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Why Do Males Have Built-in Weapons?

The stunning array of weaponry brandished by male animals—be they antlers, horns, mandibles, spurs, or claws—is driven by each species’ individual fighting style, scientists have revealed.

The finding, which may solve a long-standing evolutionary puzzle, is thanks to perhaps the most impressive weapons proliferator of them all, the male rhinoceros beetle—also the world’s strongest animal. (Related: “Pictures of Animal Record-Breakers.”)

Rhinoceros beetle in Gunung Palung National Park, Borneo.
A rhinoceros beetle in Gunung Palung National Park, Borneo. Photograph by Tim Laman

A study of the heavily armed insects by researchers at the University of Montana, Missoula, directly linked the males’ elaborately shaped horns, which they use for jousting over females, to their method of combat.

From the antlers of a giant elk to those of a stag beetle, it’s long been suspected that different fighting styles drive weapon diversity in males, study co-author Erin McCullough noted.

“What’s exciting and new about this study is that we can actually test this hypothesis,” she said.

The study team did this by creating biomechanical models of the horns of three rhino beetle species with very distinct weaponry. (Watch a video of the “weightlifting” rhinoceros beetle.)

Virtual Battles

The head horns of Trypoxylus dichotomus, for example, are shaped like a long pitchfork, and are used to pry and twist their rivals off branches.

Dynastes hercules, meanwhile, goes into battle with horns that resemble the serrated claws of a crab in order to pincer their opponents and toss them off trees. And Golofa porteri uses its long, slender horns as fencing swords in skirmishes over mating rights. (Also see “Big Testes or Big Horns? It’s One or the Other for Male Beetles.”)

Conversely, when these 3-D horns were subjected to fight stresses that mimicked those of the other two species, the models showed that the horns would buckle or snap. The team’s models of these horns found that they were precisely structured to withstand the specific combat stresses that each species puts on them.

“We argue that 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,” said McCullough, whose study appeared September 8 in the Proceedings of the National Academy of Sciences.

And since beetles can’t repair or regrow their horns, she added, a male rhino beetle that doesn’t have the optimal weapon is unlikely to pass on its genes.

Elegant Study

The scientists are also curious as to what extent their finding applies to other armed creatures, including ones that use their weapons for other purposes such as defense or display.

Male fiddler crabs, for example, use their extravagant fighting claws to advertise themselves to females, and the horns of various mammals are thought to play a similar signaling role.

But even in such animals, McCullough said, “I think that the same things that we are arguing for the rhinoceros beetles will apply.”

University of Florida biologist Christine Miller agreed that the new study elegantly demonstrates that “diversity in animal weapons is clearly related to fighting style.” (See National Geographic Channel’s “Fight Like an Animal.”)

She added that other factors, such how females respond to male ornamentation, also influence the size and shape of weapons across species.

The study “begs the question, ‘Why hasn’t this already been done?’” added Miller, who wasn’t involved in the research.

Mammals Included?

Miller said it would be exciting to see similar studies looking at fighting styles and functional performance in animals such as African antelopes, which have horns in many shapes and sizes.

Jakob Bro-Jorgensen, who studies mammalian behavior and evolution at the U.K.’s University of Liverpool, also sees great potential in the horn-modeling approach of the University of Montana team. (Watch video: “Amazing Antlers.”)

“For instance, in bovids [antelopes, buffaloes, goats, and sheep], horn shape can be linked to the style of fighting against competitors, but it is still debated to what extent horns are also used in defense against predators,” Bro-Jorgensen, who also wasn’t involved in the new study, said by email.

“Now, by calculating the physical performance of horns under different fighting styles and as weapons against predators, we may achieve a deeper understanding of the interplay between these evolutionary forces,” Bro-Jorgensen said.

Or, as the rhino beetle might put it, “Let the battle commence.”