Animal Weapons

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Animal Weapons Page 13

by Douglas J. Emlen


  Training, horses, weapons, and armor were extraordinarily expensive, unattainable for all but the wealthiest sons of noble families.

  * * *

  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 stand 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.19 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.20

  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 could no more fake a giant rack of antlers than I could buy an Azimut 40S.

  Suppose for the sake of argument that I did go buy that boat. Say I threw caution to the wind, and I bought it. I couldn’t afford to register it, or buy fuel to drive it, or repair it if it got dinged. Similarly, 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.21

  The price of big boats, like big weapons, increases exponentially with size, and people tend to buy the biggest boats they can. Next time you stroll through a marina or harbor, pause for a minute. Take a gander at the variation in the sizes of the boats people amass for their recreation. Little itty-bitty boats, in-between boats, big boats, and, occasionally, parked by itself way out in the harbor, or on its own dedicated pier, a really, really big boat—a 150-foot floating mansion of a yacht; the top of the pack. Yachts are status symbols for a reason. Like weapons, their size clearly and accurately reflects the resource pools available to their owners.

  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.

  9. Deterrence

  Suddenly I was awake, the moist floor of the tent ballooning around my face. As I sat up on my knees the nylon fabric whooshed to fill the void where I’d been, squeezing up gently around my wrists and knees, gurgling like a giant waterbed. I could hear rain smacking onto the roof and sides of the tent. A storm had blown in and it was pouring; but that wasn’t it. There was something else. Waves. We were surrounded by waves. Somehow, the tide had risen farther than expected and waves crashed into our tent, engulfing us as they raced up the shore.

  We hopped into the darkness, rain streaking down in sheets, to find ourselves shin deep in warm water. The three of us grabbed corners of the tent and yanked, uprooting the whole thing in a jerk, frame bending precariously from the weight of our sleeping bags sagging in the bottom. Holding the dripping contraption high enough to clear the surf and laughing hysterically at the ridiculousness of our situation, we raced for higher ground. (We learned the next week that a hurricane hundreds of miles to the north had triggered a record-high tide that night.)

  Soggy exodus aside, this had been one of the most magical nights of my life. My wife, Kerry, our friend Lisa, and I had read about this pristine Costa Rican beach in our guidebook. The five-mile hike kept all but the occasional hardcore surfer away, so we had the place almost entirely to ourselves. The scenery was postcard perfect, tropical forest abutting white sand and blazing blue water, palm fronds fluttering in the breeze. After exploring in the afternoon, we’d set up camp at the edge of the trees so our tent peeked out over the water. But the real treat came with darkness.

  With no lights or buildings for miles in any direction, the night was completely black and stars in the sky dazzled. The surf dazzled, too. Churning waves glowed blue-green from phosphorescence in the water. Each wave lit up as it curled, bright sheets of brilliant green lifting to the sky, before collapsing into the shore and sending glittering foam across the sand. All the way up the beach, waves lit the night and the sand shone green. As we skipped through the darkness our footprints and the soles of our feet glowed, so we choreographed neon paintings in the sand with our tracks.1

  The beach came alive in another way, too. Ghost-white crabs skittered everywhere, nickel-sized bullets shooting from underfoot as we danced. At the water’s edge the little crabs fed on detritus in the glowing froth, bits of phosphorescence sticking to their bodies. Inshore from the water, thousands of thumb-sized holes sprinkled the beachscape. Spaced maybe a foot or so apart from one another in a dotted grid, they covered the beach. Beside each burrow hovered a crab, poised and ready to dart for cover when predators ventured too close. Thousands of additional crabs flurried about, approaching and occasionally challenging owners of the burrows. If we stood absolutely still, they darted between our legs and over our toes.

  We were witnessing a spectacle few ever notice even though it unfolds on tropical beaches across the Pacific, Atlantic, and Indian Oceans. Night after night, day after day, tiny crustacean fighters face off in the sand. Ten thousand battles unfold each night on this beach alone, and beaches just like it all over the world. Ghost and fiddler crabs aren’t hard to find, or to observe. Anyone spending any time on a beach has seen them, but few of us take the time to watch what they do.

  One person who does watch is John Christy. For more than thirty-five years, John has studied the behavior of fiddler crabs, spending countless hours watching them fight and court on strips of beach and tidal flats of Panama. As a doctoral student in the 1970s, John lived by himself on a tiny island in Charlotte Harbor, Florida, with no one to talk to except his pet smooth-billed ani and the crabs. Dirt-poor, as graduate students almost always are, he convinced the manager of a nearby field station to let him live on the island for free. The only building, a premade one-room aluminum box lacking electricity and plumbing, became his home. Two thousand feet long and a thousand feet wide, the smudge of land called Devilfish Key was hot, humid, and thick with mosquitoes—a far cry from our idyllic Costa Rican paradise. But its tiny beaches were covered with millions and millions of crabs.

