In my recent post about giraffes, I forgot to cover an important part of their wonderfully bizarre appearance – those “horns” on their head. I say “horns” because they’re not technically horns – they’re called ossicones. When I found that out, it got me wondering: how are ossicones different from horns? And are horns different from antlers? Why do animals have horns or antlers or whatever at all? It’s come to my attention that a lot of other people also don’t know the difference between horns and antlers and other things that stick out of animals’ heads, or even that there is a difference. So this week, I’m breaking all of that down.

What are horns?

Horns are permanently affixed to the skull (although they aren’t actually part of the skull). They consist of a bone core covered in a layer of keratin, and they never separate from the animal (unless one is broken off, in which case it won’t grow back). Horns come in a huge variety of shapes and sizes, from the tiny spikes of the duiker to the enormous corkscrews of the kudu. Horns are grown by bovines, including cattle, sheep, goats, and antelope, and can be found either on just males or on both sexes, depending the species. Horns from certain antelope species (such as the saola) may be partially responsible for legends of unicorns. Some chameleons also have horns!

What are antlers?

Antlers are pure bone structures (no keratin) that grow from certain points on the skull every year in the spring, and then are shed in the winter. Antlers are branched and can get extremely complicated. Only cervid species – deer, moose, elk, etc. – have antlers, and only the males grow them, except for caribou (aka reindeer). Interestingly, male reindeer shed their antlers at the beginning of winter while females keep theirs through the winter, which implies that the antlered reindeer that are usually shown pulling Santa’s sleigh are all female! In the spring, antlers start growing, fed by blood vessels in a thin layer of skin and fur called velvet. Once the antlers are fully grown, the velvet is shed (which can look gruesome, but is normal and healthy). As winter approaches, the growth hormones stop coming, and the connection weakens and the antlers fall off. Dr. Seuss wrote about this phenomenon in one of my favorite books, Thidwick the Big-Hearted Moose (although the book uses the terms "horns" and "antlers interchangably for rhyming purposes).

What about other types of headgear?

Ossicones are bony protrusions unique to giraffes and okapis. They consist of bone that is permanently fused to the skull, but instead of being covered in keratin, they’re covered in skin and fur. Young giraffes and females have tufts of hair at the tips of their ossicones, and males usually have bald tips (and sometimes have a third ossicone in the center of the forehead).

Rhino horns are different from other horns in that they’re made entirely out of keratin – they’re not a sheath over a bony core. A rhino’s horn is essentially a mass of extremely modified hair (not unlike a porcupine’s quills or a pangolin’s scales) that grows continually. If a rhino’s horn is cut off safely, then the horn will grow back. Some private game owners in South Africa have even started farming their rhinos’ horns for sale to Asian markets, to flood the market, devalue poached rhino horns, and raise money for conservation efforts.

Pronghorn antelope, a species found in the American west, have a sort of odd hybrid horn/antler situation. They have large permanent bony protrusions covered in keratin, but the keratin sheaths are branched and are shed every year.

Elephants have neither horns nor antlers, but they do have tusks, which are essentially giant elongated teeth. Narwhals also have an enormous elongated tusk that protrudes through the upper lip.

Certain small cervidae species, like the Chinese water deer, grow very small antlers or none at all, so they have evolved a different sort of head gear – fangs!

What’s the purpose of horns/antlers/whatever?

Horns, antlers, and other things serve several different purposes, but the main one is combat. Sometimes they’re used for defense against predators, but more often, males with horns, antlers, ossicones, and tusks use them to fight each other for mates, or at least to display their dominance. They also serve as a secondary sex characteristic, particularly antlers. If a male deer can expend tons of energy into growing a huge rack of antlers, then females know that he’s healthy and will pass on strong genes to offspring.

Isn’t it inconvenient to have all that stuff attached to your head?

Sometimes, yes! Horns can get in the way and antlers can get caught in branches, or sometimes tangled together during fights. Several years ago, I visited Denali National Park in Alaska, where they had on display two entangled and inseparable moose skulls that had been found in the park, leftover from where two bull moose had gotten locked together while fighting for dominance, had been unable to separate, and presumably starved together. Sometimes horns and antlers can also grow impractically large – there’s an extinct deer species called the Irish elk that had the largest antlers ever known, so large that the size may have contributed to their extinction (along with hunting from humans and environmental shifts). But it's all a trade-off with the advantages of gaining mating rights and keeping away predators

Horns and antlers can also be very heavy, as can elephant tusks. The Smithsonian’s natural history museum (where I used to work) has an iconic taxidermy African elephant on display in the main rotunda. That elephant (whose name is Henry) is the real deal, except for the tusks – Henry’s display tusks are fiberglass because the real ones weigh about 90 pounds (41 kg) each, and would have ruined the rest of the taxidermy job. Tusk ivory is of course the main reason why elephants are poached, which has recently led to an interesting outcome: elephants are evolving to not have tusks anymore! The ones with smaller tusks or no tusks survive and can pass on their genes – it’s accidental “artificial” selection.