**2.2 Animal fibers**

The most commonly used animal fibers are the wool of domestic sheep and the silk. Other common animal fibers are alpaca fibers and mohair from Angora goats. The silk fiber is secreted by glands located near the mouth of some specific insects during the preparation of their cocoons.

All animal fibers contain specific proteins such as collagen, keratin and fibroin. Collagen is the main structural protein in connective tissues and the most abundant protein in mammals, see [2]. Keratin belongs to the family of structural proteins and is a key structural component of hair, horns, claws or skin in vertebrates, see [3]. In silk, keratin confers excellent tensile strength to the fiber, in the range 650 MPa– 750 MPa, see [4]. Regardless of their great importance in bioscience, both wool and silk are less used in the fabrication of composite materials for industrial applications compared with plant fibers.
