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Beetle Battle Video: http://www.npr.org/templates/story/story.php?storyId=103775784
Nature is reputed to be red in tooth and claw, but many arms races across the animal kingdom are characterized by restraint rather than carnage.
Douglas J. Emlen, a biologist at the University of Montana, has assembled ideas on the evolutionary forces that have made animal weapons so diverse.
Sexual selection was Darwin’s solution to a problem posed by the cumbersome weapons sported by many species, and the baroque ornaments developed by others. They seemed positive handicaps in the struggle for survival, and therefore contrary to his theory of natural selection. To account for these extravagances, Darwin proposed that both armaments and ornaments must have been shaped by competition for mates.
In his view, the evolution of the armaments was driven by the struggle between males for females, whereas the ornaments arose from the choice, largely by females, of characteristics they prized in males. Modern biologists have devoted considerable attention to female choice and how it has led to such a riotous profusion of animal high fashion, from the plumage of birds to the colors of butterflies. Less attention has been paid to the equally rich diversity of animal weaponry.
Dr. Emlen said he became interested in animal armament after studying a species of dung beetle in Panama that specialized in monkey scat. He broadened his studies to dung beetles worldwide and noticed a pattern in their weaponry. Dung beetles may have started their highly successful career feeding on dinosaur ordure, and seem then to have diversified to that of mammals. They have two principal strategies. Some, like the scarabs, cut out pieces of dung and roll it away for private consumption. Other species dig under a deposit and draw it into their tunnels.
Dr. Emlen noticed that only the tunneling species of dung beetles had evolved horns, which the males use to protect their tunnels from other males. The beetles that push balls of dung away also fight all the time with other males, but are hornless.
“I became fascinated by animals with strange morphologies that make you wonder how in the world they could possibly have mated,” Dr. Emlen said. After collecting papers on “anything that had funky structures,” he began to see a pattern in who developed weapons and who did not. Whenever there was some resource that could be monopolized and used for reproductive advantage, males would develop weapons to fight off other males.
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.
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