They're blundering beasts, covered in refuse, and they sport impressive prongs on their heads and shoulders. 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.