Snake Scale Keels

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Snake scale keels: a three-dimensional investigation of function Justin Bernstein: Rutgers University-Newark, Boyden Hall, Newark, New Jersey 07102 Expected Graduation Date: May, 2022 Squamate reptiles (lizards and snakes) represent a large group of organisms with great levels of diversity in their morphologies, genetics, diets, behaviors, reproductive strategies, and habitat preferences. Before the advent of molecular techniques, traditional taxonomy used morphological data to describe and delimit species. Scale counts, scalation, the presence and absence of structures (e.g. tubercles, horns, keels), dentition, and size ratios of body parts have all been used to increase the known diversity of reptiles. However, despite centuries of morphological studies and recent advancements in molecular and computational technologies, the function of particular morphological characters still remains largely unknown, such as keels. Keels, in squamate reptiles, are raised ridges that run lengthwise on the surface of the scale and range in presence/absence, number, and positioning between species and on a single organism. Keeled scales are largely associated with semi-aquatic lifestyles, so we ask “what effect do keels have on moving water around them?” We hypothesize that keeled scales change the hydrodynamics of scales and reduce drag to facilitate swimming. We first microCT scanned 15 snakes with smooth and keeled scales from different habitats (e.g., terrestrial, aquatic) and 3D printed the CT scans. Printed scale models were then placed in a flow tank for volumetric particle image velocimetry analysis (3D PIV). These results suggest that keels accelerate water flow around scales and may help to reduce drag, facilitating swimming (Figure 1). Imaging of scale surface topography (Gelsight) shows diversity of scale microstructure (Figure 1). The results of this study represent a new hypothesis for the function of snake keels and the first study to assess the functionality of snake keels. Our results show that having keels or ridge like structures on scales can facilitate swimming by reducing friction and drag. This function may be essential for the survival of taxa that frequent bodies of Figure 1. PIV and Gelsight results for 4 specimens in this study. A) water to escape predators or catch Left to right: Agkistrodon contortrix, Nerodia sipedon, Lampropeltis getula, Myrrophis chinensis. B) Microstructure of quick-moving prey like fish. As snake scales of above snake using Gelsight. Numbers indicate the highest keels are convergent and found in a point (µm); hot/cold colors represent higher/lower areas variety of terrestrial, arboreal, respectively. C) Water structure around scales (PIV). Ring structures indicates vortices which reduce friction and drag (keeled semiaquatic, and marine taxa, there are scales: Agkistrodon and Nerodia). likely multiple functions for the evolution of these structures. Indeed, future research will include testing the function of terrestrial and arboreal keeled snakes, but the results of this study sheds light on how the evolutionary “answer” of evolving a keeled structure may allow an organism to enter an uninhabited niche (aquatic systems) and provides insight into the age-old question of what the functions of scale keels are. .

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