Animal Weapons

by Douglas J. Emlen

  The American Revolution would not have been named as such if the larger army had won. American rebel forces avoided direct, open-field battles with the better-trained and better-organized British army, opting instead for fleeting skirmishes, picking off troops as they marched or attacking when the larger army passed through choke points—narrow passages or river crossings that spread the army out, diminishing their numerical advantage.18 Similar tactics were used against U.S. forces in Vietnam and the Soviet Army in Afghanistan. Today, U.S. forces grapple daily with sneak attacks by insurgents in Iraq and Afghanistan.

  Guerrilla forces are “sneaks” for several reasons. In addition to breaking conventional rules of engagement, they use stealth and concealment to get close to the enemy, ideally approaching unnoticed until the moment of attack. They also “hide” in the sense that they rarely wear military uniforms. By blending in with the civilian population, guerrilla forces make it difficult for the invading army to tell friend from foe, a “lose-lose” situation for the army; erring on the cautious side risks death from attacks, while overreacting kills noncombatants and undermines political support for the campaign.19

  The biggest, most expensive weapon technologies sometimes succumb to sneak attacks. Foot soldiers don’t stand a chance in a direct confrontation with a tank, but a grenade or Molotov cocktail slipped into a hatch can change the odds considerably. Land mines and improvised explosive devices (IEDs) cheat, in the sense that they avoid a direct confrontation and lurk, instead, in the shadows, hidden in rubble or under the soil. These tiny weapons can disable multimillion-dollar tanks and armored vehicles, and subsurface mines can sink billion-dollar battleships. In October 2000, a tiny craft sailed alongside the USS Cole, a five-hundred-foot-long, $900 million guided-missile destroyer. The craft appeared benign, but was, in fact, laden with explosives, blowing a forty-foot hole in the hull of the destroyer, killing seventeen, wounding thirty-nine, and inflicting $150 million in damages.20

  The most dangerous form of sneak attack, at least against our modern forces, may also be the least appreciated. Cyberattacks don’t sound very scary, and it’s tough to imagine how they could threaten our security beyond the occasional hassle of a usurped credit card password or identity theft. But hacking may prove to be this country’s greatest danger, capable of crippling the entirety of our military forces.

  Over the past few decades, military technology has become increasingly computerized. Everything from the guidance systems on our missiles to the navigation and handling of our submarines, aircraft carriers, and aircraft relies entirely on high-tech software. Modern aircraft fly at speeds and in maneuvers that push the limits of human capabilities, and the most modern craft are impossible to fly without the aid of computer-enhanced flight controls.21 Targeting, flight-control, navigation, and even command and control, all depend critically on sophisticated electronics and software.

  Hackers sneak into our military mainframes, bypassing firewalls and surreptitiously inserting foreign code. Between 2003 and 2006, for example, Chinese hackers launched a series of coordinated cyberattacks against U.S. defense and aerospace installations.22 Before they were finally detected and the wormholes filled, these “Titan Rain” attacks pulled masses of sensitive military data from the U.S. Department of Defense, the Pentagon, NASA, Los Alamos Laboratories, Boeing, Raytheon, and other sources. Titan Rain demonstrated with startling clarity how China could use cyberwarfare as an asymmetric tactic to undermine conventional forces of an adversary.23

  In 2013, it became clear that China was at it again, this time worming into the control systems for an alarming number of our most advanced weapons, including, among others, the F-35 joint strike fighter and the V-22 Osprey tilt-rotor aircraft, the Terminal High Altitude Area Defense missile system, the Patriot Advanced Capability antimissile system, the Aegis Ballistic Missile Defense System, and even our Global Hawk unarmed aerial vehicle system.24 The fact that these crucial weapons were compromised is scary enough, but the truly terrifying part of these events was the realization that the Chinese were not simply pirating information. It now appears their plan was to inject code that would, when activated, give them complete control of our systems.25

  “Zero-day” attacks, as these are called, are the hacker’s most difficult and dangerous weapons, codes so deeply embedded that they lurk invisibly until the day they’re needed, exploiting vulnerabilities that even the software makers are unaware of.26 Had they not been discovered, codes inserted during the 2013 sneak cyberattack could have utterly incapacitated the most expensive and advanced weapons in the history of mankind, possibly even turning them against us.

