Animal Weapons

by Douglas J. Emlen

  The fangtooth, umbrella eel, and anglerfish all have massive jaws and teeth.

  Early praying mantis species were skinny, generalist hunters who stalked along the ground or through vegetation. Their forelegs were slightly enlarged, facilitating a rapid snatch of spiders and insects they happened across. From these ancestors, mantises became increasingly specialized as sit-and-wait hunters.6 Once balancing selection for efficient locomotion waned, their forelegs became bigger and bigger, enabling mantises to capture prey at greater distances.7

  Mantises have long “raptorial” forelegs, which they use to snatch ambushed prey.

  Mantis shrimp provide an underwater parallel to these terrestrial graspers. Mantis shrimp are neither mantises nor shrimp, but they get their name from their striking resemblance to both; they have shrimplike bodies and enlarged, raptorial limbs that look like those of a praying mantis. These thumb-sized crustaceans hide inside burrows in rocks or coral on the sea floor and ambush snails, other crustaceans, or bivalve shellfish. They are called “smashers” because they use their distended forelimbs to spear and smash their prey in a highly effective manner: single strikes from these weapons can be deadly. Mantis shrimp hurl their legs with incredible speed during these blows, which is nontrivial considering they are underwater. Sheila Patek and Roy Caldwell, studying the mechanics of the predatory strike of the peacock mantis shrimp, found that they accomplish this feat using a “click” mechanism in which a rigid latch locks the limb into a spring-loaded and cocked position. When this lock is released, the weapon snaps forward using elastic recoil from energy stored in the bending of the skeleton, exactly like releasing the recoil stored in a fully drawn archer’s bow. Once released, their legs blaze through the water at strike speeds of up to sixty miles per hour which, when scaled down to the size of these “shrimp,” means that the entire strike takes just two one-thousandths of a second.8 These little predators are contenders for the fastest weapon strikers in the animal kingdom.

  The limbs of mantis shrimp (and the related “pistol” shrimp) sweep through the water so fast they create explosive vacuums in their wake. The gaps they leave behind them as they move pull dissolved gasses out of the water into cavitation bubbles. These bubbles release additional energy with a crack when they collapse. This audible pop is both loud and deadly—a noise topping 220 decibels and a flash with temperatures approaching that of the sun (8,500°F).9 Although the flashes from these cavitation bombs are too small to see, the shock waves they generate can stun nearby fish. The combined impact of an incredibly swift strike, followed by the explosive release of energy from collapsing cavitation bubbles, can crush the exoskeletons or shells of prey instantly.

  Other extreme-weapon predators creep along stealthily and surprise prey. This stalking method is similar to sit-and-wait ambush hunting in that it relies less on rapid chases or efficient locomotion and more on swift and lethal strikes from close range. Many stalker fish have extraordinary mouths. Alligator gar, for example, have long, thin jaws packed with razor-sharp teeth that enable them to snatch prey with a sudden, sideways sweep of their mouth. Caimans and crocodiles have similar jaw shapes for precisely the same reason.

  Alligator gar sweep long jaws to the side to strike prey.

  * * *

  The success of all of these ambush and stalking predators hinges on a quick strike with a weapon that immediately incapacitates its victim. Speed matters, but it’s not the sprint speed of the whole animal that determines the success or failure of these hunts. Instead, it’s the speed of the appendage that matters—how fast it springs out from the body. Here, bigger weapons are almost always better: longer claspers or jaws can reach out farther from the body; bigger appendages can house stronger, thicker skeletal elements incorporating greater elastic recoil and bigger, faster muscles; and hooks or claws at the ends of these appendages move faster the farther they are from a joint or hinge; longer appendages fly through air or water faster than shorter ones.

