Vanishing beetle horns have surprise function

Horned beetles from the genus Onthophagus are found all over the world, including the US. Click here for more information.

BLOOMINGTON, Ind. -- The function of horned beetles' wild protrusions has been a matter of some consternation for biologists. Digging seemed plausible; combat and mate selection, more likely. Even Charles Darwin once weighed in on the matter, suggesting -- one imagines with some frustration -- the horns were merely ornamental.

In this month's American Naturalist (Dec. 2006) and the Nov. 2006 issue of Evolution, Indiana University Bloomington scientists present an entirely new function for the horns: during their development, Onthophagus horned beetles use their young horns as a sort of can opener, helping them bust out of thick larval shells.

The finding will surprise anyone who assumed hornless Onthophagus adults (usually the females) never form the horns in the first place. They do, the scientists say, but the nubile horn tissue is reabsorbed before the beetles' emergence as adults.

"The formation of horns by beetle pupas that soon lose them just doesn't seem to make sense, so obviously we were intrigued," said IU Bloomington evolutionary biologist Armin Moczek, lead author of both papers. "It appears these pre-adult horns are not a vestigial type of structure, which many of us thought was the case. Instead we have shown these horns actually serve an important function regardless of whether they are resorbed in the pupal stage or maintained into the adult."

Because all the Onthophagus beetles the scientists examined form horns during development, Moczek and colleagues also argue the evolution of ornate horns in the adult beetles may actually have happened second -- that is, some time after their initial evolution as larval molting devices.