  * * *

  Coursers, squirters, satellites, and female mimics—there are many ways to cheat. For animals, this means that in addition to traditional confrontations with armed rivals, dominant males now face the more insidious threats of males breaking the rules. Similarly, guerrilla forces, land mines, IEDs, and cyberhackers all can undermine the effectiveness of conventional military forces. As long as the impacts of these cheaters are relatively minor, not much changes. When cheaters begin to get too effective, however, they can end an arms race.

  11. End of the Race

  During the height of the Middle Ages, the strength, weight, and cost of armor reached unprecedented extremes. In staged tournaments where knights fought similarly armed rivals in one-on-one jousts, bigger was better, and knights with the best equipment usually prevailed.1 The advantages of armor held on traditional battlefields, too, where men-at-arms advanced to face opposing armies head-on. Suits of armor eventually got so heavy that men and horses became clumsy,2 forcing formations to march straight ahead into battle without sudden—or, indeed, any—changes in direction.3 But as long as they clashed with comparably armed opponents facing similar limitations, the best-trained, best-equipped men in the best armor still won, and the protection offered by armor justified its excessive cost. In fact, the knights were so well protected that most battles of this sort were settled with surprisingly few casualties. Loss of honor sufficed in lieu of death or dismemberment.4

  New types of weapons changed all of this, including the crossbow and, later, the English longbow.5 Like sneaky males, these new technologies “cheated,” breaking the rules of engagement in ways that eroded the benefits of expensive armor. Prior to the invention of the crossbow a knight could ride into battle immune to all but other knights. He could sweep through a field of peasants, slashing from on high as he charged, with sheets of mail and plate armor, shield, and helmet protecting him from anything hurled up by foot soldiers below. Knights alone had this advantage. They towered over the ill-protected, shoddily armed peasant fighters, dismissing them as a waste of effort and seeking out knights of status from the opposing ranks instead.6

  Armed with crossbows, however, ordinary farmers could shoot down the best-trained, best-armed knights of the day. Suddenly, sitting astride a horse was a problem, rather than a tactical advantage, because mounted knights made easy targets. Bolts fired up at them could slip beneath armor plates—into the armpits, for example—and direct hits penetrated regardless. Horses could be toppled, too, bringing the mass of muscle and metal crashing to the ground so that a knight lay underfoot, as helpless as an overturned turtle.7

  Crossbows and longbows broke every rule of engagement that mattered for the evolution of armor. Unlike armor, these weapons were cheap and relatively easy to use. They were not the exclusive purview of the wealthy elite. They didn’t require a lifetime of training, and as a result, they were not accurate reflectors of status or class.8 Most important of all, crossbows and longbows changed the structure of military engagements. Armies adopted new tactics to incorporate these weapons, and battlefield match-ups between rival knights gradually disappeared.9

  At the battle of Crécy, for example, English forces positioned themselves in terrain designed to concentrate fire from their archers onto the advancing French.10 Edward III chose a flat agricultural field flanked by forest and other n
atural obstacles, and ordered his men to dismount and wait, rather than advance on horseback. Three divisions, each containing one thousand men in armor (called “men-at-arms”), were arranged six deep in rows flanked on either side by roughly five thousand archers. A thousand men-at-arms were held in reserve as cavalry, ready to pursue the French in the aftermath.

