Vertical Burrowing in the Saharan Sand Vipers (Cerastes)
The sand vipers of the genus Cerastes vertically burrow into the soft sand of the Saharan desert. The mechanics of vertical burrowing in Cerastes cerastes and Cerastes vipera were explored using a combination of morphology, videography, radiography, and experimental manipulation. The results suggest that the key to vertical burrowing is localized unilateral rib abduction. The ribs terminate in expanded calcified costal cartilages, which, when abducted, aid in scraping sand from below the snake and transporting it laterally. The transport of the sand is enhanced by long-axis torsion of the vertebral column. The specialized lateral scales of Cerastes do not appear to function in vertical burrowing.Abstract
Burrows created by Cerastes cerastes, starting from a level smooth surface. (A) Deep open pit (arrow) formed using the lateral surface of the head; note that the excavated sand has been formed into a raised berm adjacent to the pit. (B) Abandoned vertical burrow; note the entrance and exit tracks, and the characteristic pattern of the sand
Isolated frames from a high-speed digital video sequence of vertical burrowing in Cerastes vipera, time given is in milliseconds. Note the long-axis torsion (evident from the white dots placed at the same scale margins) and the unilateral expansion of the body produced by rib abduction (arrow)
Radiographs of Cerastes vipera during vertical burrowing. (A) Note the localized regions of burrowing activity evidenced by the dark color under the snake (caused by cavitation of the sand), the lighter color of the adjacent sand, and the change in body outline (× 0.5). (B) Radiograph showing a burrowing focal point, note that the abducted costal cartilages of the rib give the body a serrated appearance (arrow). (C) Radiograph showing a burrowing focal point, note the apparent long-axis torsion of the vertebral column (arrow)
Anatomy of the costal cartilages of Cerastes cerastes. (A) Cleared-and-stained body segment (× 5). Note the expanded shape of the costal cartilages and the presence of the darker staining calcified zone within each cartilage. (B) A single costal cartilage (× 10), showing the inner core of calcified cartilage (c). (C) Frontal section (× 100) through a costal cartilage showing the abundance of hyaline cartilage, and the darker stained region of calcified cartilage (c)
Histological sections through the integument of Cerastes cerastes stained for nerve fibers. (A) Note the prevalence of nerve endings (arrow) on the dorsal surface of this keeled scale, the remaining β-keratin can be seen on the right (× 200). (B) Surface of a dorsal scale (the β-keratin was removed during processing) showing the prevalence of the free nerve endings (× 300)