The Adaptive Significance of Sexually Dimorphic Scale Rugosity in Sea Snakes

Abstract

In terrestrial snakes, rugose scales are uncommon and (if they occur) generally are found on both sexes. In contrast, rugose scales are seen in most sea snakes, especially in males. Why has marine life favored this sex-specific elaboration of scale rugosity? We pose and test alternative hypotheses about the function of rugose scales in males of the turtle-headed sea snake (Emydocephalus annulatus) and conclude that multiple selective forces have been involved. First, rugosities may aid male positioning during courtship, because histology shows that tubercles are more highly innervated than adjacent flat areas of each scale and hence are presumably more sensitive to tactile cues, and because biomechanical tests show that rugosities enhance friction between the bodies of males and females. Second, the occurrence of rugosities over the entire body of males and (albeit less well developed) in females as well suggests that rugosities also play a hydrodynamic role by modifying water flow across the snake's surface. Flow tank tests show that rugosities reduce the thickness of the boundary layer by almost 50% and create turbulent flow that should massively enhance rates of cutaneous oxygen uptake and hence prolong maximal courtship duration by males.

Phenotypic divergence between seasnake (Emydocephalus annulatus) populations from adjacent bays of the New Caledonian Lagoon

“…The straightline distance between the two sites is 330 m, and the shortest aquatic connection (around an intervening headland) is 1.15 km. A snake could travel between the two sites in 38 min at maximum swimming speeds (30 m min −1 ; see below), and in 5 to 10 h at speeds typical of snakes in the field (< 2 to 4 m min −1 ; Shine, ; Avolio et al ., ; Lukoschek & Shine, ). The intervening area consists of shallow reef, where E. annulatus is common.…”

“…We have conducted ecological research on this taxon in two adjacent bays (Anse Vata and Baie des Citrons) beside the city of Noumea, in New Caledonia (Shine et al ., , b, , ; Shine, ; Avolio, Shine & Pile, ; Shine, Brischoux & Pile, ; Brischoux et al ., ) (Fig. ).…”

An investigation into the stranding of Australian sea snakes

“…The heart in sea snakes is located more caudally than their terrestrial counterparts with the head-heart distance estimated to be 23 -42% of total body length (compared with 15 -23% in terrestrial snakes) 39 . Among the sea snakes, species-specific differences include variation in venom apparatus development (fish-egg eating Emydocephalus annulatus have atrophied fangs and venom glands 168 ), head size (some genera are microcephalic) 3 , scale rugosity 169 and skin permeability to water 170 .…”