For animals, and humans, having bigger weapons can keep you from having to fight at all

To hear Doug Emlen describe the evolution of the weaponry and behavior of the dung beetle is alternately fascinating and uncomfortably familiar.

Emlen is the author of the new book, Animal Weapons: The Evolution of Battle, and a professor of biology at the University of Montana. The dung beetle, he says, is the “unheralded champion” of the animal arms race.

Emlen has been working for years to solve the riddle of why certain animals develop “crazy weapons.” Looking closely at the dung beetle helped Emlen figure it out, he says. “When you start looking across different groups of animals, from things like caribou, to fiddler crabs, or dung beetles, the same set of conditions tend to tip the balance in favor of an arms race,” he explains.

The crucial missing link that took him so long to stumble on is the nature of the fight itself, Emlen says. “The fights have to play out as predictable duels, one-on-one battles of strength. In those kinds of contexts, the male that's bigger and in better condition — and almost always has the bigger weapons — is the male that wins.”

In the case of dung beetles, “they might be smaller than a caribou or a mastodon, but relative to their body size their weapons are enormous,” Emlen says. One species has a pair of curving horns that arc up and over the back of a male beetle, extending beyond the end of its body. Another species has a huge horn that comes forward from between the shoulder blades, like a jousting spear — and these spears can be longer than the rest of the beetle.

Why would an animal that spends its life picking up pieces of dung and rolling them away need horns and spears? That was part of the mystery, Emlen says, because, obviously, it really doesn’t.

In East Africa, Emlen points out, you can see hundreds and hundreds of these beetles pushing their balls of dung across the dirt, and none of them have weapons. Yet if you peer beneath the pile of dung, you will find a network of tunnels and burrows extending down into the soil beneath the dung.

“That makes all the difference, ”Emlen says. “Inside the tunnels, the males will line up and face each other one at a time. You don’t have ten rivals piling onto a male at one time in a tunnel because only one can fit in the entrance at a time — and it has to approach face to face. … Most of the beetles with the crazy weapons are these guys that fight inside the burrows, in duels, underground.”

What are they fighting over? Take a wild guess.

“These are contests of strength and the beetles with the smaller weapons get pushed out of the burrow and they leave — they lose in the reproductive battle. They fail to acquire access to the female,” Emlen explains.

But in some cases, Emlen writes, the price these animals pay for developing these “extreme weapons” is high. 

As in the human arms race, the weapons become very “expensive.” In order for an elk or a moose, for example, to develop their large, deadly antlers, they “actually have to leach crucial minerals out of their skeletons and shunt them into the antlers,” Emlen says.

Some of the beetles Emlen looked at have stunted eyes. In others, the cost is even higher: some males with big weapons have stunted testes and smaller genitalia. That’s a pretty hefty price for these males to pay, Emlen notes. But interestingly, the price “keeps things honest,” he says.

As the weapons get bigger and more “expensive,” they become “honest signals, and they end up starting to function for deterrence.” Males begin to assess rival males without actually lunging into battle. They can size up the weapons of their opponent and “get an awful lot of accurate, honest information about whether that opponent is likely to clobber them or not — and most fights end up stopping before they actually start,” Emlen says.

“So there's this bizarre paradox that the species with the biggest weapons end up fighting the least frequently,” Emlen concludes. “Almost all of the ‘battles’ end up resolving themselves before they escalate into a dangerous battle — because literally sizing up the weapons side by side is enough to resolve it.”

“So you end up getting deterrence as an interesting sort of step along the way in these arms races in animals — and it all comes from those costs.”

Sound familiar?

This story is based on an interview that aired on PRI's Science Friday with Ira Flatow