Animal Weapons

by Douglas J. Emlen

  Deterrence, in turn, feeds back on the arms race, driving weapon evolution forward at a faster and faster rate. The moment weapons start functioning as cues of fighting strength, a whole new incentive for extreme size arises. Now, males with the biggest weapons win for two reasons: because they defeat rivals in battle, and because their weapons are big enough to end most confrontations without a fight.16 Fight costs saved by deterrence compound already-impressive rewards for males with the largest weapons.

  Of course, males aren’t the only ones who pay attention to weapon size. In many species females study them, too.17 And why not? They’re obvious and reliable advertisements for the quality of a male. Female crabs are more likely to approach males with bigger claws,18 and they prefer claws with bright colors.19 Female stalk-eyed flies prefer males with longer eyestalks;20 female earwigs prefer males with longer forceps;21 and red deer22 and topi23 females prefer males with bigger antlers and horns. (Attracting the girl is yet another reward for having impressive weapons.)

  Finally, the nature of deterrence—the way males assess each other before battle—reinforces the one-on-one nature of male contests, strengthening the conditions conducive to an arms race. Repeatable sequences of assessment behavior work like a branch or a tunnel in the sense that they force males to duel. Even if fights occur in the open, as they often do with caribou and antelope, the stages of sequential assessment align contestants so that full-blown battles always end up head-to-head and one-on-one, ensuring that when outright battles do occur, the better-armed male wins.

  As weapons get bigger they select for increasingly elaborate deterrence, and deterrence, in turn, selects for bigger and bigger weapons. Arms races and deterrence push each other forward, escalating in an evolutionary spiral. Like a figure skater pulling her arms to her chest, the evolution of extreme weapons gets faster and faster.

  * * *

  In its prime, the British Empire controlled a fifth of the world’s population, with colonies and territories on every continent. This was a remarkable accomplishment for a tiny island nation, and it would not have been possible without the absolute supremacy of her navy.24 Throughout the eighteenth and nineteenth centuries, the Royal Navy was the largest in the world, filled with fleets of sailing fortresses—magnificent sailing ships with tall masts and solid oak hulls stuffed with cannons.

  The cost of building warships was enormous. The hull of a single seventy-four-gun ship required 3,500 oak trees—mature hardwood trees at least a hundred years old each—and a one-hundred-gun ship needed almost six thousand trees.25 With European nations largely deforested already, these hardwood costs were incredible; only countries with extensive networks of shipping routes and colonies could afford to import the necessary lumber, and, most of the time, they simply assembled the ships in their colonies. Add in the costs of shipyards, engineers and shipwrights, labor crews, cannons, rigging, and trained officers and crews, and the price of the biggest warships was way out of reach for hundreds of the world’s nations. Fleet size and ship size became signals of a country’s fighting ability—perfect cues for deterrence.

  Just like crabs, when navies clashed, ships sought out rivals of similar size. Fleets lined up single file, leading with their largest ships and assembling them in descending order down the line.26 Opponent fleets did likewise, matching the largest ships against one another. Navies with the greater numbers of bigger ships had an edge since their string of “heavies” extended farther down the line. Even when battle lines devolved into frantic melees, like still sought out like. Big ships could always beat smaller ships, but their bulk and weight slowed them down, and smaller ships shied away. Medium ships escaped larger ones, but were too slow to catch the small ones, and so on. Warships inevitably chased down rivals of comparable bulk and speed.27

  The flagships of these navies were the “first-rate” ships of the line—the most massive ships in existence in their day. First-rates were potent weapons, capable of shattering the hulls of lesser ships with single broadsides of cannon fire. They were also stunning to look at—paragons of power. One glance at these splendid ships was enough to know just how effective they could be in a battle. Simply sailing a ship this big into troubled harbors could quell uprisings or settle disputes anywhere in the world.28

  At a time when most of the world had no first-rate ships at all, Britain kept dozens and dozens in her fleet. During the Napoleonic Wars, for example, Britain had 180 warships of seventy-four guns or larger.29 The Spanish, Dutch, and French navies all vied for power with Britain at one time or another,30 but none could match the British navy and, by the end of the Napoleonic Wars, Britain alone ruled the seas. For most of the nineteenth century, a period called the “Pax Britannica,” the majority of local conflicts were settled without escalation simply because of the presence of a British warship. As with crabs and caribou, big weapons and deterrence ushered in a window of relative peace.

