Animal Weapons

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by Douglas J. Emlen


  Huge teeth got in the way of more than just running; they made eating and other basic activities difficult. The simple act of ingesting food was awkward because the enormous canines got in the way. Sabertooths had to place their faces sideways against their kills, gnawing food in through the sides of their mouths to get around their massive daggerlike teeth.14

  Because of the complications extreme size introduces, the weapons of most predators remain small. Teeth, claws, and claspers are sharp and lethal, but not particularly large or spectacular. Such are the canines of lynx: longer than the surrounding teeth and effective for separating the spinal vertebrae of hares, but not so large as to hinder overall agility or head angle in any way, and definitely not large enough to impede the speed and coordination so essential to lynx survival.

  Teeth face trade-offs in shape as well as size. One tooth cannot excel at all tasks. Long and slender teeth such as canines are very effective at piercing skin, muscle, or viscera, but these same teeth can snap if they strike bone.15 Sturdy, bladed teeth, especially if they line up precisely with other sharp and bladed teeth on the opposite jaw, are great for shearing through muscle and sinew. But these blades fracture if they are used to crush or grind bone, and even accidental contact with bone can dull the blades, rendering the teeth useless at their primary function. Alternatively, wide, solid, dome-shaped teeth are great for cracking bone to reach the nutritious marrow, but they are useless at slicing, piercing, or puncturing.

  Enhanced performance in one context can detract from performance in another, forcing a compromise. In this case, the inability of a single style of tooth to pierce, slice, and crush has been a fundamental obstacle hindering the evolution of an increasingly specialized predator weapon.

  The success of mammals may be attributed in no small measure to the fact that they stumbled on a mechanism to circumvent this trade-off, at least partially. Mammalian carnivores uncoupled the evolution of subsets of their teeth, so that each set evolved to function like a different tool. This way the mammalian jaw could carry three or four tools (for example, canines, molars, and premolars), and each could tackle a distinct task.

  This was no easy evolutionary feat, and other types of predators never managed it. For example, theropod dinosaurs, notorious flesh-specialist predators (including Allosaurus, Carnotaurus, and the infamous Tyrannosaurus rex) had no obvious parallels to molars or premolars, no bladelike edges for shearing, no dome-shaped caps for crushing. Instead, all of their teeth were roughly similar in shape to canines. As a result, even though they did diverge in body size, allowing some partitioning of prey resources, theropod dinosaurs never diversified to fill the breadth of ecological roles seen in carnivorous mammals. In other words, there were no bone-cracking or saber-toothed theropods.16

  Tyrannosaurus rex and other flesh-eating dinosaurs lacked specialized subsets of teeth like molars and premolars.

  By uncoupling the shapes and functions of particular subsets of teeth, carnivores became incredibly successful and specialized hunters. But this solution was far from perfect, and the fundamental limitations of these trade-offs remain. Canines, premolars, and molars are all still housed side by side in the same jaw, which is rather like opening all the tools in a Swiss Army knife at the same time. This means that their distinct functions can only be utilized by careful chewing, positioning bone over the dome-shaped molars, sinew and meat over the bladelike premolars, and keeping the canines out of the way.

  The luxury of careful chewing that we might experience while enjoying a fine steak at a French restaurant is rarely afforded to free-ranging top predators, who face constant and intense competition from rival predators attempting to steal their kills. So the reality is that animals must slash and crush fast, and in this real-world haste, mistakes get made. Blades get worn and teeth crack. A survey of both living and extinct carnivores shows an astonishing frequency of natural tooth breakage, with one out of every four teeth chipped, cracked, or shattered.17

  * * *

  A similar balance between size and performance can be found in the teeth and jaws of predatory fish, especially in the cruising predators of open waters such as tuna and bluefish. Like mammalian carnivores, these animals are often the top predators in their communities, and they can be enormous. Large fish have big jaws and teeth, enabling them to gulp down big prey.18 Smaller fish with small jaws cannot swallow large prey simply because it’s physically impossible to fit them into their mouths. Predatory fish must chase and catch their prey while swimming fast and, like the lynx, these predators often fail. In fact, they miss their mark more than half of the time,19 placing a premium on body shapes that enhance swimming speed.

