Headgear

"Check out the ossicones on that giraffe." OK, that may sound a little weird, but it's better than "Look at the tooth on that narwhal."

Either way, we're talking about headgear here--from the relatively small ossicones on the head of a giraffe to the large left incisor that projects from the narwhal whale's upper jaw as a tusk. Strangely enough, as many mammals that developed horns, antlers, tusks or ossicones--from deer and sheep to cattle and goats--no early mammals had horns on their heads.

Despite that original lack of headgear, some mammals, like the male moose, evolved antlers as wide as a car over millions of years. Others, like Embolotherium andrewsi, developed horns as support systems for their giant noses (and to head-butt rivals).

Even without its impressive horns, the Marco Polo sheep is the world's largest sheep at three to four feet tall at its shoulder.
AMNH/D. Finnin

But why? Why do we now see these extreme examples of headgear when mammals once roamed the earth with plain-old heads, bones and teeth?
 
Defense, recognition and mating--three common reasons threaded throughout evolution. Nearly all mammals with headgear are prey animals and sometimes use their headgear as defense against would-be attackers.
 
Most mammals with headgear live in social groups rather than alone--using headgear as a quick way to recognize kin. For male mammals, head "decor" can be an eye-catching way to advertise vigor and desirability to females and strength and dominance to males.

Moose antlers are shed and regrown each year. Male moose use them to attract mates, intimidate rivals and fend off predators, including packs of wolves.
iStockphoto

The ossicones on a giraffe are relatively small compared to some mammalian headgear. They start as a small knob of cartilage in young giraffes and later become bony.
Shutterstock

A male narwhal's (NAR-wall's) incisor tusk is more flexible than it looks: in living animals, it can bend a foot in any direction. Also, the tusks are usually found only in males; they may use them to attract females.
Richard Ellis/Photo Researchers

Fun Facts

• The ossicones on a giraffe are made up of bony cores that develop first within the skin as a knob of cartilage. Ossicones later "ossify," becoming bone. A baby giraffe's ossicones lie flat against the head at birth, popping upright about a week later.
• Made up of branched bones shed annually, antlers regrow each year from a permanent bony base. At first, antlers have thin, hair-covered skin called "velvet" that eventually falls off, leaving antlers sharp and shiny. Antler bones grow incredibly fast--up to an inch a day.
• The Marco Polo sheep is the world's largest sheep (three to four feet or 1-1.3 meters tall at its shoulder). At home on the steep hillsides and mountains of Central Asia, a male will fight for females by rearing up from a distance and then racing toward its rival, crashing into him horns-first.