Animal Weapons

by Douglas J. Emlen

  Part 3, “Running Its Course,” explores what happens after an arms race is triggered, describing in detail the predictable sequence of stages characteristic to the evolution of nature’s biggest weapons. Staggering costs, deterrence, and cheating all appear as intuitive milestones along the way, and costs and cheaters erode payoffs to huge weapons in ways that can set the stage for collapse. Appreciating how arms races unfold reveals a great deal about the weapons themselves, about the conflicts and contests surrounding their use, and about what happens when weapon evolution goes too far.

  Finally, Part 4, “Parallels,” tackles head-on the evolution of extreme weapons in our own checkered past. Although I am a biologist (not a military historian), and this book is primarily concerned with diversity and extravagance in animal weapons, the parallels with our own armaments are too striking and exciting to ignore. Every element of arms races, from the conditions critical to launching a race, to the sequence of stages unfolding along the way, lends itself to comparisons between animals and people. Manufactured weapons are not inherited in the strict sense—instructions for their production are not encoded in DNA—and they are assembled in factories rather than wombs. The competitions they resolve are less likely to yield opportunities to mate, and success is often measured in currencies other than increased numbers of offspring. Nevertheless, human weapons change in shape, capability, and size over time, and the directions of this change are sculpted by forces of selection astonishingly similar to the forces shaping weapon evolution in animals. Arms races are arms races, and the natural history of extreme weapons, it turns out, is precisely the same.

  PART I

  STARTING SMALL

  1. Camouflage and Armor

  It is November 1969, and dark. Moonlight glints off of tree branches, casting small shadows beside pebbles and twigs on the fresh dirt. A tiny metal door opens and two mice rush out, like gladiators released into the Roman Colosseum. They dash into the darkness in search of cover, but there isn’t any, and only one of the mice will survive this race. Above them an owl sits on a perch, watching. Its head snaps into position as it spots a mouse and, with a graceful swoop utterly devoid of sound, it attacks. One moment both mice are running. The next, just like that, one of them is gone. Droplets of blood on the dirt are the only evidence of what has occurred.

  Six concrete enclosures lie side by side, encased in chicken wire to keep the owls from escaping. Twelve feet wide and thirty feet deep, they are more vast to a mouse than Mile High Stadium is to you or me. In three of them the soil is rich and dark, imported from a nearby field. In the others, the soil is sandy and pale, trucked from coastal dunes. Otherwise, the enclosures are the same, and each houses an owl, patiently waiting. Over and over the race is repeated, as pairs of mice—one brown, one white—sprint across the dirt. All told, almost six hundred mice will rush into the South Carolina night, all to answer one question: which color mouse will the owls catch first?

  Owls eat astonishing numbers of mice. When owls feed, they pack fur and bones and other indigestible parts of their prey into their gizzards, coughing them up later as dense little pellets that they spit onto the ground. Diligent biologists can harvest these pellets and pick through them, counting and identifying bones, to reconstruct an owl’s diet on any given night. A single owl can eat four or five mice a night, and well over a thousand in a year.1 Scaled up to the landscape, owls kill between 10 and 20 percent of mouse populations in a typical year—up to one-fifth of all mice die in the talons of an owl.2

  Despite the brutal toll exacted by owls and other predators, oldfield mice thrive across the southeastern United States. They live in abandoned corn and cotton fields, along hedgerows, in forest clearings, and throughout all sorts of shrubby fields. These mice also live along coastal beaches in sand dunes tufted with coarse grasses, and they have colonized many of the small offshore barrier islands of Alabama and northern Florida.

