Horn (anatomy)

A horn is a permanent, pointed growth on an animal's head, made of a keratin and protein sheath wrapped around a core of living bone. This sets them apart from antlers, which are not permanent. Among mammals, true horns are mostly found in ruminant artiodactyls, specifically the families Antilocapridae (pronghorn) and Bovidae (cattle, goats, antelope, and others). In cattle, horns start in the subcutaneous connective tissue beneath the scalp and later fuse to the underlying frontal bone.

Most animals have one pair of horns, but a few wild species and some domesticated sheep breeds can have two or more pairs. These multi-horned, or polycerate, sheep include the Hebridean, Icelandic, Jacob, Manx Loaghtan, and Navajo-Churro breeds. Horns are often curved or spiral, and may have ridges or fluting. In many species, only males grow them.

Horns begin growing shortly after birth and continue throughout the animal's life, except in pronghorns, which shed the outer layer each year while keeping the bony core. Partial or deformed horns in livestock are called scurs. Similar growths elsewhere on the body are usually called spurs, claws, or hooves, depending on their location.

Different types of true horns have distinct shapes. Bighorn sheep have horns that sweep backward and then curve forward. Bison have horns that curve upward from the sides of the head. Mountain goats have curved horns that taper backward from the forehead, typically about 20 to 30 centimeters long.

Other hornlike growths

The term "horn" is also used for other hard, pointed head features in various animal families. Giraffes have one or more pairs of bony bumps called ossicones, covered with furred skin. Most deer have antlers, which are made of bone, are not true horns, and lack a horn or skin covering when fully developed; they are shed and regrown each year, usually by adult males (except reindeer). Rhinoceros "horns" are made of keratin—the same substance as fingernails—and grow continuously, but have no bone core.

Many chameleons, notably Jackson's chameleon, have horns on their skulls with a keratin covering. The "horns" of the Triceratops were extensions of its skull bones, though it is debated whether they had a keratin covering. Various abelisaurid theropods, such as Carnotaurus and Majungasaurus, had extensions of the frontal bone likely covered in keratin.

Horned lizards have head horns with a hard keratin covering over a bony core, similar to mammalian horns. Some insects, like rhinoceros beetles, have hornlike structures on the head or thorax made of hard chitin, while stag beetles have enlarged jaws also made of chitin. Golden jackals were once thought to occasionally develop a horny growth on the skull, associated with magical powers in southeastern Asia, but no evidence of its existence has been found, though the belief persists in South Asia.

The triassic azendohsaurid archosauromorph Shringasaurus had two massive, forward-facing conical horns on its skull, likely covered in cornified sheaths. The horned screamer has an entirely keratinous spine loosely connected to its skull. Many mammal species, including musk deer, wild boars, elephants, narwhals, and walruses, have tusks—oversized teeth that often serve similar functions as horns. Polled animals, or pollards, are those of normally horned species whose horns have been removed or never grew; small horny growths in the skin where horns would be are called scurs.

On humans

On humans, the only examples of horns are cutaneous horns. Cases of people growing horns have been historically described, sometimes with mythical status, but researchers have not found photographic evidence.

Human cadaveric specimens with outgrowths exist, but these are classified as osteomas or other excrescences. The phenomenon has been observed in countries lacking advanced medicine, and there are living people, several in China, with cutaneous horns, most common in the elderly. Some people, notably The Enigma, have horn implants—silicone placed beneath the skin as body modification.

Animal uses of horns

Animals use horns and antlers for various purposes, including defending against predators and fighting members of their own species for territory, dominance, or mating priority. Horns are usually present only in males, but in some species, females also have them. Researchers theorize that taller species living in open areas are more visible from a distance and more likely to benefit from horns for defense. Female bovids that are not hidden from predators due to their large size or open habitat are more likely to bear horns than small or camouflaged species.

Horns may also be used to root in soil or strip bark from trees. In courtship, many animals use horns in displays; for example, male blue wildebeest ream bark and branches to impress females and lure them into their territory. Some animals with true horns, like goats, use them for cooling, as blood vessels in the bony core allow them to function as a radiator.