  Nobody ever visited Devilfish Key, so John could set up experiments without interference. He placed hundreds of colored flags in the sand, marking and mapping the locations of burrows. He excavated tunnels to see how deep they went and discovered females tending broods inside some of them. Super Glue had just been invented, so he stuck tiny colored tags onto the backs of five hundred crabs, recording the body size and claw size of each animal and watching to see what they did. He recorded who approached whom; how they interacted when they faced off at close range; whether they fought; who won if they fought; and who succeeded in breeding.2 Most of all, John wanted to see what they did with their extraordinary weapons.

  What he found was they wave them. Up and down, up and down, again and again they raise their claws high and drop them. Dozens of times each minute, thousands of times per hour, hour after hour. When contests get intense, claws clearly function as weapons—dangerously strong claspers capable of inflicting real harm. Yet they also appeared to serve as signals. In fact, for every few minutes of outright fighting, males spent dozens of hours waving. Mos
t of the time, fiddlers were employing their claws as warnings rather than instruments of battle. They were using them for deterrence.

  Male fiddler crabs wave their claws, using them as deterrents first and, only occasionally, as actual weapons.

  * * *

  The best way to tell if you can beat opponents in battle is always to fight them. Charge with everything you’ve got, hold nothing back, and, at the end of the brawl, you’ll have your answer. The problem with fighting is it’s dangerous. Sometimes the risk is simply a by-product of being distracted. Crabs are well protected from one another, since their exoskeletons are like armor, but fighting crabs are distracted crabs and they make easy targets for gulls and grackles. Other times distraction is deadly because of where the fight takes place. Bighorn sheep and ibex smack heads on narrow ledges of steep cliffs, and a single misstep can be disastrous. Even a broken leg is fatal, so males pay constant attention to their opponents and their footing as they fight.

  Most of the time, the fight itself is dangerous. Elephant seal bulls are always covered in gashes. I’ve seen bulls with six-inch slabs of fat and skin dangling from cuts inflicted in battle. Tines on horns and antlers catch and parry an opponent’s thrust, but tines get tangled, and bulls whose weapons lock die helplessly intertwined. Rhinoceros beetles with holes in their cuticles are commonplace, punctured by barbs on a rival male’s horns. Jabs and stabs from tusks can shatter bones, inflict deep stab wounds, and trigger infections. Almost every fight brings with it some risk of real harm, and males with battle scars are so ubiquitous in the animal world that we tend to take them for granted.

  What if there was a way to tell who was going to win before entering into dangerous battle? If a male could discern that he was likely to lose beforehand, he might opt to just walk away. A male ceding a fight outright forfeits the chance of mating, but he lives to fight another day. Walking away when he might have won would be crazy, but ceding fights he would have lost saves time, energy, and risk. This is particularly true for the smaller contestants in poorer condition, since they’re the ones most likely to be maimed in the fray. The trick, of course, is to accurately predict who’s likely to win, and to do that, rivals must have an easy way to size each other up. They need a conspicuous indicator of the fighting ability of potential competitors.3

  When fiddler crabs size up rival males, they look at their claws. For one thing, they’re huge and, in many species, brightly colored, making them supereasy to see. They’re also the most variable part of a crab, ranging from tiny to extreme and including everything in between. Finally, like all big weapons, claw growth is sensitive to parasites, disease, and nutrition, so the male with the bigger claw really is the most likely to win.4 This makes claws worth paying attention to, and males use them to evaluate one another before battle.

  * * *

  Fiddler fights revolve around burrows. Resident males defend ownership of a burrow, cleaning and widening and perfecting it each time they have a moment to spare. Females visit a succession of males, inspecting each burrow before picking just the right one. After choosing, they mate with the male and lock themselves belowground for several weeks while their broods develop. Females are consistent in their choice of burrows, picking tubes of the right width and depth, positioned just the right distance from the water—high enough to escape flooding by the highest high tides, but low enough that the bottom of the burrow stays moist.5

  Males battle for burrow ownership, guaranteeing that the biggest, best-armed males end up residing in the most attractive burrows. At any point in time, the beach will be blanketed with guarding males, stationed as if tethered by threads to their tunnels, waving claws like banners in the air. But beaches are full of wandering males, too, and animals cycle between these two states. Guarding males cannot eat, since their food lies at the waterline farther down on the beach. They live off of stored nutrient reserves, depleting them a little more each day. Eventually, even the best males run out of steam and are forced to abandon their burrows to go feed and refuel. The instant they leave others will claim their burrows, so they’ll have to work their way back up the beach as wanderers, challenging males in attempts to claim new burrows as they go.