  In the mid-1920s, Francis Bertody Sumner, the leading mouse biologist of his day, heard about the strange white mice of Florida beaches. He worked his way across their range, meticulously sampling animals from population after population. Some he brought into his lab to breed, but most he killed, stretching their little pelts for archiving in museum collections. The pattern he documented was striking: mice from inland populations—stubble fields and clearings across Alabama, Tennessee, South Carolina, Mississippi, Georgia, and interior Florida—were dark brown in color, much like that of other field mouse species found elsewhere in the United States. But along the coasts and out on the sandy offshore islands, the mice were white. And, if you marched a line from inland to coast, there was an abrupt transition separating brown mice from white. The boundary fell about forty miles inland from the shore, and it tracked the coast like a contour line on a map.3

  Sumner noticed that around this transition zone, the soil also changed color. Inland, the soil was loamy and dark, filled with organic detritus from decaying vegetation. Near the shore, soils were sandy and white—in some cases mice lived on dunes of bleached sand so bright they resembled giant mounds of sugar. Ninety years later, Lynne Mullen and Hopi Hoekstra, biologists at Harvard University, retraced Sumner’s steps and sampled the populations again. A thousand mouse generations separated the two samples, but the pattern held. Soil color changed abruptly from brown to white, and mice matched this transition with a shift in fur color.4 Brown mice lived inland, and white mice lived on the beaches.

  In all other respects, inland and beach mice are similar. They make the same kind of burrows, for example. They cut in at an angle, leveling off into a horizontal nest chamber about a foot below ground. Many of them also make an “escape hatch”—a vertical tube that extends from the nest chamber straight up, stopping just an inch below the surface.5 If a snake or weasel pokes into their burrow entrance, they can “explode” through the thin soil capping this shaft to escape. Inland and beach mice eat the same foods, including insects, seeds, and the occasional berry or spider. By all metrics except color, these mice are the same. So why are the coastal mice white and inland mice brown?

  This was the question Donald Kaufman sought to answer with his gladiatoresque doctoral dissertation experiment that November back in 1969. Over and over, night after night, he released dark mice and white mice into cages side by side. Each time the owl snatched one of the mice, Kaufman recorded which one died, and which survived. He showed that both soil color and mouse color mattered. When the mice dashed across dark soil, the white mouse was most often taken. When the soil was pale, the pattern was reversed. Owls snatched the darker mouse. There were additional nuances to the owls’ behavior. For example, on the darkest nights the pale mice fared especially poorly on the dark soil. Their white fur contrasted starkly with the blackness of their surroundings. On the other hand, bright, moonlit nights and light soils made the dark mice stand out most sharply. Mouse survival depended to some extent on ambient moonlight and local conditions but, overall, the pattern was clear: mice whose fur color contrasted with their backgrounds got eaten.6

  Hopi Hoekstra and her colleagues completed this story by tracking down the genes, and even the particular mutations to these genes, responsible for fur color in mice.7 Once Hoekstra’s team knew the molecular machinery responsible for genetic variation in mouse color, they could reconstruct precisely how mice evolved in response to recent changes in the direction of natural selection. Most oldfield mice are brown, and this color is favored by selection across the majority of fields inhabited by this species. At some point in the past—possibly as recently as a few thousand years ago—mice spread into open areas along both the Gulf and Atlantic coasts, where they dug their burrows into sand dunes and grassy embankments. Beach mice now raced across a vastly different background than their inland ancestors, and in these new environs dark mice got plucked from the sand.

  By chance, some of the beach mice carried in their DNA new mutations to one or both of two genes involved in the production of dark pigments. Mice inherit
ing these mutations carried copies of the pigment-influencing genes that were just a little bit different from the copies carried by other mice (alternative versions of a gene are known as alleles), and as a result they developed with lighter fur. Mice bearing the new alleles survived better than mice inheriting the ancestral versions of the genes, and these survivors populated the beaches with their pups. Over time, mice with the new alleles increased in frequency, while those with the original alleles disappeared, and the result was an evolutionary shift from dark to white.

  * * *

  Camouflage might seem an odd place to begin this book. But weapons come in many forms, and not all of them function offensively. A U.S. Army infantry soldier marching into conflict carries all sorts of gadgets that contribute to his or her efficacy in battle. Not counting specialized weapons such as grenade launchers or squad automatic weapons (SAWs), the primary weapon is an M4 carbine assault rifle with removable bayonet.8 Along with this, soldiers carry fragmentation grenades, knives, food, water, and first aid kits. They wear body armor (vests with plates of finely woven Kevlar designed to protect against bullets and heat), a helmet, and cloth uniforms—“camo”—with color patterns designed to blend with the surrounding landscape.