  The physics behind this last point is called the “law of the lever,” and a good way to think about it is to consider a seesaw. If the pivot sits in the middle of the seesaw, then each end of the board moves the same total distance up or down, and each end moves through the air at the same speed (they travel the same distance in the same amount of time). But slide the pivot closer to one end of the board, and two things happen. The distance traveled by the two ends begins to differ, since the long end travels a more dramatic arc than the shorter end. And the speeds of the ends of the board diverge. Both ends of the seesaw complete their respective arcs in the same span of time (presuming that the board doesn’t bend). But the long end travels farther than the short one, which means that the long end also travels faster than the short end of the board. The farther an object like a tooth sits from the hinge of a lever, the faster it moves when that lever rotates.

  Carnivore jaws illustrate the other half of this principle. Cats and hyenas each sacrifice jaw speed in favor of jaw strength, with squat faces and canine teeth migrating relatively closer to the hinge. Here it helps to think of a nutcracker, or a pair of pliers: the closer an object is placed to the hinge, the more powerful the closing force that can be applied. Wolves, on the other hand, have longer jaws than hyenas or cats. Their canines bite with less force, but what is lost in force they gain in speed. Their canine teeth are situated farther from the hinge of the jaw and, as a result, move faster when the jaw closes.

  In ambush predators and stalkers we find this logic carried to an even greater extreme. A specific ecological situation has relaxed the counteracting forces of selection that normally constrain the evolution of large weapons, and selection for snatching predominates.

  * * *

  Highly social insects, particularly ants and termites, have escaped the weapon size versus locomotor performance trade-off in a different way: through a division of labor. Colonies of these insects can be immense, often with millions of individuals all functioning together as an efficient whole. Part of this colony efficiency arises from a subdivision of tasks among workers with specialized body shapes, not unlike the different subsets of teeth in carnivore jaws.

  The ability to uncouple development paved the way for independent evolution of carnivore teeth—canines diverged from premolars, and premolars from molars. In social insects, an analogous uncoupling permitted soldiers and workers to evolve independently, and to diverge considerably in form.10 Soldiers didn’t need to run efficiently, fly, conduct colony maintenance tasks, or even reproduce. They had only to perform as soldiers. Their release from these other tasks meant that the negative consequences of enlarged weapon sizes were minimal.

  Within the ant genus Pheidole (the “big-headed” ants), for example, individuals fall into one of several very different “castes,” including winged reproductive males and females (who disperse from the colony in huge, synchronous, mating swarms), small workers, large workers, and soldiers. Soldiers in these ants have evolved to become fighters with grossly enlarged heads, jaws, and teeth.

  In the trap-jaw ant, soldier jaws are long and curved, with sharp teeth. These ants use a lock-and-release mechanism of jaw closure quite similar to the appendage strike of the mantis shrimp. Jaws of this ant can close at speeds of up to 143 miles per hour, snapping from completely open to shut in less than a thousandth of a second.11 These jaws are so fast that if the ants snap their jaws when their faces are aimed against the ground, they can launch themselves backward twenty body lengths into the air—which turns out to be a very effective escape tactic.

  Army ant soldiers have giant jaws and thick, distended heads. En masse, these formidable little warriors can topple scorpions, lizards, and birds. They also can be handy to humans, as at least a few tropical biologists can attest. As a graduate student I spent three weeks in Belize with a biology class, living in the muddy forest understory in tents learning how to conduct field experiments. I wore a machete in a cheap plastic sheath hanging from my belt, and one afternoon as I stripped for a
swim the handle of the machete caught on my pants. I must have been talking or otherwise distracted, because I never noticed the blade sliding across my thumb until it was too late—I’d sliced clean through to the bone. We were miles away from civilization with no easy way to get to a hospital. So we sterilized the wound with rum and sutured it with ants. One person held the edges of the cut closed, while another carefully positioned the ants. Angry soldiers reach out with their huge jaws agape, but as soon as you place them against skin, the jaws clamp shut. Pop the rest of the body from the head and you have a surprisingly effective suture. A row of five or six ants worked nicely, once I got used to the idea.