  The cost of state-of-the-art weapons is still staggering today, and only the richest can afford the best. More than one thousand feet long and displacing one hundred thousand tons of water, Nimitz-class nuclear-powered aircraft carriers are home to ninety fighter jets, batteries of antiaircraft missiles, and more than six thousand crew members.31 A ship this size costs $4.5 billion to build, not counting the aircraft. Modern fighters such as the F/A-18 E/F Super Hornet cost $67 million apiece,32 bringing the total ship cost closer to $10.5 billion. Add the price of training and paying six thousand military personnel, and the cost climbs still higher. On top of this, each carrier by itself is vulnerable to attack, so they never travel alone. Carrier strike groups consist of a carrier plus a miniarmada of support craft, generally two guided-missile cruisers, between two and four antisubmarine and antiaircraft destroyers, and, often, a submarine. The purchase cost for a carrier strike group exceeds $20 billion, and one recent study estimated operating costs of maintaining a strike group at $6.5 million per day.33

  The United States has ten Nimitz-class carrier strike groups; no other nation has anything even close to this. We employ our navy in ways similar to nineteenth-century Britain. Massive, powerful, and prohibitively expensive, our carriers function both as weapons and deterrents, portable projections of military power shuttled like chess pieces to stabilize troubled regions.

  10. Sneaks and Cheats

  During my final year in Panama, after dawn dashes into the forest to find monkeys and in between generations of the artificial selection experiment, I spent my days in the dark under a tent of thick cloth strung from the ceiling of my office. This time, my objective was simply to watch. Anything and everything the beetles did, I wanted to see—no one had ever studied their behavior before, and I was eager to see how the males used their horns.

  The problem was, everything interesting happened belowground. I didn’t have thousands of crabs fighting at my feet or jacanas prancing on mats of floating lettuce. These guys disappeared into pencil-sized burrows in the clay. At the end of the nineteenth century, the French naturalist Jean-Henri Fabre overcame a similar challenge while studying the underground breeding behavior of a European dung beetle species. He secured a pie tray with a hole in it on top of a vertical soil-filled glass tube.

  A hundred years later I’d graduated from glass tubes to glass sandwiches. I built a series of “ant farms,” packing soil between panes of glass. Instead of pie trays, I used Plexiglas boxes with slits cut into the floors, securing one to the top of each ant farm. When the beetles tunneled, they had no choice but to excavate in between the panes of glass, permitting me to peek inside. Bright lights bothered the beetles—the insides of tunnels don’t normally get much sunlight—so I had to simulate darkness. Fortunately, dung beetles are like harlequin beetles in that they cannot see the color red. Using red-filtered lights, I could watch without disturbing them from inside my black cloth tent.

  For four-hour stints I scribbled notes, squinting in the dim light while watching pea-sized beetles tussle inside tiny tunnels. Heat from the lamps
cooked the little tent, and the smell of must and manure was overpowering. But the beetles thrived inside the glass sandwiches. They fought and mated and provisioned their young, and I got to see it all.

