Horns evolved independently in many animals to meet similar needs—first as weapons, and then as defenses against rivals, says Don Moore, director of the Oregon Zoo in Portland.
Horns likely initially inflicted body blows, but became larger and more elaborate as they absorbed blows to the head. This strategy led some animals, like pronghorns, to essentially wrestle, whereas others, like sheep, ram their opponents.
Horns can also communicate power—big ones, for instance, may identify a herd's dominant male so that newcomers can avoid fighting him and risking injury, Moore says.
Male horns may also attract females looking for the strongest mate, though in most ungulate species, females also have horns, says Patrick Bergin, chief executive officer of the African Wildlife Foundation. The beautifully striped bongo is one example, he says.
Cincinnati Zoo staff collect the eggs of a giant eland to help with reproduction of the species in Africa. PHOTOGRAPH BY MICHAEL NICHOLS, NATIONAL GEOGRAPHIC CREATIVE
Species with horned females are usually large and live in open areas, making it difficult to camouflage themselves and likely driving the evolution of their weaponry. They're often species in which females fight each other for territory, according to a 2009 study.
In particular, Africa's “diverse ecological niches” sustain a huge variety of hoofed animals with many different horns, Bergin says.
They range from the jackrabbit-size dik-dik, with three-inch (eight-centimetres) horns, to the giant eland, whose curling horns can reach nearly four feet (just over a meter).
The dik-dik (above, an animal at the Kansas City Zoo) is a type of dwarf antelope with tiny horns. PHOTOGRAPH BY JOEL SARTORE, NATIONAL GEOGRAPHIC CREATIVE
It's not always clear whether horns help African animals in their habitat, but scientists have some clues. The nyala, for instance, may use its spiraled horns to move brush aside and help the shy creature hide.
The red forest buffalo’s horns are "smaller, tighter to the head, so they can move through the forest” easily, Bergin says.
Bighorn males, called rams, are famous for their large, curled horns. These impressive growths are a symbol of status and a weapon used in epic battles.
That's not the case for the Cape buffalo, whose grassland lifestyle allows for big, curved horns. The horns sometimes grow so large they fuse in the center, creating “one big solid plate of horn” called a boss. It can be quite a defense in a fight with a lion, Bergin adds.
Cape buffalo horns sometimes grow so large they fuse in the center, creating one solid plate, called a boss. PHOTOGRAPH BY SERGIO PITAMITZ, NATIONAL GEOGRAPHIC CREATIVE
Some horns carry some useful information—Africa's walia ibex and Europe’s Alpine ibex both have three-foot (nearly a meter long) horns whose rings reveal their ages.
Horns differ from antlers in that they’re permanent, fused to the skull, and covered with keratin, the same material that makes up our fingernails. Antlers are shed annually and are made only of bone.
Rhinoceros horns are the exception: A recent study found they are made of keratin with deposits of calcium and melanin in the core, instead of bone. The horn gets its shape when the animal rubs it, softened by sun exposure, on the ground.
Rhinoceros horns (pictured, white rhinos in Kenya) are made of keratin, the same substance that makes up human fingernails. PHOTOGRAPH BY ROBIN MOORE, NATIONAL GEOGRAPHIC CREATIVE
Also, a rhino horn can regrow—depending on how much is lost—while other animals' horns can’t.
But when the animals die, their horns don’t always go to waste. A very specialized insect native to Africa, the horn moth, “lays its larvae in the horns of these deceased animals,” Bergin says.
The bugs then spin silken tubes and dine on the horns' keratin coating.
Who wants keratin cake?