What Is the Difference Between Horns and Antlers? One Grows Back

A true horn consists of a bony core growing from the skull, permanently covered by a sheath of keratin, the same protein as hair and fingernails. It grows from the base throughout life, is never branched, and is not shed, so a horn that is broken is not replaced. Both sexes usually carry them in horned species, though the female's are commonly smaller. An antler is bone throughout, grown from a permanent bony base on the skull called the pedicle, branched in most species, and shed and regrown every year. During growth it is covered in velvet, a highly vascular skin that delivers the blood supply and nerves needed to build it, and once growth is complete the blood supply is cut off, the velvet dies and is rubbed off, leaving dead bone. Antlers are carried almost exclusively by males, the reindeer being the notable exception where females grow them too and keep them through winter, which is thought to help them compete for feeding craters in the snow.

Several structures resemble one or the other and belong to neither category, which is why the distinction is worth stating carefully:

• •A rhinoceros horn has no bony core and is a solid mass of compacted keratin fibres growing from the skin, closer to a mass of fused hair than to a cow's horn
• •A giraffe's ossicones are bone covered in skin and fur, fused to the skull in adults and never shed
• •A pronghorn carries a branched keratin sheath over a permanent bony core and sheds the sheath annually, which makes it a genuine intermediate case and unique among living mammals
• •Tusks are teeth, being elongated incisors in elephants and canines in walruses and wild boar
• •The horns of horned lizards and many other reptiles are skin or bone outgrowths unrelated to either mammalian structure
• •Bighorn sheep and ibex horns record growth in annual rings, so a horn can be used to age the animal much as tree rings date a trunk

Building a new set of bones every spring is extraordinary. Large antlers can grow more than two centimetres a day, making them the fastest growing tissue in mammals, and a full set in a large deer can weigh many kilograms. The calcium and phosphorus required exceed what a deer can obtain from food during the growth period, so the animal draws minerals from its own skeleton, effectively developing a temporary and reversible osteoporosis and then rebuilding the bone afterwards. That expense means antler size is a genuinely honest signal, because only a male in good condition on good ground can afford a large set, and females and rival males can read it directly. Shed antlers are not wasted in the ecosystem: rodents, and sometimes the deer themselves, gnaw them for minerals, which is why complete sheds are harder to find than the number dropped would suggest. The regeneration itself is studied medically, since it is the only case of a mammal regrowing a complex organ containing bone, cartilage, skin, nerves and blood vessels, and understanding how it is controlled has obvious implications.

Both are primarily weapons and signals used in competition between males for mating access, and both are shaped by how the fighting works. Species that fight by ramming head on, including sheep and goats, have horns built to absorb repeated impact, with sinus-filled skulls acting as shock absorbers. Species that wrestle, including most deer, have branched antlers designed to interlock and allow a pushing contest that tests strength without frequently causing injury. A great deal of competition is settled without contact at all, by assessment: two males display, compare size and condition, and one withdraws, which is far cheaper than fighting and is why the structures are so conspicuous. Secondary uses include defence against predators, thrashing vegetation to mark territory and display, and in some species digging. The annual cycle in deer is driven by testosterone and day length, which is why castration disrupts the pattern and why the timing of casting and regrowth is so consistent within a population.

A horn has a permanent bony core inside a keratin sheath, grows from the base for life, is unbranched, is never shed and is usually carried by both sexes. An antler is bone grown fresh each year from a pedicle, is usually branched, is covered in living velvet while growing, and is shed after the breeding season, almost always on males only. Rhinoceros horn is pure keratin with no core, giraffe ossicones are skin-covered bone, and tusks are teeth. Antlers grow so fast that deer take minerals from their own skeletons.