The cost of developing and carrying the weapon, Dr. Emlen inferred, was outweighed by the greater access to females gained by owning some prized possession like a food source or tunnel where females could lay eggs.

Dr. Emlen noticed a tendency for weapons to start out small, like mere bumps of bone, and then to evolve to more ornate form. The small weapons are actually quite destructive since their only role is to attack other males. But the more baroque weapons, even though they look more fearsome, seem to cause lesser loss of life.

The reason is that the more menacing weapons have often acquired a signaling role. Instead of risking their lives in mortal combat, males can assess each other’s strengths by sizing up a rival’s weapons, and decline combat if they seem outclassed. The ornate weapons also lend themselves to ritualized combat in which males may lock horns and assess each other’s strength without wounding each other.

“The most elaborate weapons rarely inflict real damage to opponents, but these structures are very effective at revealing even subtle differences among males in their size, status or physical condition,” Dr. Emlen writes in the current Annual Review of Ecology, Evolution and Systematics.

Since the weapons still have to be used from time to time, they are realistic signals of a male’s fitness. This information is of greatest interest to females, which are always looking for true, unfakeable signals of a male’s quality. Geerat Vermeij, an evolutionary biologist at the University of California, Davis, and an expert on the arms race between mollusks and their predators, said he agreed with Dr. Emlen that weapons began as soon as there was something to be defended. But Dr. Vermeij said he was “skeptical of the conclusion that initially harmful weapons turn into display weapons.” Crabs, for instance, lose claws a quarter of the time in combat, he said.

Scott Sampson, a dinosaur expert at the Utah Museum of Natural History, said that among hoofed animals, weapons often became less dangerous when the animals started to live in larger herds and males would cooperate in defending females against predators.