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.
Competition among males is often expressed in the form of elaborate weapons made of bone, horn or chitin. The weapons often start off small and then, under the pressure of competition, may evolve to attain gigantic proportions. The Irish elk, now extinct, had antlers with a span of 12 feet. The drawback of this magnificent adornment, though, was that the poor beast had to carry more than 80 pounds of bone on its head.
In a new review of sexual selection, a special form of natural selection that leads to outlandish armament and decoration, 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.
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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.
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