  The sheer numbers of crabs—hundreds of thousands per beach—and the constant turnover between wanderers and guards, result in an astounding number of face-offs. Each resident male faces hundreds of challenges per day, and evictions and turnovers are commonplace. These many encounters would be dangerous indeed if they all involved pitched battles. But they don’t. Most confrontations settle long before they escalate that far.

  Imagine the beachscape from the vantage of a wandering crab, eyes perched on stalks a mere inch above the sand. Everywhere he looks are claws jerking up and down, bursting above the horizon and back again. Over and over they rise and fall, surrounding him with an incessant barrage of motion. The wandering male doesn’t simply march up to the first burrow he encounters. Nor does he approach at random. As he weaves between waving males, he sizes them up—bigger claws reach higher than smaller claws—advancing only toward burrows guarded by males whose claws are equal in size or just a bit smaller than his own.6

  This is remarkable not only because it implies males can tell how big their own claws are (how else could a male discern at a distance that some claws were too big?), it also means that males opt out of most confrontations from afar. An overwhelming majority of contests end before they ever begin, without anything even resembling a fight. A mere glance at a big claw is sufficient to deter smaller males.

  Only when a male finds an appropriately sized rival will things advance to the next level. As a wandering male approaches, the resident turns to meet him head-on. The guarding male extends his claw forward, and the wanderer pushes back against it in a gentle sequence of shoves, claw to claw.7 If the intruder guessed poorly on his approach and the resident’s claw is too big, he’ll back down at this stage. If not, then the pushes get a little rougher. Each male rubs his claw along the length of his opponent’s, sliding claw against claw in the process. Here, too, many males opt to depart peaceably. Unless they’re very evenly matched, the smaller male walks away.8

  If both males hold fast past this stage, then the confrontation escalates once again. This time it includes much more forceful shoves and clasping—squeezing with the full strength of the claw. Finally, if neither backs down, males enter into unrestricted combat, attacking with powerful strikes and grabs in a fierce war of attrition. The resident may back into the safety of the burrow and shield himself while the intruder hammers his claw against him and continues to strike until one of them finally gives up and flees.9

  Fights reaching the pinnacle of this sequence are furious and all-consuming, energetically demanding and dangerous. But fights this intense are rare. Given the incredible number of male-male encounters that take place on beaches each day, it’s remarkable how few reach actual battle. For every contest settled in a full-blown brawl, hundreds are resolved peaceably. Fiddler crabs have the largest weapons relative to their size of any living animal but, because claws act as deterrents, they almost never have to use them in fights.

  * * *

  Whenever there is an adequate cue that conveys information about a male’s fighting ability, it will always benefit other males to pay attention.10 There really is such a thing as choosing battles wisely. Males who fight every battle to the fullest, attacking indiscriminately and never holding back, end up mangled and exhausted, or dead. By assessing the abilities of rivals before fights get costly, and attacking only when the likely payoffs exceed the risk of injury or failure, males allocate resources more efficiently.

  Like fiddler crabs, male bamboo bugs use their weapons as deterrents first, and only occasionally as instruments of battle. Huge hind legs bulge out from their flanks. Thick, strong, and armed with sharp spines, these legs can squash a rival male, crushing and puncturing cuticle in the process. Males guard harems of females crowded onto new shoots of bamboo, shepherding the
m together and facing off with intruder males. As rivals approach, guarding males wave their big legs in the air, flashing them in the faces of the intruders. Usually this is a sufficient deterrent. Only when males are very evenly matched do confrontations progress into actual battles.11

  Ibex have the longest horns of any ungulate, yet they, too, almost never fight. Males size one another up constantly, strutting and sometimes running side by side, and they spar. But smacking heads in full-blown battle is extremely rare.12 Caribou are similarly cautious. One study followed more than 11,600 male-male contests over two years; only six escalated into outright battle—less than one-twentieth of 1 percent.13

  Bamboo bugs wave their weapons to deter rivals, and only escalate to fights if they are evenly matched.

  Ibex rams size each other up, comparing weapon sizes; most confrontations end without escalating to battle.

  In one of nature’s more amusing paradoxes, the most extreme weapons are also the least likely to be deployed in pitched battle.14 The biggest, best-conditioned males wield massive weapons capable of deterring most rivals on the spot; the mere presence of these weapons is sufficient to dissuade all but the biggest competitors. For the rest of the males with intermediate or small weapon sizes, battles are engaged only when rivals are equitably matched.

  * * *

  Deterrence is an integral and intuitive stage in the unfolding of an arms race. As weapons increase in size they get more expensive.15 Fewer and fewer males can afford to pay the price, widening the gap between rich and poor. By pulling the extremes farther apart, evolutionary increases in weapon size magnify the rifts between haves and have-nots. Zero stays zero, but the biggest weapons keep getting bigger. As signals, weapons become more honest and more visible simultaneously, fueling the evolution of deterrence.

 

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