  The combined strength of the English was approximately twenty thousand, of which four thousand were men-at-arms. The French, on the other hand, advanced with three times this number, including twelve thousand men-at-arms who stayed on their horses.11 The first French to advance were six thousand crossbowmen, mercenaries hired for the occasion. Behind them advanced the ranks of men-at-arms. The army marched to within 150 yards of the English, and the crossbowmen fired their bolts, but most fell short of the emplaced army. So they advanced again, only to be met by a shower of longbow arrows, shattering their ranks and triggering panic. Impatient to meet their equals in battle, the French cavalry charged forward anyway, plowing through and over their scrambling mercenary crossbowmen only to find themselves trapped in a deadly crossfire. Horses stumbled and collided, tripped over dying men, or were shot, and knights toppled to the ground. The few who reached the English line were routed, but the French kept advancing. More than a dozen waves surged forward, all tangling in the jumbled piles of bodies, trapped in a slaughter. When the French finally gave up, they left more than fifteen thousand dead. The English lost only two hundred.12

  Seventy years later, Agincourt ended the same way. Although the French outnumbered the English by five to one, their numerical advantage vanished in showers of metal-tipped arrows, as piles of bodies tripped up knights in the fray.13 Unhorsed and stuck in the mud, fully armored knights proved easy prey. The peasants they detested and ignored in traditional battles shot them point-blank with arrows, and what had been the ultimate protection before—magnificent armor untouchable by the masses—become a deadly liability. An inexpensive new weapon made elaborate armor obsolete.14

  Unhorsed knights were vulnerable to weapons that broke traditional rules of engagement, like the English longbow and, later, muskets.

  * * *

  No arms race lasts forever. As weapons get bigger they also get dramatically more expensive. Eventually, populations reach a new balance where the now-higher costs neutralize the reproductive benefits. Bigger stops being better, and the arms race stalls. Populations stabilize, hovering at the new weapon size. How big the weapons are at this stalling point depends on where the balance is finally reached; animal weapons under strong sexual selection, for example, may attain astonishing proportions before costs catch up and place the process in check.

  Populations in such a balance can persist for long periods of time, holding fast with massive structures. Were you to examine one of these populations and measure the strength of selection on weapons, you might find it is weak at best, or absent.15 When people look at species with huge weapons the last thing they expect to find is weak selection for weapons, but it shouldn’t be surprising. In theory, every population reaches such a balance eventually, and, once they get there, they should stay there. Considering how rapidly weapons can evolve and how many species appear to have had them for millions or even tens of millions of years, it’s only logical to assume that many have reached their balancing point, and their arms races have stalled. The tape on their tug-of-war rope holds fast, powerful forces canceling each other out as they strain in opposite directions.

  Cheating males work just like costs, offsetting the reproductive advantages of large weapons. For males to reap the full benefits of their weapons, success in battle must translate into success siring offspring. In a perfect world, victorious males sire all the offspring of females they defend. In reality, sneaky males surreptitiously mate with guarded females, stealing fertilizations and undermining the effectiveness of the guarding tactic.

  If a sneaky male dung beetle sires one-fourth of the guarded female’s offspring, for example, this slashes the payoffs to the dominant male by the same amount. He still pays to produce his horns, and he still pays to use them, fighting constantly to keep intruders at bay, but now the reward he earns is only 75 percent of what it otherwise could have been.16

  Averaged across the population, some fraction of male success is always likely to go to sneaky males. After all, cheating tactics are ubiquitous in the animal world; nearly every population has them. As long as sneaky males steal only small amounts of sired offspring, their effect on weapon evolution is likely to be minimal. However, when sneaks start doing well, they may erode the payoffs to fighting males substantially. Together with traditional costs, reproductive success lost to cheats can put the brakes on continued weapon evolution, helping define the point where populations begin to stabilize. In fact, if cheaters start doing too well, they may erode payoffs to weapons so drastically that the direction of selection reverses; big weapons become a liability. Instead of stalling, these races collapse.