  In principle, a fish should be able to increase jaw and tooth size without having to get larger overall. Then it could catch disproportionately large prey—possibly even prey bigger than itself—without sustaining the metabolic demands of an especially large body size. Again, the problem is the balance between opposing forces of selection. Jaw dimensions affect individual performance in two contexts: swallowing prey and catching prey in the first place. Larger jaws are definitely better for ingesting bigger and more diverse prey. But they are selected against due to the drag they incur as they’re pushed through the water.20 For many open-water predatory fish, natural selection for faster swimming speed acts in the opposite direction from selection for swallowing larger prey. Because the same fish must accomplish both tasks, the result is functional but unspectacular jaws and teeth, and modestly proportioned weapons.

  * * *

  As a boy I used to splash through the muddy waters of Bull Run Creek in eastern Tennessee where my mother and stepfather have a small farm, to climb up to the crest of a hill on the far side where our neighbor grew tobacco. Surrounded by sticky leaves, I’d gently push between plants taller than I was and walk down the rows, studying the mounds of exposed earth as they sloped away from the base of each plant. I was scanning for shiny pieces of black obsidian or blue-gray flint freshly cleaned by rain. A notch on one side, or flaked edges that came together at the right angle—some pieces just looked right, and I could tell even from the smallest bits of exposed stone that beneath the surface lay a masterpiece. Of course, most of these “masterpieces” turned out to be nothing; but every once and a while I’d pluck from the dirt a beautiful work of art.

  Two thousand years ago some hunter sitting on this Tennessee hill smacked a hammerstone against a fist-sized obsidian “core,” shearing off a slender, two-inch flake. Then, knapping gently with the hammerstone, he chipped off small pieces, roughing out the overall shape of the point. Finally, pressing a piece of antler bone against the edges and marching in tiny steps, he popped a precise row of slivers away from each side, working the point until it was symmetrical, with a razor-sharp blade along the length of each edge. The result: a three-fourths-inch-long tip for a lethal and efficient predator weapon.

  My neighbor’s field was filled with arrowheads. Discarded flakes lay everywhere, suggesting that this flat-topped hill had once been a village where the points were made, rather than a battlefield or hunting ground. Most of the arrowheads were broken, but when I did find an intact point poking out of the mud, I’d close my eyes, sweet-smelling tobacco leaves rustling in the breeze beside me, and squeeze the arrowhead tightly in my hand. The last person to touch it had been the man who made it. For the briefest of moments it felt like I’d touched the past.

  Two thousand years may seem like a long time, but even by North American standards the points I pulled from my neighbor’s field were young. Stone points and the spears, atlatls, and bows and arrows that delivered them were the primary weapons of humans for tens of thousands of years.21 Millions have been collected and archived, exposed by plows or eroded from lakeshores and the banks of streams, allowing archaeologists to trace how their shapes and sizes changed through the ages.22 Remarkably, almost all of these weapons are small. Like the canines of lynx or the jaws of fish, the sizes of stone projectile points reflect a balance
between killing power and portability.

  As early as fifteen thousand years ago, North American hunters began to throw their spears with a throwing stick, or atlatl, and to tip their spears with points flaked from sharp stone.23 Spears function well only if their tips are balanced with the rest of the shaft, and if the blade is wide enough to create an incision in an animal’s hide sufficiently large for the attached shaft to enter.24 This puts strict limits on the sizes of tips that will function on particular wooden shafts: bigger shafts require bigger stone points. Large spears are also heavy, hitting their prey with greater force and deeper penetration than smaller ones. Not surprisingly, large spears and points enable the takedown of large prey.25

  But just as with mammals, the benefits of large weapon size are offset by considerable costs. Larger points require rarer materials such as large obsidian or flint cores with no internal imperfections, and they take substantially longer to make.26 Large spears are also bulkier, and hauling them around can be tiring. Early hunter-gatherers traveled extensively as they tracked the seasonal appearance of fruits and tubers and the movements of large game—possibly as much as three to six miles a day and more than two hundred miles every year.27 These nomadic people had to carry their weapons with them as they walked, along with everything else they owned.

