Derived Morphology of Ventral-Scale Nanostructure in the Sidewinder (Crotalus cerastes)

Diaz-Cruz, Kelimar

School of Physics

Georgia Institute of Technology

Atlanta, GA USA

Mendelson III, Joseph R.

Zoo Atlanta

Atlanta, GA USA

School of Biological Sciences

Georgia Institute of Technology

Atlanta, GA USA

Li, Tai-De

Advanced Science Research Center

City University of New York

New York, NY USA

Ventral surfaces of snakes typically contact the substrate during locomotion, with lifting being an important component especially in sidewinding. Studies of the functional and physical properties of the ventral scales of many snakes have found no clear ecological or phylogenetic patterns to contextualize observed variation. We used Atomic Force Microscopy to study the 3-D nanostructure of the ventral scales of the Sidewinder, a species known for distinctive lateral and lifting locomotion and exceptional abilities on granular substrates. Examination of a diversity of related pitviper species documented a conserved morphology of regularly ordered, uplifted, sharply pointed, posteriorly protruding nanostructures with typical dimensions of 3 µm x 0.5 µm x 0.15 µm (length, width, height); small epidermal pits with typical diameter of 0.3 µm but of unknown function are present as has been found in other snakes. This morphology is generally representative of the ventral nanostructure anisotropy seen across virtually all snakes, clearly associated with the plesiomorphic mode of anteriorly directed locomotion. Our outgroup comparisons identified the distinctive morphology of Sidewinders as a derived condition, with a flat surface and modified (1 µm x 1 µm x 0.05 µm), blunted, posteriorly oriented vestiges of the rearward projections of other pitvipers; epidermal pores are present, but larger (300 nm diameter) than any snake yet examined. This unique, phylogenetically derived 3-D nanostructure with low directionality could provide isotropic friction for the derived pattern of locomotion of Sidewinders involving lateral forces, abrupt directional reversals, and lifting.

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