Vanishing beetle horns have surprise function

Most scientists have assumed the sexual dimorphism of some Onthophagus beetles was borne of differential growth; flamboyantly horned male beetles grow them, hornless females simply can't. But the American Naturalist report shows that even within sexually dimorphic horned beetle species, both sexes initially form the horns, even if one or both sexes reabsorb the horn tissue sometime before adulthood.

In the American Naturalist report, Moczek examined the development of four Onthophagus species. Both species nigriventris and binodis exhibit typical sexual dimorphism -- adult males have horns, females are hornless. In sagittarius, the sexual dimorphism is reversed. Adult females and males of the fourth species Moczek examined, taurus, are both hornless.

Despite the differences in adult appearance, all four species begin to grow horns as larvae -- regardless of sex, Moczek found. The hornlessness of some adult beetles is therefore not the result of an inability to make horns, Moczek says, but the reshaping or reabsorption of horn tissue before the beetles become adults.

"I think these findings illustrate quite clearly the importance of development to evolutionary biology," Moczek said. "By including studies of your organism's development, at the very least you stand to gain fundamental insights into its biology. More often than not, however, you may prevent yourself from making big mistakes when drawing up evolutionary histories. In this case, I think we did both."

Research reported in both papers was supported by grants from the National Science Foundation and the Howard Hughes Medical Institute. Moczek , graduate student Tami Cruickshank and undergraduate student Andrew Shelby coauthored the Evolution paper. Moczek is the American Naturalist paper's sole author.

To speak with Moczek, please contact David Bricker.

"When ontogeny reveals what phylogeny hides: gain and loss of horns during development and evolution of horned beetles," Evolution, vol. 60, no. 11 (November 2006)

"Pupal remodeling and the development and evolution of sexual dimorphism in horned beetles," American Naturalist, vol. 168, no. 6 (December 2006)