  Many of these items function for defense, rather than offense, but they are no less critical to troop success in combat; for this reason they can be considered weapons. Although this book is principally about extreme weapons—the biggest tools in nature’s arsenals—it begins with these other weapon types, including animal analogs to camo and armor, which we’ll cover in this chapter, and to lightweight and portable small arms, which we’ll get to in chapter 2. Each of these animal examples has been studied unusually thoroughly, providing clear insights into the processes of selection and evolution. All of them also have intuitive parallels with man-made manufactured weapons.

  Obviously, blending with backgrounds is essential for soldier survival for precisely the same reasons that it is in mice (imagine conducting a night operation wearing white winter camo). In fact, in 2003 the U.S. Army used a process not unlike Kaufman’s experiment with owls to determine the most effective camouflage patterns for our troops. More than a dozen color and pattern types were assessed against urban, desert, and woodland environments, to identify uniforms least likely to stand out.9 Some of these tests were conducted at night, where they showed—just like Kaufmann—that being too dark on moonlit nights could be deadly. Modern enemy soldiers, it turns out, are a lot like owls. They have phenomenal nighttime vision thanks to the spread of night-vision goggles and other technologies. As a result, black has been eliminated from most camouflage patterns.

  Ideally, the uniform selection process should have unfolded just like owls selecting for mismatched mice, with the population—in this case, the army—evolving toward the best camouflage possible. Unfortunately, politics and the economics of mass production intervened. Rather than choose several different types of uniforms, each the best available for a particular habitat, the army opted for a single Universal Camouflage Pattern (UCP).10

  This may have solved logistical problems of production and distribution, but it also caused our troops to sometimes stand out when they were supposed to be blending in. After all, the solution with mice was two colors, not one, and the reality of diverse combat habitats is that no one pattern blends well in all places.

  It didn’t take long for our troops to complain,11 and by 2009 it was obvious to everyone that the UCP was performing terribly in Afghanistan.12 The army then rushed to develop a new pattern, called “Operation Enduring Freedom Camouflage Pattern” (OCP) for soldiers deployed in Afghanistan, which it began issuing in 2010.13 Special Forces soldiers, incidentally, are not subject to the same constraints of mass production, and these units have diverse and effective uniforms to choose from, depending on the mission. Military units in other countries also base pattern choices on advanced tests of detectability.14

  The brutal reality of life and death on the battlefield has provided a sort of natural selection for military uniforms. Many versions are tried, some perform better than others, and patterns performing the best are (usually) selected for further use. Despite various hiccups along the way, few would disagree that modern uniforms are vastly improved over those worn in earlier wars. WWII uniforms were better than those of WWI, and uniforms today are better than those used in Korea or Vietnam.

  * * *

  From lizards and desert beetles that look like little pebbles, to giant tropical katydids resembling decaying leaves, camouflaged animals are evolutionary legacies of the same basic process: natural selection by visually searching predators. Predators don’t just drive their prey to match colors with their backgrounds; they drive them to behave in new ways. When and how an organism moves can influence how vulnerable it is to predators. Animals that panic, dashing from their hiding places at the wrong time, or animals that walk or fly with the wrong gait, can break camouflage with deadly consequences. A leaf-mimicking katydid would stand out if it whirred through a forest clearing in broad daylight. In fact, these katydids forage at night. During the daytime, they rest on branches nestled amid similar-looking leaves. If they do need to move in the daytime, they lurch with a swaying gait just like the back-and-forth flutter of a leaf in the breeze. Place one of these animals on an exposed, flat table, and this gait looks absurd. Place it on the branch of a bush and the animal vanishes; the katydid’s movements, combined with its shape and color, make it almost indistinguishable from the surrounding leaves.

  Resembling a leaf is a relatively passive defense—hardly a “weapon” proper—as is mouse fur blending with soil. Other animal defenses are much more formidable. Many animals use chemical weapons against their predators, either synthesizing toxins, or extracting (and sometimes modifying) toxins from their food.15 Some caterpillars ooze droplets of poison from glands near the bases of tiny barbed, needlelike hairs. Poison-dart frogs pack toxins into their skin, foam grasshoppers disgorge foul-tasting bubbles from their armpits, and bombardier beetles spray jets of exploding acid from their anuses.

