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A horn is a pointed projection on the head of various animals consisting of a covering of keratin and other proteins surrounding a core of live bone. In mammals, true horns are found mainly among the ruminant artiodactyls, in the families Antilocapridae (pronghorn) and Bovidae (cattle, goats, antelope etc.). One pair of horns is usual; however, two or more pairs occur in a few wild species and 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 hoofs 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. 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. * Horned lizards (''Phrynosoma''): These lizards have horns on their heads which have a hard keratin covering over a bony core, like mammalian horns. * Insects: Some insects (such as rhinoceros beetles) have horn-like structures on the head or thorax (or both). These are pointed outgrowths of the hard chitinous exoskeleton. Some (such as stag beetles) have greatly enlarged jaws, also made of chitin. * Canidae: Golden jackals are known to occasionally develop a horny growth on the skull, which is associated with magical powers in south-eastern Asia.〔''Sketches of the natural history of Ceylon'' by Sir James Emerson Tennent, published by Longman, Green, Longman, and Roberts, 1861〕〔''Mammals of Nepal: (with reference to those of India, Bangladesh, Bhutan and Pakistan)'' by Tej Kumar Shrestha, published by Steven Simpson Books, 1997, ISBN 0-9524390-6-9〕 Many mammal species in various families have tusks, which often serve the same functions as horns, but are in fact oversize teeth. These include the Moschidae (Musk deer, which are ruminants), Suidae (Wild Boars), Proboscidea (Elephants), Monodontidae (Narwhals) and Odobenidae (Walruses). Polled animals or ''pollards'' are those of normally-horned (mainly domesticated) species whose horns have been removed, or which have not grown. In some cases such animals have small horny growths in the skin where their horns would be – these are known as ''scurs''. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Horn (anatomy)」の詳細全文を読む スポンサード リンク
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