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On the Horns of a Dilemma

23 February 2001 7:00 pm
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Horny beetles. These male dung beetles pay for big horns (colored in this image) with reduced wings.

Male dung beetles are the Hell's Angels of the insect world. They're blundering beasts, covered in refuse, and they sport impressive prongs on their heads and shoulders. Ever since Darwin, biologists have been puzzled by why these armaments seem to crop up in so many different places: Some species have horns on their thoraxes, while some have them on the front or back of their heads. Now, a study in the 23 February issue of Science takes a stab at solving this mystery.

Evolutionary biologist Douglas Emlen of the University of Montana, Missoula, measured horn length in three species of the dung beetle Onthophagus. Males use their horns to defend their mates against competitors. One species, O. sharpi, has its weapons at the front of the head. The second, an unidentified species from Ecuador, has them at the back of the head, while the third, O. sagittarius, wields a horn on its thorax. For each of more than 200 specimens, Emlen also measured the weight of the wings, the antennae, and the size of the eyes.

Growing outlandish horns seems to come at a cost, Emlen discovered, because they correlate with smaller organs nearby. Large-horned males of O. sharpi, with its horns at the front of the head, had smaller antennae than did less endowed males of the same species, presumably affecting their sense of smell. Beetles of the species with the horns at the back of the head had smaller eyes if their horns had grown larger, while O. sagittarius paid with puny wings for having a large horn on the thorax. Apparently, energy invested in growing horns during the pupal stage is siphoned away from neighboring organs.

Sometimes, the costs can be too much to bear. When Emlen compared the position of horns in 74 species of nocturnal dung beetle with those in 87 species that are active by day, he found that the nocturnal species tended to avoid having horns at the base of head, where they would affect the size of their eyes. Presumably, such night walkers need large eyes, Emlen says, and he suggests that similar trade-offs may have helped determine the location of other horns.

Many scientists study the evolution of ornamentation in animals, says evolutionary biologist Gerald Wilkinson of the University of Maryland, College Park, but few have looked at the reasons for the place on the body where such structures form. "I think it is a very nice piece of work," he says.

Related sites
Douglas Emlen's home page
Darwin on dung beetle horns, in his book The Descent of Man and Selection in Relation to Sex

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