  “Curling up” defensive postures evolved in tandem with armor in trilobites, pill bugs, cuckoo bees, armadillos, and pangolins.

  Still other animals use armor to protect themselves. Like the jointed metal breastplates and shields sported by Roman centurions and medieval knights, many animals cover their bodies with tough plates of compacted hair, bone, or chitin (the main ingredient of insect and crab exoskeletons). Turtles and crabs may be the most familiar examples, but armor plates also protect armadillos, pangolins, pill bugs, and tortoise beetles, and similar shells protected the extinct glyptodonts and ankylosaurs as well.

  My personal favorite among the wild array of defensive weapons are spikes and spines that jut from the flanks and backs of prey—blades of bone or chitin sharp enough to puncture the mouths of predators and tear delicate linings of digestive tracts. Daggerlike spines protect all sorts of animals, ranging from porcupines and hedgehogs to spiny crabs, long-spine porcupine fish, and katydids.

  Three-spined sticklebacks swim in shallow waters along the coasts of Europe and North America. These finger-sized fish rely on both sharp spines and armor to protect them from predators. Rigid spines project along their back and from their pelvis, and a row of bony plates adorns their flanks. Here, as with oldfield mice, biologists understand both the genetic basis for variation in defensive traits—the genes responsible for heritable variation in spine length and plate size or number16—and they understand how this variation has contributed to rapid weapon evolution in the face of natural selection. This time, however, the traits are rigid bony outgrowths rather than pigments altering the color of fur. Understanding how these weapons evolve sets the stage for the much larger outgrowths we’ll consider in later chapters.

  Porcupine spines are effective defensive weapons.

  As with all evolutionary tales, the stickleback story begins with variation. Some sticklebacks invest more in defensive weaponry th
an others, resulting in fish-to-fish differences in the length of pelvic spines and in the size and number of body-armor plates. Not surprisingly, this variation in weapon size influences fish survival. Long spines make sticklebacks difficult to swallow (think splintered chicken bones lodged in a dog’s throat), and armor plates protect sticklebacks whenever predatory fish make the mistake of trying to bite them. Almost 90 percent of attacks on sticklebacks fail. But before spitting them out, predators chew sticklebacks rather harshly. A stickleback’s armor plates act like shields, reducing the extent of injuries from these bites.17

  While most sticklebacks live in the ocean where predators are common, some inhabit freshwater lakes, and here their evolutionary story is different. Ocean levels fluctuate greatly over time, and during periods of high water fish spill into inland reservoirs, where they end up trapped as the water recedes. Inland fish experience very different patterns of selection from their marine ancestors, and in lake after lake, their weapons have changed as populations adapted to their new locales.

  Fossils provide a road map of this weapon evolution. In fact, so many stickleback fossils have been preserved that they provide an almost unparalleled paleontological record of change in weapon size through time, as layer upon layer of fish corpses piled into the mud at the bottoms of lakes. Michael Bell, a biologist at Stony Brook University, studies this temporal progression of fish in a Nevada lake bed, where he and his colleagues reconstructed approximately one hundred thousand years of stickleback evolution in 250-year slices.18

  In the beginning (well, the first eighty thousand years of their one-hundred-thousand-year window), Nevada lake sticklebacks had almost no protective weapons (only one dorsal spine, rudimentary pelvic spines, and very few lateral plates). But then, eighty-four thousand years into the time sequence, this type of stickleback was replaced entirely by armored sticklebacks, meaning three long dorsal spines and full pelvic spines. Bell suspects that marine fish flooded into the lake around this time, because both forms co-occurred for about one hundred years before the early fish type disappeared. Remarkably, over the following thirteen thousand years, the defensive structures in this new fish regressed: in graded steps through time, the spines got shorter and shorter, until by the end of this period the new sticklebacks resembled the earlier form that they’d replaced. Lake-bound fish lost their weapons.