From clashing antlers to gnashing mandibles, male animal weapons are among nature’s most diverse structures. So it stands to reason that males from different species have different weaponry since they fight in different ways, yet there’s no experimental evidence that an animal’s apparatus performs better at its own style of fighting than it does at others.
Now, a team studying rhinoceros beetles established a link between weapon form and function. Their findings, published in Proceedings of the National Academy of Sciences this week, suggest that variations in the shape of sexually selected weapons reflect structural adaptations for species’ specific fight styles.
Male rhinoceros beetles wield some very showy weapons -- long pitchforks, robust pincers, and thin spears, to name a few -- and they’re used to combat other males for access to females. But there’s no evidence that females choose males based on their horn shape and size, suggesting that these aren't just flashy adornments. If not female choice, then what drives the diversity of these sexually selected weapons?
To test if the shape of head horns are structurally suited for various fighting styles, a trio of University of Montana researchers led by Erin McCullough used micro-CT scans to construct 3D biomechanical models of the horns of three species of rhinoceros beetles: Trypoxylus dichotomus, Dynastes hercules, and Golofa porteri. When they’re fighting with rival males, these three have head horns that are bent vertically and twisted, bent vertically only, and bent vertically and laterally, respectively.
Trypoxylus males have head horns that are long and forked, and they work like pitchforks, prying and twisting opponents off the trunks and branches of trees. The head horns of Dynastes males are long and work together with another horn (called the thoracic horn) like pincers on pliers to lift and squeeze opponents off of trees, tossing them to the ground. Finally, head horns of Golofa males are long and slender, and they’re used like fencing swords to lift opponents and push them sideways off balance on narrow shoots.
Using simulations, the team evaluated the functional performance of these horns in response to both species-typical fighting loads (their own style) and species-atypical fighting loads (having a pitchfork-fighter try fencing, for example). Their model calculated the strains and stresses experienced by the weapon when different forces are applied to it, Science explains.
They found that horns are both stronger and stiffer in response to species-typical fighting loads. “Species have the type of horn that they do because those types perform best, and if they didn’t then they would be more likely to break,” McCullough tells National Geographic.
Each species has evolved weaponry that works best for their own battle tactics, suggesting that selection for improved performance with different fighting styles played an important role in the diversification of weapons.
Images: Didier Descouens via Wikimedia CC BY-SA 4.0 (top), E.L. McCullough et al., PNAS 2014 (middle), David J. Tuss, PNAS 2014 (bottom)