Edition: U.S. / Global

A Lance, a Lunch or a Mate, All for the Price of a Horn

Published: April 27, 2004

What could be more useful than a nice set of horns?

They seem perfect for driving off predators, handy for clobbering rivals and fine for signaling subtly to potential mates. Ideally, everyone should have a pair, instead of Pan, Lucifer and Hellboy having all the fun.

But come to think of it, if they're so great, why are they so rare?

Although humans who visit zoos or farms might assume that horns are the norm, they are in fact largely confined to a small and modern slice of mammals, hoofed ungulates, and some unusual reptiles and insects.

Cattle, oxen, sheep, goats, yaks and buffalo have them, as do African antelopes, from the prongs of the tiny duiker to the elegant spirals of the kudu. Morphologically extremely different, but also usefully sharp, are the antlers that annually adorn the foreheads of deer, moose, elk and reindeer. Rhinoceroses have horns of compressed hair that ride like gun sights between their notoriously weak eyes.

Meanwhile, many other hoofed herbivores like camels, llamas, hippos and horses lack horns. (Unicorns don't count.)

Why evolution produces horns in some species but not others is not known, but an intriguing partial answer was offered this month in a study that had nothing to do with mammals.

The study, in the journal Science, dealt with a rare species, the horned lizard, in which the horns seem decidedly more useful against predators than against rivals. Fourteen species of the potbellied reptiles live in American deserts, eating ants. Also known as horny toads, the lizards have sharp skull spurs that pierce the throats of birds and snakes that swallow them. The new study suggests that those spurs can also scare off diners before they attack.

An important lizard predator, the loggerhead shrike, has a grisly habit that earned it the name ''butcher bird.'' Shrikes impale their kills on thorns or barbed-wire fences to create a larder that signals to females that they are good providers. Biologists measured the horns on 155 living lizards and 29 impaled ones. Those on the live lizards averaged 10 percent longer. Moreover, any lizard with horns more than 80 percent as long as its head seemed to be safe.

''You can look at is as 'bigger is better,' '' said Dr. Edmund D. Brodie III of Indiana University, an author of the study.

Selection for long horns is so powerful, Dr. Brodie said, that he calculated that it might take only 20 to 30 generations for average length to grow 10 percent. ''That's only 20 to 30 years,'' he said. ''Very fast.''

In the last 65 million years, mammals have come a long way from shrew-like creepers on the forest floor. Dr. Robert Wiese, director of animals at the Fort Worth Zoo, said most species broke off the mammal line that led to ruminants before horns evolved.

''They didn't really have that option,'' he said. ''You can't evolve them just because it's helpful.''

Horns are unknown among felines, pigs, dogs, rodents, bears, monkeys and apes. No bird or fish has them. The horned owl, horned puffin and horned screamer are all named for their feather tufts. The vagaries of evolution can plant a glowing ''light bulb'' on the forehead of deep-sea anglerfish, but nary a horn. The ivory prong that precedes the narwhal whale is a tusk.

Only among the almost infinite varieties of reptiles and insects do horns begin to crop up. The triceratops of the reptile world is the Jackson's chameleon, which has three thick horns jutting out of its face. Chameleons, however, are not known for ferocity. At a predator's approach, they change color and freeze.

The horns are strictly for mating battles, which are hardly epic because chameleons move so slowly. In captivity, the Jackson's may fight without even locking horns; two goggle-eyed males confronting each other on a branch can simply stare at each other till one dies of stress.

Horns also appear on dung beetles, where, among the 30,000 or more species, some look like the psychedelic nightmares of metalworkers. Dr. Douglas J. Emlen, a professor of biology at the University of Montana, said almost all those so adorned are male, and they duel on bamboo shoots, over tiny sap-oozing tree wounds and in tunnels beneath dung patties -- virtually anywhere a big set of horns will block a narrow path.

Females mate with the handiest male when they are ready, Dr. Emlen said, ''so the horns aren't there to impress her with their looks; they're there to keep other males out of the tunnel.''

But the battle is not always to the strong. While a big-horned guy is taking on all comers as his intended packs dung into the chambers where she will lay eggs, a small ''sneaker'' male may tunnel in quickly from the side and cuckold him. ''You don't want big horns for that,'' Dr. Emlen said. ''They get in the way.''

Still, horns must lead to breeding success, because they are ''darned expensive,'' as Dr. Emlen put it. They use a lot of the food energy that a beetle larva consumes.

''They can be 15 percent of body weight,'' he said. ''That's like producing another leg and wearing it on your head.''

Dr. Christine Janis, who teaches evolutionary biology at Brown, said there was ''no unifying reason'' for the development of horns across species. The fossil record of the transformation of mammals, she warned, shows that it is dangerous to generalize about evolution from visible animals. Only 10 of 91 ungulate families, many of them extinct, ever evolved bony horns. A proto-camel had one; a proto-rhinoceros had two, side by side.

Dr. Janis theorizes that most ungulates' horns were responses to the climate changes that created open woodlands and savannas. With more food available and breeding gone seasonal, males began to practice polygamy and needed something to help defend territories and harems.

Horns also seem related to bigger bodies. Mammals weighing less than about 40 pounds apparently did better in the love wars by biting than butting. Even today, the mouse deer uses its antlers to ward off bites and to wrestle himself into biting position.

By contrast with horned lizards, there is little evidence that mammals' horns evolved because of predators.

In danger, most horned mammals choose flight. Rhinoceroses and cape buffalo are well-known exceptions, and females with calves are often the most dangerous. Lions and sable antelopes have been found locked together in death, the cat impaled on the scimitar-shaped horns of the sable whose neck it has clawed open.

In most cases only males have horns, which suggests that they are for mating. Why both sexes have them in some species like cattle, reindeer and wildebeest is not known, but they form year-round mixed-sex herds. One theory is that when food is scarce, females also have to fight for it. Another theory is that more mothers survive to procreate when predators cannot tell females from males.

Photos: Scientists say there is no one reason why horns have developed in such varied animals as a Jackson's chameleon, left, a duiker, white rhinoceroses and an iridescent dung beetle. (Photos by Reuters, left; G.C. Kelley/Photo Researchers Inc.; Agence France-Presse; and E.R. Degginger/Photo Researchers Inc.)