Why do bull elk grow such big antlers?

How come whitetail deer antlers have tines growing off a single main beam, while mule deer antlers fork? And why don’t either of them flatten out like moose antlers?

Douglas Emlen can tell you all about that first question, and he loses sleep at night pondering the others. But he’s pretty sure the same biological principles work for tiny New Guinean antlered flies as with the now-extinct mastodons that swung their 16-foot-long tusks around North America.

“The drive to be a breeder is so strong,” the University of Montana biologist explained. “It’s one of the most astonishing things about sexual selection. About 90 percent of males never breed their whole lives. But they do anything and everything in the world to get a chance to mate. It’s expensive, dangerous, they’re more likely to get eaten, but they’ve got almost no shot at breeding if they don’t do that.”

Emlen lays out the science behind that seemingly self-destructive system in his new book “Animal Weapons,” which hits bookstore shelves Nov. 11. While his personal specialty is entomology – the study of insects – he’s found the rules governing growth and use of those big displays of virility apply with surprising uniformity throughout the animal kingdom. Even stranger, he can hold a fascinating mirror between animal weapon development and our own human military arms race.

“I actually was amazed to find these military historians making the same observations about the animal world as I was seeing in the human world,” Emlen said. “Robert O’Connell wrote about weapons evolution 25 years ago and drew the same parallels with animals that I’m seeing.”

For example, the most successful animal weapons almost never get used. A 7-point bull elk often protects his harem of cows simply by parading his big rack around, deterring others from fighting him. It’s the same way the U.S. Navy deploys a Nimitz-class aircraft carrier in the South China Sea, knowing nobody else has a ship in the same league.

But just as a Nimitz-class carrier costs billions of taxpayer dollars, that big rack extracts a huge toll from the animal under it. Moose spend up to 50 percent of their food energy just growing antlers. They even rob their own skeletons for the calcium and phosphorus needed for construction. That leaves their bones brittle and weak just at the time they start crashing into one another in mating duels.

Growing big weapons requires lots of other things to fall into place. A slight change in food supply – say a drought – can stunt even a mature breeder’s headgear. So can an injury or illness. Natural selection can show effects quickly, as predator pressure bears down on the big guys. That includes us humans.

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The Boone and Crockett Club archives hunting trophy records for all major species of game animal in North America. Assistant director of big-game records Justin Spring said the logs show a variety of changes to different species. Pronghorn antelope trophies have steadily increased in size, which indicates their habitat quality has improved. But mule deer rack sizes no longer match those recorded in the 1960s and ’70s, in concert with noticeable habitat degradation across the West.

“For some particular species, especially thin-horned sheep such as the Dall sheep, the habitat is more accessible to hunters,” Spring said. “So we’re seeing a decrease in the trophy size as we’ve selected for the large horns. The basis of that is why we keep records.”

Humans aren’t the only trophy-hunters on the landscape. In Emlen’s scientific terms, “Increased exposure to the risk of predation turns out to be an almost universal cost to males for the burden of producing and wielding elaborate weapons.”

Consider the fiddler crabs. It grows one claw that outweighs the rest of its body. That claw is good only for fighting, which means the crab has only one other claw for feeding while females and immature males have two. It makes them slow to escape, and more attractive to predators who want to eat the big muscle inside the big claw.

There are two exceptions Emlen keeps in mind. First, big weapons serve very different purposes when they’re actually intended for killing. Prehistoric sabertoothed cats sported canines the size of Ruana knives, but they weren’t for attracting mates. They were for getting fed.

The second is that in the mating game, somebody always cheats. Emlen documents a multitude of ways wannabes outmaneuver the alphas. Bighorn sheep have “coursers” who lurk at the edge of a big harem.

“These sneaky males charge into a territory for a few seconds at a time, forcing quick copulations with females before getting hammered by the dominant males,” Emlen wrote. The coursers sire up to two of every five lambs born the following spring.

Dung beetles grow big horns that they use to defend dirt burrows where they trap females for mating. But some small males didn’t produce horns. Instead, they dug little tunnels right beside a big beetle’s burrow. And when the big guy was distracted, the hornless wonders would punch a side burrow in underneath him and get lucky. Emlen theorized that skipping horn growth actually helped, because it made the little beetles more maneuverable on their raids.

“They may not be able to mate as often as the weapon-wielding guarding males, but they make the most of the opportunities they get,” Emlen wrote. “Fighting wasn’t working, so these little guys switched to Plan B.”

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Fortunately for the general reader, Emlen spent a lot of his energy and resources turning years of research into an intriguing and brisk master class in natural wonder. And he covers the whole arsenal, especially the freakish productions of his favorite beetles.

In his lab at UM’s Bio Research Building, dozens of jars full of wood compost nurture Japanese rhinoceros beetles as big as chicken eggs (with a horn). One of his graduate students, Erin McCullough, recently finished a study using an engineering technique called finite element analysis to see why three similar beetle species grew such different weapons.

Finite element analysis was developed to test bridges and other structures for weakness. You poke and twist and bend a model until you find where it breaks or fails, which reveals how a design works. McCullough’s research found each species’ horn or jaw was ideal for its particular fighting style.

That may seem obvious, but it’s what keeps Emlen up at night. Why does nature endow animals with such a plethora of weapons variations? Why do mule and whitetail deer have different antler structures when they’re otherwise so similar? Why haven’t they settled on the AK-47 of antlers, with one form proving most efficient and deadly?

“I’ve always been interested in animals with crazy structures, that look so bizarre they shouldn’t be possible,” Emlen said. “I wanted to study the extremes, and beetles just blew me away. They have this incredible diversity of species, and thousands of them have spectacular weapons. We really can’t explain the nuances. But these differences of species are beautiful.”

Reporter Rob Chaney can be reached at 523-5382 or at rchaney@missoulian.com.

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