

Taking into account the West African Gaboon viper's biology, velvet black colouration suggests to be a tool for a perfect camouflage. The disruptive geometrical colouration pattern of pigment based colours conceals the body contours and makes the snake on the ground of the forest indistinguishable. Such colouration was interpreted as common concealing pattern in large stationary snakes73. In addition, the West African Gaboon viper is camouflaged by areas with alternating reflectance due to different microornamentation at the scale surfaces, the obliterative shading. This pattern of alternating reflectance coincides with that of colouration. Thus, the viper features black spots with low reflectance, that purport spatial depth and pale areas that reflects the incident light. This high-contrasting pattern is a perfect camouflage in the richly-structured forest ground with variations of light and shade originated by the canopy. Beside optical characteristics of skin and visual system of the observer, colour appearance of an animal also depends on the spectrum of ambient light7,74,75. In the West African Gaboon viper reflectance contrast of pale and black spots is higher for short wavelengths (corresponding with bluish and greenish colour) than for longer ones. Data about ambient illumination have been collected in numerous studies. Under the open sky, regardless whether cloudy or clear, radiance spectra are dominated for wavelengths longer than 500 nm76,77. Under these conditions differences in reflectance between black and pale spots would be rather low. However, considering the snake′s habitat, spectral reflectance characteristics of its skin are quite beneficial: Radiance spectra of forests at many continents show a radiance intensity peak at 550 nm (green), due to direct transmission and reflection of the canopy77,78. Under clear sky, woodlands and forests with larger gaps in their canopies shift the spectra of ambient light towards shorter wavelength (blue)77. Low sun angles at dusk and dawn in combination with the absence of clouds lead to deficiency of the wavelength range from 630 to 750 nm and consequently overrepresentation of bluish light and long wavelengths corresponding to the red colour. Although these spectral data77 do not include the viper′s African habitat, based on similarity of data in previously forest studies, the results can be presumed for the African rainforest. All mentioned spectra share a high contribution of short wavelengths. Especially this range is reflected differently by pale and black areas. This yields a high-contrast in reflectance, which camouflages the snake especially during ambushing in the evenings. But the system also provides high-contrasting appearance at day time in the greenish light of the forest or the bluish light of woodland and open anthropogenic vegetation.

In our measurements dorsal pale and black scales were transparent in the measured UV range, especially for shorter wavelengths. Because radiation of this spectral range was less reflected by these scales, it can be assumed that UV radiation is at least able to pass through the shed upper skin layers. For a species that lives at the bottom of forests and woodlands, where the main part of the destructive UV radiation is reduced; such properties may ensure sufficient UV supply for vitamin D3 synthesis. Our findings do not correspond with data on transmissivity78 and reflectance79,80 of the epidermis in other reptiles showing that reptile skin strongly absorbs the ultra violet and transmit the visible range of the solar spectrum. Comparisons with transmittance spectra of colubrid, boid and pythonid snakes show that even the epidermis of arboreal species is not nearly as transparent for ultra-violet and blocks less visible and infra-red radiation as the epidermis of the West African Gaboon viper78.

However, different characteristics of reflectance in dorsal and ventral skin of different lizard and snake species suggested that, beside solar protection, also thermoregulation and concealment are selective factors for epidermal reflectance in reptiles79,80.

The West African Gaboon viper′s low reflecting properties were preserved even when scale structuring was covered with gold-palladium. Thus, the structure based velvet black effect could also be potentially transferred to other materials. In science and technology such high-absorbing and low-reflecting surfaces are of prime importance and can be applied in many fields, such as solar thermal collectors or optical systems. Increasing interest in such materials is reflected in a growing number of recent publications about ultra-black surfaces that avoid any reflectance and absorb nearly any incident light81. The currently darkest commercially established surface is a nickel phosphorous alloy82. The low-reflecting properties of this material can be enhanced by modifications of the surface topography. Different phosphorous contents cause different surface structures appearing in the acid etching process. The reflectance can be less than 0.4%82. The currently darkest surface is made of vertically aligned carbon nanotubes (VACNT) which reflect 0.045% of incident light83. A large amount of recent publications studied the optical properties and optimization of fabrication of these materials84,85,86. The structures are fragile. But research continually creates new ultra-dark high absorbing materials such as silicon nanowires arrays (SiNWAs)87, laser textured metals88, nanoporous anodised aluminium oxide-coated polycarbonate surfaces89 and graphene sheet stacks90. Overall, these studies demonstrate the strong influence of surface topography on material′s reflectance and absorption. The surface topography of the West African Gaboon viper's velvet black scales resemble the ultralow reflectance ultrafast laser textured metal surfaces of Iyengar and his co-workers88. Both surfaces feature leaf-like structures or pillars88 in a similar range with an average diameter of about 20 μm and an average height of about 30 μm. Both materials feature nearly angle independent reflection characteristics. Further, in both cases the coverage with an absorbing layer decreases the reflectance.

Nonetheless, the West African Gaboon viper′s microornamentation might enhance the darkness of artificial materials. Especially varying orientation of the nanoridges could be an inspiration for engineering ultra-black designer. The microornamentation on the snake′s velvet black scales is a further example that the same physical law applies to both nature and technology and leads consequently to similar constructions. Regarding to the relation of durability and weight the fine shaped structures of the West African Gaboon viper are not inferior to artificial ultra-black surfaces considering that the snake slithers several month until the next shedding with its fine structured skin on the undergrowth.