A horn is a permanent pointed projection on the head of various animals that consists of a covering of keratin and other proteins surrounding a core of live bone. Horns are distinct from antlers, which are not permanent. In mammals, true horns are found mainly among the ruminant artiodactyls, in the families Antilocapridae (pronghorn) and Bovidae (cattle, goats, antelope etc.). Cattle horns arise from subcutaneous connective tissue (under the scalp) and later fuse to the underlying frontal bone. One pair of horns is usual; however, two or more pairs occur in a few wild species and in some domesticated breeds of sheep. Polycerate (multi-horned) sheep breeds include the Hebridean, Icelandic, Jacob, Manx Loaghtan, and the Navajo-Churro. Horns usually have a curved or spiral shape, often with ridges or fluting. In many species, only males have horns. Horns start to grow soon after birth and continue to grow throughout the life of the animal (except in pronghorns, which shed the outer layer annually, but retain the bony core). Partial or deformed horns in livestock are called scurs. Similar growths on other parts of the body are not usually called horns, but spurs, claws, or hooves, depending on the part of the body on which they occur. Other hornlike growths The term "horn" is also popularly applied to other hard and pointed features attached to the head of animals in various other families: Giraffidae: Giraffes have one or more pairs of bony bumps on their heads, called ossicones. These are covered with furred skin. Cervidae: Most deer have antlers, which are not true horns and made of bone. When fully developed, antlers are dead bone without a horn or skin covering; they are borne only by adults (usually males, except for reindeer) and are shed and regrown each year. Rhinocerotidae: The "horns" of rhinoceroses are made of keratin, the same substance as fingernails, and grow continuously, but do not have a bone core. Chamaeleonidae: Many chameleons, most notably the Jackson's chameleon, possess horns on their skulls, and have a keratin covering. Ceratopsidae: The "horns" of the Triceratops were extensions of its skull bones, although debate exists over whether they had a keratin covering. Abelisauridae: Various abelisaurid theropods, such as Carnotaurus and Majungasaurus possessed extensions of the frontal bone which were likely covered in some form of keratinous integument. Horned lizards (Phrynosoma): These lizards have horns on their heads which have a hard