Snake Scale Microstructure Phylogenetic Significance and Functional Adaptations

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Language: English

This study investigates whether micromorphological features of the oberhautchen of snake scales show adaptations to the animals' life habit. For the first time, the ventral scale microstructure of 41 snakes belonging to three different families (Pythonidae, Boidae, and Elapidae) is described. Species adapted to four different life habits (terrestrial, fossorial, arboreal, aquatic) were examined using scanning electron microscopy.

Snakes (Serpentes) occur almost world-wide and occupy a variety of terrestrial, fossorial, arboreal, and aquatic habitats. Macromorphological adaptations to different life habits, such as body shape and size, are well known from the literature. Previous studies on snake scale microstructure were focused on dorsal scale microstructure. However, in spite of macroscopic similarity of the ventral scale surface among snakes, the ventral surface is extremely important from the functional point of view. Due to the loss of extremities, the epidermis of the ventral scale of a snake is in continuous contact with the substrate. Therefore, there is a special interest in revealing whether certain characteristics of ventral microstructure are determined by the biology of the species or if they reflect phylogenetic relationships.

Careful analysis according to the author's newly revised terminology of microstructural features reveals that at least two major patterns of microstructure have evolved within snakes. The first comprises the ancestral microstructural pattern found in most of the snake families examined. The second pattern is derived and has evolved at least twice, independently. These two distinct types are assumed to represent different structural solutions for the same functional demands, such as propulsion generation due to frictional anisotropy, and minimization of wear.

Modifications of certain features of these two major patterns appear to correlate with different strategies of habitat occupation.

This study is of great interest to biologists studying animal integument, functional morphology, ecomorphology, and evolution of reptiles. In addition, the described diversity of microstructures might inspire surface scientists to mimic similar features in polymer surfaces, in order to generate innovative biologically-inspired surface-active materials with novel properties.

Contents

1 Introduction 3 

2 Literature review 3 

2.1 Morphological adaptations to life habits 3 
2.2 The integument of lepidosaurs and its function 4 
2.2.1 Lepidosaurian scales 5 
2.2.2 The epidermis of lepidosaurs 6 
2.3 Microstructure of lepidosaurian skin 8 
2.3.1 Previous studies on lepidosaurian 
microstructure 8 
2.3.2 Morphological variation of lepidosaurian 
microstructure 10 
2.3.3 Problems of microstructure homology 11 
2.4 Phylogeny of extant snakes 11 
2.5 Motivation and goals of the present study 13 

3 Materials and methods 14 

3.1 Species examined 14 
3.2 Sample preparation 16 
3.2.1 Museum specimens 16 
3.2.2 Frozen specimens 17 
3.2.3 Shed skin 17 
3.3 Scanning electron microscopy of scales 17 
3.4 Terminology of microstructural features 18 
3.4.1 Cell shape 18 
3.4.2 Boundary morphology of longitudinally 
adjacent cells 18 
3.4.3 Denticulations 18 
3.4.4 Arrangement of denticulations 19 
3.4.5 Cell and denticulation surface 19 
3.4.6 Multicellular structures 19 
3.5 Dimensions of microstructural features 20 
3.5.1 Regular denticulation length 20 
3.5.2 Irregular denticulation length 20 
3.5.3 Regular denticulation width 20 
3.5.4 Distance of regular denticulation tips 20 
3.5.5 Length of strap-shaped cells 20 
3.5.6 Length of non-strap-shaped cells 21 
3.5.7 Cell width 21 

4 Results 23 

4.1 Variation of microstructural features on a single 
scale 23 
4.1.1 Microstructure on different sites of the 
scale 23 
4.1.2 Microstructural transition in craniocaudal 
direction of a scale 26 
4.2 Description of dorsal and ventral microstructure 31 
4.2.1 Pythonidae 32 
4.2.2 Boidae 39 
4.2.3 Elapinae 53 
4.2.4 Hydrophiinae 66 
4.3 Comparison of dorsal and ventral microstructure 72 
4.3.1 Similarities in microstructural features 
of dorsal and ventral scales 72 
4.3.2 Consistent diff erences in microstructural 
features of dorsal and ventral scales 72 
4.3.3 Inconsistent diff erences in microstructural 
features of dorsal and ventral scales 76 

5 Discussion 77 

5.1 Scale microstructure 77 
5.1.1 Variation at the scale level 77 
5.1.2 Boundary morphology of longitudinally 
adjacent cells 78 
5.1.3 Clear layer imprints 79 
5.1.4 Dorsal microstructure 79 
5.2 Taxonomic and phylogenetic correlations of 
ventral microstructure 81 
5.2.1 Conservative characters of ventral microstructure 81 
5.2.2 Ventral microstructure variability and 
evolutionary trends within families 82 
5.2.3 Phylogenetic implications 84 
5.3 Functional and ecological correlations of snake 
scale microstructure 88 
5.3.1 Dorsal microstructure 89 
5.3.2 Functions of ventral microstructure 91 
5.3.3 Possible adaptation of ventral microstructure 
to life habits 91 
5.3.4 Differences in dorsal and ventral microstructure 95 

6 Summary 96 

Acknowledgements 97 

References 98 

Appendix 101

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