The Structure of the Water-Holding Feathers of the Namaqua Sandgrouse
The morphology and fine structure of the feather barbules of the Namaqua Sandgrouse Pterocles namaqua are investigated histologically and experimentally by means of light microscopy, scanning electron micrography and X-ray diffraction. Proximally the barbule is helically coiled for three and a half turns and has a kidney-shaped, concave/convex transverse section. The inner concave surface is pitted, the outer convex surface smooth. The barbule is solid, consisting of three layers, and bears a number of appendages at its distal end, where it is more rounded in transverse section. The uncoiling of barbules from the abdominal feathers on contact with water may be initiated by water uptake and further facilitated by the number of helical coils at the base of the barbules. The keratin is fairly crystalline when dry. This crystallinity is somewhat reduced on wetting. The uncoiling mechanism is related to the expansion of the polypeptide chains of Il-keratin in order to accommodate additional water bound to the side chains.