Illustrations by David J. Tuss

^{Excerpted from Animal Weapons: The Evolution of Battle,* November 2014, published by Henry Holt and Company, LLC (2014 © Douglas J. Emlen)}

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By any account, elk are magnificent beasts, icons of power and beauty. But most of what impresses us sticks out from the tops of their heads. It’s the antlers that inspire our wonder—and our covetousness. The racks of elk, red deer, moose, and caribou have added regal splendor to the walls of royal halls for centuries. Indeed, no self-respecting chateau or castle would be complete without them. Antler-wielding stags are one of the most pervasive symbols in heraldic coats of arms and even in corporate branding. No matter how you stack it, we love antlers and horns.

But why are antlers so impressive? It’s not just that they’re weapons—most animals have weapons of one sort or another. Tigers and lions have claws, eagles have talons, snakes have fangs, wasps have stingers, and even our household dogs have a respectable set of teeth. What strikes us about antlers is that they are conspicuously big. The rack on a bull elk is forty pounds of bone erupting from the head in two curved beams, each adorned with as many as seven sharp tines. And, although most of us never stop to think about it, we all know at some level that anything that big must also be expensive. In fact, the price bulls pay for their antlers is extraordinary, and they pay this price again and again, since they shed and regrow their antlers anew each year. Antlers grow faster than any other bone in any animal. Growing antlers suck up so much calcium and phosphorus that males must leach these vital minerals out of other bones and shunt them to the antlers, depleting the rest of their skeleton so severely that they experience a seasonal form of osteoporosis.

Such is the reality of extreme animal weapons. Brutal and beautiful, extreme weapons have cropped up repeatedly during the unfolding of the history of life. All told, some three thousand species now wield them. That’s a drop in the bucket considering that there are 1.3 million described types of animals, but it’s a collection packed with remarkable creatures. Antlers in flies, forelegs in harlequin beetles, tusks in narwhals and elephants—these structures are alike in far more ways than simply being large. To better understand the evolution of these weapons, one can look at variation between individuals in a single species.

Pick any one of the heavily armed species, and look closely at the weapons as they are expressed from male to male. Here, within populations, lurks a pattern: not all males produce extreme weapons. Measure a sample of one hundred males, and you’ll find that most of the weapons aren’t all that big. Sure, some males strut with monstrous racks. Organizations such as the Boone and Crockett Club keep meticulous records of these super-stud bulls and bucks. But they do this precisely because such magnificent specimens are rare. The majority of bulls are not Boone and Crockett caliber. They produce weapons, but they’re middling.

Even though a history of sexual selection has led to evolution of extravagant weapons in a species, only a very few individuals actually achieve full weapon splendor, and lots of the males produce pathetic renditions of the structure. If males with the biggest weapons win in every sense of the word—they win the fights, they get the girls, and they sire the offspring—then why don’t all of the males produce full-size weapons? The answer is simple. They cannot afford them.

Animals are born with different resource pools. Elk calves born to the biggest, best-fed parents start life with an edge. They weigh more at birth, and have more stored nutrients and stronger immune systems. They have access to the best environments, including the safest, least stressful places and the best food. Other calves begin life under duress, with poor-quality parents in poor physiological condition. They’re born smaller and weaker, in substandard habitats. They grow more slowly and are quickly outpaced by the others. Because of their small stature, they lose contests over food, weakening them still further, and the stress they experience increases the risk of  infection. All of these experiences reinforce the differences in  size that they started with, magnifying the gap between dominant and subordinate, large and small. Even subtle differences early in life compound as the animals grow, and by the time these calves reach adulthood they’ll differ hugely in available resources. Only a very few will be able to produce the biggest, most extravagant weapons.

What’s more, weapons are discretionary. They are not necessary for survival. Females, for example, fare just fine without them in many species, as do small males. In stark contrast with the rest of the body, which must be built regardless, no weapon growth need occur at all. This means that the sizes of weapons should be far more sensitive to the availability of resources than other, mandatory body parts. My colleagues and I tested this a few years ago by perturbing the amount of food available to growing rhinoceros beetle larvae. By restricting access to nutrients, we experimentally altered the sizes of the resource pools available to developing males, allowing us to measure just how sensitive the different body parts were.

