From antlers to horns, humans have long been fascinated by animals' ability to defend themselves with their natural-born weapons. But until now, no studies have directly tested whether those weapons perform better at the animals' own style of fighting than they would using the fighting style of another species.

Researchers at the University of Montana recently discovered each species' weapons are structurally adapted to meet their own functional demands of fighting.

The groundbreaking research, conducted over the past year by UM doctoral student Erin McCullough and designed with the help of UM researchers Doug Emlen and Bret Tobalske, is published in the journal Proceedings of the National Academy of Sciences this week.

"Animal weapons are some of the coolest and most exaggerated and diverse traits that we find in nature, and I think a big question for biologists is understanding why these structures are so diverse," McCullough said.

"Intuitively, different animals have different weapons because they fight in different ways, and I think my research provides the first rigorous test of this hypothesis."

McCullough, who earned her Ph.D. in March, studied three different species of rhinoceros beetles with three different horns using three different fighting styles.

She took micro-CT scans of each beetle and used the images to construct 3-D models of each species' horn. By using the same software engineers use to design and test bridges, she tested the stresses and strains on each species of beetle in battle.

"You can't get the animals to do it in the wild; they don't cooperate," Emlen said.

"You can't get a whitetail deer to fight the way a caribou fights." The biomechanical modeling approach allowed McCullough to find out whether each species' weapon performs better at its own style of fighting than it would using a different species' style of fighting. It's something that can't be tested in the field.

McCullough's research found that horns are stronger and stiffer when exposed to species-typical fighting styles, which suggests that performance in battle played an important role in the diversification of each weapon form.

"Even though people have been interested in animal weapons for a long time, and a number of really bright people have tackled the riddle of why weapons are diverse, nobody's been able to test it directly until now," Emlen said.

To watch a video of McCullough and Emlen discussing their research, please go here.