  * * *

  Once the rewards for big weapons plummet, selection starts to favor rapid reductions in weapon size. In theory, if populations with costly weapons fail to lose them fast enough, they may even suffer extinction. We will never know exactly what happened to the majestic Irish elk, for example, but the latest attempts to model costs of antler growth indicate that antlers had reached a very expensive extreme, in which even the most successful males barely managed to recoup their calcium and phosphorus losses before the onset of winter.17 This, combined with climate reconstructions showing a dramatic decrease in the mineral contents of the plants the elk foraged, raises the possibility that a change in habitat pushed populations into a place where they could no longer afford to pay the price of their extravagant weapons. All we know for sure is that this drop in food quality coincides with their extinction.18

  More often, I suspect, populations persist but their weapons disappear. When a few species of stalk-eyed fly stopped roosting on hanging rootlets in Malaysian streams, two of the three arms race ingredients vanished; females stopped gathering in harems on hanging threads (they were no longer economically defensible), and fights stopped being duels. With two of the race-stimulating ingredients gone, the flies’ arms races collapsed. They lost their extreme weapons.19

  When one species of stag beetle dispensed with battles over sap flows, scrambling instead along the broad surfaces of the insides of hollow trees, resources stopped being economically defensible, and mandible sizes shrank.20 Similarly, males in three additional stag beetle species started forming stable, long-term pair bonds with single females and helping them raise their young—causing two of the three arms race ingredients to disappear. Now, not only were males not battling over localized sap sites, they weren’t even battling at all. The reproductive turnaround times of males were so similar to those of females that there was almost no competition. Today, their mandibles are tiny.21

  Environments change, and there is no a priori reason to presume that conditions conducive to arms races will last indefinitely. In fact, most clades of heavily armed species show ample evidence of weapon loss. Reconstructions of the histories of these groups reveal a multitude of evolutionary gains and losses, suggesting that weapon evolution is a dynamic, even cyclic, process of escalation and collapse. When my colleagues and I explored patterns of weapon evolution in a sample of fifty dung beetle species, for example, we found new horns had been gained fifteen separate times—the cauldron of preconditions sparked one arms race after another. But horns had disappeared, too. Nine times the race collapsed, and species lost their weapons.22 Similar studies of horns in antelope show both increases and decreases in weapon size.23 Arms races are like houses of cards, magnificent and fragile.

  * * *

  Whenever a small number of wealthy states invests in exorbitant weapons that nobody else can afford, someone, somewhere, is sure to come up with a cheap way to bring them to their knees, and, every so often, the smallest of weapons manages to topple a Leviathan. Fire ships brought panic
and disorder to naval fleets as far back as the fifth century BCE, when the Syracusans took an old merchant vessel, packed it with pitch and pine, set it aflame, and released it to drift downwind into the Athenian navy.24

  Two millennia later, fire ships were still used in essentially the same way. Wooden hulls and stores of gunpowder necessary for loading cannons made sailing ships of the line extremely vulnerable to fire.25 Small, disposable vessels packed with flammable materials or explosives could wreak havoc, especially if they were set adrift to blow into a full line of battle. A mile-long string of warships, all sailing in strict formation one behind the other, made an easy target even for a drifting vessel, and the fire ships were much too small for crews to hit readily with cannon fire, permitting them to slip dangerously close to the giant vessels.

  Although fire ships only rarely sank a ship of the line, they often forced fleets to break formation in ways advantageous to the instigators.26 In 1588, for example, the English sent eight fire ships drifting into the anchored fleet of the Spanish Armada, 140 warships clustered together in darkness off the Calais shore. The English could not afford to let the Armada stay moored, since reinforcements were likely on the way, and they needed to force them out into the open. The Spanish saw the fire ships coming—indeed they had been expecting them—and were able to catch and divert two away from the fleet. But the remaining six slipped past the outer defenses and dispersed the Armada. None of the Spanish ships caught fire that night, but the fleet was forced to weigh anchor in darkness. By dawn the next morning, the English had engaged them in battle.27