Rhinoceros beetle larvae feed on decomposing logs. We made an artificial diet for them by fermenting sawdust in giant composters, mixing in healthy doses of year-old maple leaves. After about a month, the goop turned chocolate-brown and smelled like a wooded streamside on a rainy day, just the way the beetles like it. These are big beetles—a typical larva is the size of a mouse—and for this experiment we placed half of the larvae each in its own pint-size jar filled with the food we’d made. We placed the rest of the larvae each in a gallon-size jar, also filled to the brim. The only difference between the habitats was the amount of food to which each larva had access. When adult beetles emerged from the jars several months later, we collected and measured them, comparing males from the two diet treatments.

Not surprisingly, nutrition had a pronounced impact on beetle growth. Genitalia in poorly fed males were 7 percent shorter than in well-fed males. Wings and legs each were about 20 percent smaller. Horns, however, differed by almost 60 percent, meaning horn growth was three times as sensitive to nutrition as wings and legs, and almost nine times as sensitive as genitalia.

All big weapons are exquisitely sensitive to nutrition. Like lottery winners upgrading to bigger houses, male beetles fed artificially supplemented diets grow into adults with bigger bodies and much longer horns. Remove the food, and you find the reverse. Well-fed male earwigs grow longer forceps than poorly fed males do, and well-fed stalk-eyed flies grow longer eyestalks. The same holds true for deer antlers, elk antlers, and ibex horns. Food is like income to an animal, filling its coffers so that it can later spend. Males able to sequester surpluses of nutrient reserves have large discretionary pools, and they can afford to produce big weapons. Other males have fewer resources to start with. Mandatory expenses may claim everything these males have, leaving nothing left to spend on weapons.

Weapon growth is extra sensitive to illness, too, for exactly the same reason. Infections drain resources from the surplus pool. Males fighting infections as they develop cannot afford to pour as much into weapon growth. Parasites gnaw away at tissues, pathogens battle the immune system, and all of this gobbles up stored reserves. Weapons and other discretionary structures absorb the brunt of these losses. Antlers grow to a much smaller size in sick males than they do in healthy ones, for example, as do Cape buffalo horns and fiddler crab claws.

Everything about weapons is expensive, from the resources pulled from the pool to permit their excessive growth to the constant drain required to keep them, carry them, and use them in battle. That is why weapon size is extra sensitive to the vagaries of life. Because differences in animal weapon sizes reflect the health, nutritional history, overall condition, and genetic quality of each male, they’re a meaningful signal—a visual proxy—for fighting ability. Of course, other body parts differ, too. The best bull elk stand taller, and have bigger heads and longer tails, than poor-quality males. But weapons make better signals for two reasons. First, they’re vastly more variable than other elements of an animal’s arsenal. No elk stands zero inches tall, for example. All elk have bodies and muscles, but there are plenty without antlers. Weapon size spans all the way from zero to enormous, resulting in a bigger range from male to male than in other body parts. It’s far easier to discern six-foot-long antlers from a pair of six-inch spikes than it is to tell that one male stands a few inches taller than the other. Even subtle differences in the body sizes or fighting abilities of males become amplified into pronounced differences in the relative sizes of their weapons.

Second, these structures are huge. Weapons are massive and conspicuous outgrowths, biological billboards advertising the quality of a male for the world to see. Best of all, these billboards are honest. A puny bull elk cannot fake a giant rack of antlers. A poor-quality bull that somehow grew big antlers wouldn’t be able to use them. He’d lack the size, strength, energy reserves, and stamina necessary for wielding his antlers effectively in battle. The effort would be futile.

Males invest as much as they can in their weapons, but not all males have the same amount available to spend, and weapon sizes differ wildly as a result. Ultimately, though, this comes in handy. Because weapons display crucial information about the health, status, fighting ability, and overall quality of a male, and because they’re so visible, they make it easy for rivals to assess each other before allowing confrontations to escalate into dangerous battle.    

--DJE
* http://animalweapons.com/