Rhinoceros beetle’s horn shape reflects fighting style

Rhinoceros beetle species all tend to look similar — except for the males’ elaborate horns. Those come in a variety of shapes and sizes, from long forks to sharp pincers. The males use the horns when they battle over females. A beetle wins when he flips his opponent off the ground or tree where they’re fighting.

But not all species use the same tactics in combat. The Japanese rhinoceros beetle (Trypoxylus dichotomus), for example, uses his pitchfork-like horn to pry and twist his enemy off a branch. A Hercules beetle (Dynastes hercules) has two horns, one longer than the other, which he employs in a pincer motion to squeeze and lift the other guy. And Golofa porteri beetles pry and push with their long, slender horns, as if they were wielding fencing swords on their heads.

Erin McCullough and colleagues at the University of Missoula in Montana had a theory that the shape of the beetles’ horns was best suited for each species’ specific fighting style, and ill-fitted for the others. So they built 3-D biomechanical models of the horns of three species of rhinoceros beetle (Trypoxylus dichotomus, Dynastes hercules and Golofa porteri) and put them to the test. The results appear September 8 in the Proceedings of the National Academy of Sciences.

Each horn was subjected to three types of force: pressure from the top or from the side, and twisting. The tests revealed that each beetle horn experienced the least amount of stress and strain when the force matched the kind that species’ fighting style creates.

The researchers speculated that the key characteristic in the horns’ morphology is not the decorative bits, like a fork or serration, but the shape of the cross-section. An elliptical cross-section is best for a rod that is bent in a single direction, such as in pliers. A beam with a circular cross-section, such as a fencing sword, is best suited when bent unpredictably. And a triangular-shaped cross-section works best when a beam needs to resist bending and twisting.

Micro-CT scans of the beetle horns indicated that the shape of the horn’s cross-section does indeed reflect those differences in how the appendage moves during a fight. D. hercules is elliptical, G. porteri is circular, and T. dichotomus is triangular.

“Relatively minimal changes in the cross-sectional shape clearly have a large effect on horn performance,” McCullough noted last month at the Animal Behavior Society meeting at Princeton University, where she presented the results of the study. “These results are really exciting because they illustrate the critical link between form and function.”