  It was immediately clear that males use their horns in fights. This was not surprising, but it was exciting to see nevertheless. Fights were wonderfully chaotic. The guarding male braced himself, leg spines lodging into the soil walls of the tunnel, the intruder pushing into him, forcing himself downward and twisting with his head and horns. The horns of the males would enmesh as beetles pried with their heads and pushed. If the males were evenly matched and the fights escalated, they would become increasingly frenetic, with males forcing the tunnel wider and flipping past each other. Back and forth they’d switch positions as first one and then the other managed to slip into the prized inside spot. Sometimes the dueling beetles backed all the way down to the female, slamming against her in the fight. Other times they’d tumble outside the tunnel onto the surface. During the craziest fights I’d lose track of who was who but, at the end of the scuffle, it was almost always the male with the smaller horns that ended up leaving.1

  After I’d watched enough of them, the fights became a blur, playing out the same way every time. Watching the winners wasn’t very exciting. In the end, it was the losers who surprised me. If a big male lost a fight he’d storm off in search of another tunnel and another challenge. In the field he’d have to travel only a half inch or so before reaching the next tunnel; in my cages he had no such luck, so he’d walk in endless circles around the perimeter of the Plexiglas boxes. But tiny males did something very different. After getting booted they’d only go a short distance away—maybe a half inch—and there they’d begin to dig their own tunnels. Excavating tunnels is typically a female behavior, but here these little males were making new tunnels, right beside the guarded tunnels.

  I got very excited when I first saw this, thinking the little male might sneak back into the main tunnel belowground. But all he did was sit there. For hours he just sat, and I sat, until I started to get restless. Of course, it was the moment when I left to use the bathroom that he did it. I came back to find the whole thing over. The little guy was back in his tunnel, but I could tell from the chamber that he’d excavated a side tunnel, drilling right across into the main burrow. I started setting up five or six chambers at a time, all with mixtures of large and small males, and sure enough, I finally saw the sneaking firsthand. After sitting still for hours, the little male suddenly stirred, pushing into the side of the main tunnel and shooting down the shaft to the female. He could mate with the female and bolt right back out again in just a couple of minutes, while the guarding male blocked the entrance above, oblivious to the intruder.

  Sneaky male dung beetle

  When I told my doctoral committee about the side tunnels they challenged me, pointing out that these beetles were digging in skinny glass sandwiches, a two-dimensional universe. Where else were the little guys going to go? Of course they’d bump into the main tunnel in an ant farm. The real question was whether they did the same thing in the field. Armed with tubes of warm silicone caulking, I squirted white goo into tunnels in the forest. Pieces of monkey dung aren’t very big—maybe the size of a silver dollar—and it was not uncommon for there to be entrances to ten or twenty separate tunnels crammed into the soil beneath them. So I filled them all with silicone, then dug the whole thing up and carted it back to the lab where I could gently wash away the soil, revealing casts of the tunnels.

  Not only did side tunnels happen in the wild, they happened often. It was clear from the casts that sneaky males could hit four or five tunnels with each of their horizontal shafts.2 Now, I understood why big males periodically patrolled their tunnels, and I was beginning to understand why small males don’t produce horns. In this species, like many of the tunneling dung beetles, the largest males all produce a pair of long horns, but the smaller males do not. They don’t even have intermediate horns. Instead, they appear to shut off horn growth entirely, and they mature looking a lot like females.3 Small, hornless males maneuver inside tunnels better than the bigger, horned males, in part because they don’t have horns to get in the way.4 Thanks to work by John Hunt, Joe Tomkins, and Leigh Simmons at the University of Western Australia, we also now know that the tiny males in many of these “dimorphic” dung beetle species are highly specialized sneaks. Quicker to mate and quicker to transfer sperm, they also have relatively bigger testes, and much more sperm.5 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. Fighting wasn’t working, so these little guys switched to plan B.

  * * *

  When only a few dominant males monopolize access to reproduction, there’s strong incentive for the rest of the males to break the rules. If you can’t win the game the normal way, cheat. Sneaky males are everywhere, lurking in populations of nearly every animal species.6 Bighorn sheep rams guard harems on sheer slopes high in the Rocky Mountains. The largest and oldest males have by far the biggest horns, and these males consistently win ownership of the harems. Yet as many as 40 percent of the lambs end up sired by smaller males.7 Called “coursers,” 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.

