Scale Shape Enables Limbless Movement

green snake on brown wood against colorful background

Biological Strategy

Naja nigricollis

Eleanor Banwell

Image: Herman Pijpers / Flickr / CC BY - Creative Commons Attribution alone

Move in/on Solids

To obtain needed resources or escape predators, some living systems must move on solid substances, some must move within them, and others must do both. Solids vary in their form; they can be soft or porous like leaves, sand, skin, and snow, or hard like rock, ice, or tree bark. Movement can involve a whole living system, such as an ostrich running across the ground or an earthworm burrowing through the soil. It can also involve just part of a living system, such as a mosquito poking its mouthparts into skin. Solids vary in smoothness, stickiness, moisture content, density, etc, each of which presents different challenges. As a result, living systems have adaptations to meet one, and sometimes multiple, challenges. For example, some insects must be able to hold onto both rough and slippery leaf surfaces due to the diversity in their environment.

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Reptiles

Class Reptilia (“creeping”): Lizards, crocodiles, turtles, snakes

Reptiles retain some of the key characteristics that first enabled vertebrates to live permanently on land. They have dry skin covered in scales made of keratin that help prevent water loss. (Amphibians still need to stick close to water to keep their skin moist.) Many reptiles reproduce by laying eggs that are watertight and have a yolk, so eggs can develop on land while staying moist and nourished. Reptiles are also ectotherms, meaning they don’t produce their own body heat. This means they burn through energy much more slowly than “warm blooded” creatures of the same size. It also means that they need to keep warm to keep active, which is why you might see them spending a seemingly inordinate amount of time simply basking in the sun.

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Scales of snakes enable limbless forward movement through directional friction.

Snakes are limbless animals and so must move over a variety of surfaces using only their flexible and slender bodies. During typical slithering movements, a large portion of a snake’s body is in contact with and sliding over the ground at all times. This causes friction, which can damage the animal’s skin. In order to manage this friction, snake scales have a number of features that increase slipperiness. The scales on the underside of the snake are smoother than those on the sides and back, and they may also release a lubricant. However, in order to move, snakes must be able to grip the surface they are travelling across and push against it. In order to do this, they require a high level of friction between their scales and the surface: snakes must generate both low and high friction during travel.

Studies on a variety of different snake species have demonstrated that the friction generated by sliding depends on the direction of travel. Belly scales have small “micropatterns” that create arrays of v-shaped feathered trailing edges. The tips of these v-shapes point towards the tail of the snake and, in some species, they are raised at the tip. In this way, as the snake slides, the surface moves easily up and over the raised tips, but in the reverse direction they act like the pawl of a ratchet, snagging the surface and resisting movement in the opposite direction. The scales also have series of parallel grooves running from the head of the snake towards its tail. These grooves work like rails, limiting surface contact area and friction as long as the snake in moving in a parallel direction to the grooves, but generating high friction and grip when the snake moves sideways. In general, the scales on the undersides of snakes are low friction when the animal is moving forward, but generate high friction and grip when it moves from side-to-side or backwards.

Snakes have multiple methods of moving, including a side-to-side slither, concertina-like (accordion-like), and “sidewinding” (flinging themselves over the ground). They have remarkable levels of control over their muscles and may even be able to control each scale’s angle and attitude independently. In this way, they are able to combine fine muscle movement with the structure of scales to generate forward travel by bracing against and pushing off of the ground in a variety of different ways.

Image: Filippov et al. / CC BY - Creative Commons Attribution alone

Patterning reduces friction in the forward direction but increases it sideways, enabling snakes to move forwards without limbs

Image: Danny Steaven / CC BY SA - Creative Commons Attribution + ShareAlike

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References

“The absence of extremities in snakes has strong tribological consequences for the material of their skin. The ventral body side of the snake is in continuous contact with the substrate. Therefore ventral scales must have optimized frictional properties. In order to generate propulsion during locomotion high friction and to slide along the substrate low friction must be generated [1]. Additionally, a minimum abrasion rate is necessary to enable long lasting optimal frictional properties [1]. Thus, to facilitate effective locomotion, the ventral body surface needs to possess anisotropic frictional properties, which can originate from macroscopic structures [2,3] such as the overlapping scales. The arrangement of scales provides the possibility of interlocking between their edges and asperities of the substrate. Also microscopic structures of the skin scales, so called microornamentation [1,4-13], and specific adaptations of the material architecture of the skin, like highly ordered embedded fibers [14], which can potentially influence material properties [15,16], might contribute to the frictional anisotropy.” (Baum et al. 2014:83)

Journal article

Friction behavior of a microstructured polymer surface inspired by snake skin

Beilstein Journal of Nanotechnology, 5: 83–97 | 24/01/2014 | Baum MJ, Heepe L, Gorb SN

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Reference

“The first striking feature of all skin areas along the snake ventral surface involved in locomotion is the presence of highly organized arrays of ‘micro-hairs’(Fig. 1). Two basic patterns are observed: either long parallel rows with light modulations along the rows and well defined periodicity of about 2 μm or triangular arrays with overlapped sheets of homogeneously oriented microfibrils (Fig. 1). In both cases, the microfibrils are oriented in the direction of motion with their ends having caudal orientation (toward the tail of the animal). All microfibrils possess uniform lateral sizes with diameter 100–400 nm and height 30–100 nm.” (Hazel et al. 1999)

Journal article

Nanoscale design of snake skin for reptation locomotions via friction anisotropy

Journal of Biomechanics, 32(5): 477–484 | 01/05/1999 | Hazel J, Stone M, Grace MS, Tsukruk VV

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Reference

“Our results show for the first time, that there is a significant anisotropy of friction coefficient for the ventral scales with higher coefficients for sideward directions.” (Berthé et al. 2009:317)

Journal article

Surface structure and frictional properties of the skin of the Amazon tree boa Corallus hortulanus (Squamata, Boidae)

Journal of Comparative Physiology A, 195: 311-318 | 01/03/2009 | Berthé RA, Westhoff G, Bleckmann H, Gorb SN

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Reference

Journal article

The mechanics of slithering locomotion

Proceedings of the National Academy of the United States of America, 106(25): 10081-10085 | 23/06/2009 | Hu DL, Nirody J, Scott T, Shelley MJ

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Reference

Journal article

Modelling of the frictional behaviour of the snake skin covered by anisotropic surface nanostructures

Scientific Reports, 6: 23539 | 23/03/2016 | Filippov AE, Gorb SN

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