• 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 [1-1.3 meters] tall at its shoulder). At home on the steep hillsides and mountains of Central Asia, males will fight for females by rearing up from a distance and then racing toward its rival, crashing into him horns-first.

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

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).

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 “décor” can be an eye-catching way to advertise vigor and desirability to females and strength and dominance to males.