  Male sunfish and salmon guard cleared patches of sand where females come to lay eggs. Females choose large, attractive males with the best territories, sidling up next to them so they can shower sperm over her eggs. Tiny males have no chance of defending a territory or being picked by a female, so they dart in surreptitiously and squirt clouds of sperm into the mix.8

  The biggest male ruffs, tall shorebirds that wade through marshes in Europe and Asia, guard territories from which they court, resplendent in gorgeous manes of fluffy black and chestnut feathers. Females consistently pick the largest, brightest males as mates, leaving few options for the rest. But smaller males cheat in two ways. One type of male dispenses with the black and chestnut plumage and dons a mane of white instead. These males are satellites, skirting around the edges of male territories and attempting to intercept females as they approach the dominant males.9 The dominant males tolerate them, to an extent, because females appear to be drawn to territories with both dominant and satellite males.

  Satellite males are obvious, decked out in white; but a third type of male mingles in the territories, too, and these males are tough to spot—so tough, in fact, that their existence in the mix was not discovered until decades after studies on ruffs began.10 Called “faeders,” these males look and act exactly like females. As a result, they’re able to encroach onto the best territories unnoticed, edging up to females right in front of the territorial male.11

  Sneaky males mimic females in a diversity of species. Marine isopod crustaceans—swimming pill bugs, for lack of a better term—guard hollow cavities in palm-sized sponges where females visit to feed and mate.12 The biggest males wield fearsome claspers: tongs they use to grapple with rival males. Males with the longest tongs win, and these males succeed in guarding the best sponges. But other males work their way into the sponges, too. Medium-sized males dispense with weapons and look exactly like females. Like ruffs, female-mimic isopods are able to hang out inside sponges undetected by guarding males.13

  Australian cuttlefish have the cleverest female mimics of all. These marine mollusks have superb color vision. Masters of camouflage, they’re able to change the color of their skin in seconds so that it blends seamlessly with their backgrounds. For most of the year they live invisible and alone but, during a brief mating season, hundreds converge, and males begin to display, changing from dull, cryptic colors to a beautiful rainbow of greens, blues, and purples.

  There may be as many as eleven males for every breeding female, so competition is fierce, and females approach the biggest, most colorful males.14 Once a female chooses, the pair will attempt to swim to the periphery of the group to sp
awn. But sneaky males accost them in the process. The fact that they can change color so fast gives these mollusks a number of options. Sometimes little males charge in bright and strong, courting the females outright while guarding males are distracted in battles. Other times, they slide in looking like rocks, blending with the ocean floor as they glide up to the pair. Often, one will approach looking like a female, so that he can swim all the way up beside the dominant male unmolested. When the sneaky male has slipped in between the dominant male and the female he’s mating with, working himself right beside the female, he will flash on his bright courtship display. However, he’ll activate his display on only one side of his body—the side facing the female. He keeps the side visible to the dominant male cloaked and femalelike.15

  * * *

  Cheating and sneaking are as pervasive in human populations as they are in other animals, and they can stall even the largest armies. “Irregular,” or “guerrilla” warfare tactics date to at least the sixth century BCE, with the writings of Sun Tzu’s Art of War.16 The basic idea is simple: if a population faces invasion by an overwhelmingly superior army of conventional forces, break the convention. Don’t fight them on their terms. By hiding out in local terrain and attacking only in swift, covert strikes, a smaller force can inflict irritating and demoralizing damage to a larger force. They’ll never defeat the larger army outright, but they don’t have to. Sneak forces “win” simply by staying in the game—by surviving, and by slowly draining the will to fight from the larger force.17 Because irregular forces refuse to confront the larger army in traditional battle, they are all but impossible to expunge, and in the face of such secretive strikes, the bulk and strength of the conventional force actually becomes a liability.