  The record shows thousands of years of stasis in point size and shape, and when these points did begin to change in size, they actually evolved to smaller, rather than larger, sizes. At least two factors contributed to this gradual reduction: changes in the sizes of prey available, and changes in the technology used to propel the weapon.

  The earliest North American points (Clovis points) reach 8 inches in length but were typically closer to 3 inches, and were consistently found alongside mammoth bones.28 Indeed, the Columbian mammoth appears to have been the primary prey species for North American hunters until approximately twelve thousand years ago, when mammoth populations crashed and hunters began to shift their efforts to the giant, and also now-extinct, Bison antiquus. These ancient bison were six times smaller than mammoths (only three thousand pounds, on average, instead of eighteen thousand pounds), and the early hunters’ appetite for them was associated with a steady reduction in spear and point sizes (Clovis points associated with B. antiquus averaged about 2 inches in length, and later Folsom points associated with this same bison species averaged 1.5 inches).29 When this species, too, disappeared, hunters tracked still smaller prey types, including modern bison (Bison bison), as well as bighorn sheep, deer, elk, and antelope, and point sizes shrunk accordingly.30

  15,000 years of weapon evolution in North America

  At the same time, several key innovations in weapons technology occurred. The addition of feather fletching to thrown spears around 7600 years ago dramatically improved the speed and accuracy of spears. But these advances worked best with smaller, lighter shafts and correspondingly smaller points.31 Then, around 2000 to 1300 years ago, the bow and arrow replaced atlatl-thrown spears, favoring still smaller shaft and tip sizes.32 Hunters could shoot arrows farther and faster with a bow than they could hurl darts with an atlatl, improving hunting success still further and making it possible to kill all sorts of new prey animals.33 Arrow tips, now much smaller than their Clovis predecessors, could be manufactured rapidly using readily available materials, and the bow and arrow was significantly more portable than a spear.34

  As with carnivore teeth, a balance between opposing agents of selection kept the stone tools of early humans modest in size. This sort of compromise with weapon size is almost always necessary for predators, and it’s why the vast majority of animal weapons are small. Yet, exceptions to this rule definitely occur. In certain special situations, the shackles of balancing selection are shattered. In these populations, weapons begin to get really big.

  3. Claspers, Graspers, and Giant Jaws

  In the fall of 1992, I reunited with another college buddy for a ten-day excursion to South America. We couldn’t afford to go to Machu Picchu, so we settled on Ecuador, a beautiful land I’d explored three years earlier in my search for rhinoceros beetles. The plan was to climb a mountain and then spend several days relaxing by a lake in the rainforest. Volcán Tungurahua was still over eighteen thousand feet—it wouldn’t erupt for another six years—and the views from the summit were spectacular. Now, sunburned and sore, we’d bounced for twelve hours in a crowded bus to the frontier town of Coca to meet up with our guides, Clever and Selfo, for the boat trip into the forest. We stashed our gear in the motorized dugout canoe and hopped into the back of their pickup to fetch last-minute supplies.

  My friend Chris spoke no Spanish. I could get by, with difficulty, and we’d just discovered Clever and Selfo spoke no English. We weren’t particularly worried until they pulled over by the side of the road and peeled a road-killed javelina from the tarmac, hacking off a hind leg and wrapping it in a sheet of plastic, placing it gently in the back of the truck next to the cooler and boxes of food. Chris and I exchanged glances. I leaned into the little window that opened into the cab of the truck and tried my Spanish, asking what the pig was for. “Cena” was the response. I was pretty sure that meant “dinner,” which was not the response we were looking for. I probed again, but the only word I could discern from their answer was “cena.”

