Vanishing beetle horns have surprise function

In the Evolution report, the scientists examined literature describing the evolutionary relationship of 47 Onthophagus species. They also studied the development of eight beetle species in the laboratory (seven Onthophagus species and one species from the closely related but hornless genus Oniticellus).

The scientists found that all seven Onthophagus species examined in the laboratory develop horns during their larval and pupal development. That finding should instigate a complete revision of the evolutionary history of Onthophagus beetles, which are largely categorized according to their adult shapes with little or no heed given to the quirks of the beetles' development.

Evolutionary biologist Armin Moczek. Click here for more information.

Despite the growing presence of developmental biology in evolutionary studies, "Even today, evolutionary theory is very much a theory of adults," Moczek said. "But evolution doesn't morph one adult shape into another. Instead there's an entire lifetime of development that we can't afford to ignore."

Curious as to whether or not the horns had a function, the scientists destroyed the horn tissue of beetle larvae using electrosurgery: minute voltage arcs that permit precise destruction of targeted cells while nearby tissues are left intact and undamaged. With the larval horn tissue destroyed, the scientists observed most larvae were unable to break the husks of their larval head capsules, resulting in young adult hatchlings whose heads were tightly (and lethally) encased within larval helmets. Altered Oniticellus, on the other hand, had no trouble breaking free of their former exoskeletons.

"It may be that these larval horns enabled Onthophagus beetles to grow a thicker carapace," Moczek said. "But it is also possible a thicker carapace made horns necessary. We are left with the commonly asked question in evolutionary developmental biology, 'Which came first"'"