Beetle Battles

On the dilemma of horns

Why do dung beetles have horns?

Insignificant as the question may seem, it has puzzled entomologists for years. Are the protuberances weapons? Are they decorations for attracting the opposite sex? Indeed, do they serve any purpose at all? Timothy Palmer, a young scientist at Britain's Imperial College Field Station, outside London, decided to settle the matter once and for all.

As his subject, Palmer turned to Typhoeus typhoeus, commonly known as the minotaur beetle. Barely larger than a pebble, this long, shiny black bug is found throughout sandy areas of Europe, where it feeds mainly on rabbit, sheep or deer droppings. It is named for its three distinct horns--two large ones separated by a smaller one--that project threateningly from the male of the species.

Palmer constructed an artificial burrow with viewing glasses on either side like a child's ant farm. Then he introduced several female beetles, plus a single horned male. For hours he watched as the little bugs burrowed, scrounged for food and copulated. But the male never used his horns.

Then Palmer introduced a second male, and, as he had expected, an entomological display of macho erupted. Battling to assert their supremacy and win a female, the two little beasts went at each, other like monsters in a Japanese sci-fi flick, pushing and shoving each other with their horns. If one beetle seemed to be getting the upper hand, the other often slumped on its side, blocking the first beetle's path. The more aggressive beetle would then use his horns as levers in an attempt to dislodge his opponent. Sometimes the defender flipped over on his back and locked horns directly with the aggressor. All the while, as the beetles lunged at each other, they made loud squeaking noises.

Each of these skirmishes lasted nearly three minutes, and the entire battle often continued for more than an hour. Finally, as one minotaur gained the upper hand, his vanquished foe either left the burrow of his own accord or was actually pushed out by the winner (who invariably turned out to be the larger beetle). Thus, Palmer reports in Nature, the minotaur's horns, and perhaps similar horns in other beetles, seem to have been evolved for only one purpose: combat.

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