  Eight hours later we glided into our campsite, having motored more than seventy-five miles down the Napo River and then another dozen up the Pañacocha tributary. There wasn’t another person for dozens of miles in any direction. Save for a raised wicker platform for our tents and a table with a cookstove, the place was pristine. By this point the javelina leg was getting ripe, and we again watched with alarm as our guides placed it beside them and began assembling cookware for dinner. Brushing away an assortment of flies, Clever pulled back the matted skin and sliced into the muscle, making a pile of sugar-cube sized chunks, scraping the lot onto a platter, and handing them to us raw. Then he unwound fishing line from a spool in his pocket, gave us hooks, and pointed to the water. The pig was for dinner after all—just not in the way we’d expected!

  Piranha fishing is so easy it’s almost not even fair. Skewer a chunk of meat onto a hook, toss it into the lake, and wham; pull out your fish. Over and over we cast, and every single toss scored another fish. In no time at all we had two dozen fillets simmering in butter over the fire. To this day I consider piranha the best-tasting fish I’ve ever eaten, and those nights in the forest we feasted like kings. (We even went swimming in the lake the next morning—exhilarating, now that we knew we were surrounded by flesh-eating fish; the trick, incidentally, was to jump straight into deep water from a boat, rather than wade in from the shore.)1

  * * *

  Not all predator weapons are small, and piranha teeth are a case in point. Piranha jaws are packed with giant, triangular blades that bulge out of their mouths in a fierce under bite, even when their jaws are closed. Piranha don’t feed like other fish. In addition to swallowing prey whole, they also rip off chunks of flesh one bite at a time. This simple change in hunting behavior released these fish from the constraint of having to be physically larger than their prey. Now they could swallow small fish as well as parts of bigger fish, including scales, fins, and gouged mouthfuls of flesh.2

  Popping mouthfuls of meat from big animals required quick lunges from short ranges, rather than prolonged chases through open water. Piranha also scavenge, stripping even human carcasses to skeletons.3 But scavenging, like lunging, works just fine with awkward jaws. Because piranha no longer needed speed to catch prey, selection favored thicker jaws and larger and larger teeth. Barracuda, incidentally, have similarly impressive teeth for precisely the same reason.4

  Sit-and-wait, or ambush, predators take weapon evolution to an even greater extreme. Sabertooths were ambush predators who dropped from the branches of trees to plunge their daggers into the necks of unsuspecting prey. Like piranhas, ambush predators no longer chase after prey to hunt them down. In fact, most of them don’
t run or swim fast at all. Instead, they lurk motionless, often blending spectacularly with their backgrounds like a hunter in a blind, waiting for prey to come to them. When unlucky edibles happen by, these predators lunge from their hiding places, striking out with a snap of their jaws or a flick of their legs to snatch and incapacitate prey before they even recognize what is happening, much less have time to escape.

  Deep-sea ambush predators often use lures—dangling globs of light that act like beacons in the immense blackness of the extreme deep. Because prey come right up to these predators, the need for cruising speed is gone. The constraints imposed by drag are minimal, and selection for increased jaw size prevails. Many of these fish, with evocative names such as viperfish, ogrefish, and the humpback anglerfish, have extremely large mouths studded with long, sharp teeth. “Umbrella-mouthed” or “gulper” eel jaws are so big that the fish’s body is literally just a giant mouth with a tail. Their gape can be as wide as their body is long. When these eels unfurl their enormous mouths, engulfing water like giant submerged balloons, they can swallow prey that are even larger than they are.5

  Most praying mantises are ambush predators, which explains their supersized forelegs with long curved spines. These insects get their name from a habit of holding their big forelimbs in front of their faces, a posture that resembles a person in prayer. In fact, these long raptorial limbs are spring-loaded with recoil and muscle, and their placement can be compared to the cocking of the hammer of a gun. These toothed limbs snap out from the body, grasping any prey that make the mistake of wandering into the “kill zone.”

 

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