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.
Types of horns Bighorn sheep have backward-sweeping, forward-curving horns. Bison have upward-curving horns that grow from the sides of the head. Mountain goats have curved horns that taper backward from the forehead and are typically about 20 to 30 cm long.
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 keratin covering over a bony core, like mammalian horns. Insects: Some insects (such as rhinoceros beetles) have hornlike 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 were once thought to occasionally develop a horny growth on the skull, which is associated with magical powers in south-eastern Asia. Although no evidence of its existence has been found, it remains a common belief in South Asia. Azendohsauridae: the skull of the triassic azendohsaurid archosauromorph Shringasaurus possessed two massive, forward-facing conical horns, which were likely covered in cornified sheaths in life. Anhimidae: The horned screamer possesses an entirely keratinous spine, which is loosely connected to its skull. Many mammal species in various families have tusks, which often serve the same functions as horns, but are in fact oversized 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.
On humans Cutaneous horns are the only examples of horns growing on humans. Cases of people growing horns have been historically described, sometimes with mythical status. Researchers have not however discovered photographic evidence of the phenomenon. There are human cadaveric specimens that show outgrowings, but these are instead classified as osteomas or other excrescences. The phenomenon of humans with horns has been observed in countries lacking advanced medicine. There are living people, several in China, with cases of cutaneous horns, most common in the elderly. Some people, notably The Enigma, have horn implants; that is, they have implanted silicone beneath the skin as a form of body modification.
Animal uses of horns
Animals have a variety of uses for horns and antlers, including defending themselves from predators and fighting members of their own species (horn fighting) for territory, dominance or mating priority. Horns are usually present only in males but in some species, females too may possess horns. It has been theorized by researchers that taller species living in the open are more visible from longer distances and more likely to benefit from horns to defend themselves against predators. Female bovids that are not hidden from predators due to their large size or open savannah-like habitat are more likely to bear horns than small or camouflaged species. In addition, horns may be used to root in the soil or strip bark from trees. In animal courtship, many use horns in displays. For example, the male blue wildebeest reams the bark and branches of trees to impress the female and lure her into his territory. Some animals such as goats with true horns use them for cooling with the blood vessels in the bony core allowing them to function as a radiator. After the death of a horned animal, the keratin may be consumed by the larvae of the horn moth.
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