  Termites also show division of labor with specialized soldier castes, although these fighters are involved primarily in colony defense, rather than offense. Incisitermes soldiers have thick, muscle-filled heads with huge jaws. Nasutitermes soldiers have completely different adaptations; they squirt sticky filamentous threads at invading ants. The sticky strings tangle the legs of ants, incapacitating them. Nasute soldiers have no eyes or mouths. Their entire head is one giant, bulbous nozzle with a snout—a walking squirt gun.12

  Division of labor has always been an integral part of human military forces, too, circumventing the same trade-off between portability and size. Light infantry can move far more swiftly than heavier artillery, for example, and the largest guns are cumbersome and difficult to maneuver. Despite attempts to overcome these limitations, such as placing catapults and cannons on wheeled carts or, much later, housing big guns on rail cars and tracked vehicles, armies have always faced limitations due to the unwieldiness of their biggest guns.13 The solution was to separate the functions of infantry and artillery units, permitting one to specialize for speed; the other for firepower.

  Four soldiers: biting and squirting termites, and army and trap-jaw ants

  During the First and Second World Wars, navies faced the same constraint, and they adopted an analogous solution. Battleships got bigger and bigger, packing more and larger guns, but increased firepower came at the expense of speed and maneuverability, so battleships relied on smaller, faster ships such as cruisers and destroyers to screen them and provide reconnaissance.14

  * * *

  In the animal world, extreme weapons are usually confined to specific, rare circumstances such as sit-and-wait hunting tactics, or within-colony division of labor. But another surprisingly potent phenomenon can lead to the evolution of exaggerated weapon sizes: competition. Nature’s biggest weapons decide the outcomes of battles over the chance to reproduce, the most precious commodity of all.

  PART II

  TRIGGERING THE RACE

  Specific circumstances must fall into place before weapons launch into an arms race. Appreciating these “ingredients” reveals much about the function and diversity of nature’s most extravagant weapons, including why some species have them and most others do not.

  4. Competition

  Jacanas are bizarre birds, especially when it comes to their weapons. Black slender bodies contrast starkly with yellow bills, long yellow wing spurs, and wrinkled, fleshy wattles—folded globs of feather-free skin that look like pieces of chewed cherry-red bubble gum squished onto their foreheads. The female I’m watching has especially big spurs, one on each elbow, and she creeps about with delicate steps on very long legs. Her slender toes splay to a span of more than five inches per foot, and her gait brings to mind a carnival stilt walker.

  This female, red-over-blue-and-white right (named for the colored ring bands on her right leg), is making the rounds of her territory, checking the nests of each of her four mates. Her territory is difficult to get to, and we have to watch at a distance from a canoe. Jacanas defend areas of floating vegetation on wide tropical rivers. Panamanians call them “Jesus” birds because they look like they walk on the water and, in a sense, they do. They tiptoe across their floating mats, dispersing weight with each step over delicate toes, balancing atop the bobbing rosettes of water lettuce and hyacinth. Most predators can’t reach jacanas out here, since they’d sink through the thin mat into the river. Crocodiles and caiman, however, swim under the mats and float up into the lettuce, snatching birds from below.

  Morning sun cuts through thick mist rising from the Chagres River, the water source for the Panama Canal, and steam lifts from the tropical forest nearby. It is unbearably humid, and biting flies nip at our ankles and the soles of our feet. We crouch in the canoe, trying to get comfortable as we follow our focal birds. I’m using binoculars, sweat already dripping down my arms as they steady my view, elbows planted on the hot aluminum rim of the boat. My father sits behind me peering through a spotting telescope. Mounted on a tripod and perched rather precariously between us, the scope is trained on one of the males, who just this morning hatched four chicks. My dad is a biologist who studies the behavior of birds, and on this morning in 1987 he was beginning a multiyear project in Panama examining the behavior of jacanas. A sophomore in college and eager for adventure, I joined him in Gamboa for a month to help out.

  Each morning we drove to the edge of the river where an old Grumman canoe lay chained to a tree. After loading water, lunch, ponchos, clipboards, and binoculars, we’d paddle a mile upriver and cross the huge channel to the far side, where a number of floating mats sat in a wide eddy, as stationary as islands floating on the surface of a river can be. Here he’d ring-marked most of the territorial birds—the ones able to hold on to the precious vegetative real estate—and we spent day after day spying on our avian actors as their lives unfolded on this patchwork of swirling, drifting stages.1

  This morning, red-over-blue-and-white right is fighting again, as she so often does. An unmarked female had darted in from the adjacent shoreline, hiding in the hyacinth leaves behind one of the males, but our territorial female spots her immediately and closes in. Now, face-to-face, the two birds size each other up. Crouching low, elbow spurs flared out to the sides, each sidesteps the other in a slow circle. Then red-over-blue-and-white right pounces, leaping into the air and striking the intruder feetfirst on her way down, slashing out with her spurs. Everything whirls into a blur as both birds leap at each other over and over, crashing together and jabbing as they flail onto the mat of floating lettuce, pop to their feet, and leap once more. And then suddenly the fight is over; the intruder flies away, and the thick air rings with raucous ka-ka-ka-kas as our focal female proclaims victory to birds nearby.

  Hundreds of vagrant females forage along the nearby shores of the river. These individuals have failed to secure an island territory of their own, and they challenge the resident females incessantly, pressing and probing, searching for weakness. For vagrants these battles are “do or die,” since failure to secure a territory is an evolutionary full stop; a dead end. Unless they can find a way to displace one of the owners, their chances of breeding are nil.

  Female jacanas are fighters, towering over the males. They are stronger than males, vastly more aggressive, and they have the larger weapons. Sharp yellow spurs jut forward like daggers from each elbow. Bigger females fare better in fierce battles and, as a rule, only dominant, top-condition females manage to hold a territory for long enough to breed.

  Jacanas are unusual because females have larger weapons than males: a pair of yellow wing spurs.

  Males also fight for territories on the floating mats, though their battles are less vicious and are independent of the wars waged by the females. Males fight with rival males, and successful individuals defend patches of floating greenery sufficient to raise a clutch of chicks. These territories pack into the floating islands like tiles of a mosaic, with the female territories superimposed on top. Some females may be able to shove rivals away from only a single male territory, but the biggest and best females own enough island to house three or four males.

  The sky cracks with a boom, and warm rain begins to pour down (it does this a lot in Panama). Torrents of water dump over us as we scramble to cover the scope and our notes with ponchos and pla
stic. The birds hunker down to sit out the deluge. We hunker down, too, and wait, exposed and shivering as lightning crackles around our little metal boat. Ten minutes later the storm has passed and we, and the birds, are back at it. Three inches of water sloshes along the floor of our canoe, so we flip a plastic milk crate on its head and use it as a table to keep our gear clear. My female is engaged in yet another fight—her fourth of the morning—and I check in with my dad. The male he’s following shepherds his brand-new chicks as they wobble from plant to plant, all feeding on little insects squirming at the water surface around the lettuce.

  The next male over, his territory also nestled within the holdings of red-over-blue-and-white right, still has eggs, and he sits on his hidden nest sheltering his clutch. A third male has nearly grown chicks, and the last male is between broods, ready to begin the process again. Our female, when she’s not battling to hold her spot, moves freely among her males, mating with them from time to time. When one of her males is ready, she’ll lay a clutch of four eggs into his nest. But then she will abandon the eggs to the male and move on, placing her next clutch a few weeks later into the nest of a different male. Male jacanas spend several months tending to young each time they reproduce, preparing nests, incubating eggs, and shepherding the chicks as they grow to independence. Females show up to provide eggs when needed, but otherwise they leave the tending of young to the males.

  * * *

  In every way that matters for this book, jacanas are backward. Females are more aggressive than males, they are larger than males, they fight more viciously and frequently than males, and they have larger weapons. Usually it’s the other way around. In flies, beetles, mastodons, crabs, and elk, males are armed, not females. Jacanas excepted, in every species with weapons confined to a single sex, males have those weapons. Why should just one sex have weapons? And why is it (almost) always the males?