Animal Weapons

by Douglas J. Emlen

  The first naval arms race, before and after: a penteconter contrasted with Ptolemy’s “forty.”

  “Fives” were created by adding a second oarsman to two of the three oars in a trireme. Fives still had 90 oars per side (in three rows of 30), but these oars were now powered by 300 men instead of 180. Bigger really was better, and fives bested threes in a fair fight. By 387 BCE “sixes” were added to the line, and within ten years ships had increased through “sevens,” “eights,” and then “nines” (nines had three men abreast at each of three oars). By 315 BCE there was a “ten,” and by 301 BCE fleets had experimented with “elevens,” “thirteens,” a “fifteen,” and a “sixteen.” By this point ships were getting unwieldy, however, and most ships above tens were considered clumsy and slow, though they made impressive displays of power. The culmination of the race was a behemoth contracted by Ptolemy V, the “forty.” This beast of a ship likely consisted of two parallel hulls bridged catamaran-style by an upper deck (more rowers could fit this way, since each hull had files of oarsmen along each side). More than 420 feet long and powered by 4,000 oarsmen, Ptolemy’s forty was the largest vessel ever built in antiquity.37 Not surprisingly, it was so extreme that it was nautically worthless.

  * * *

  Together with competition and economic defensibility, duels tip the evolutionary balance in favor of extreme weapons. This simple insight has terrific explanatory power because it provides a general rule for why particular animal species have extreme weapons, and why their close relatives may not. Like uncovering the key to an ancient encrypted cipher, we can now make sense of so much animal diversity. Specifically, we can look to the histories of groups of species to see when behaviors changed in ways that caused the arms race ingredients to fall into place. This can help explain why isolated species stand out from the rest, and it can also explain why whole groups of related species appear to enter into arms races together. Sometimes, one or more of the arms race ingredients are part of the inherited physiology of a group of species, and this can predispose them all to arms races.

  As groups of organisms diversify through time, spawning new lineages that turn into new species, they carry with them legacies of their past. Mammalian carnivores all inherit the same basic types of teeth (canines, premolars, molars, and so forth) because all carnivore species descend from a common ancestor that had that set of teeth. There are just over fifty species of field mouse (Peromyscus) scattered around the world, and they all use the same enzymes to infuse pigments into their fur because their ancestors, a mouse species that lived roughly ten million years ago, used these enzymes to pattern its fur.

  Sometimes the characteristics inherited by a group of species (species that trace their ancestry back to a shared common ancestor are called a “clade”) cause many members of the clade to all experience similarly strong selection for large weapons. For example, female African elephants invest impressive amounts of time, energy, and nutrition into their offspring during multiyear bouts of pregnancy and nursing. These are major features of their physiology and behavior, and this extreme form of investment was likely shared by many other species within the elephant clade. In fact, we know from specimens hacked out of glacial ice and pulled from anoxic tar pits that female woolly mammoths, Columbian mammoths, and mastodons also nurtured their young through a prolonged pregnancy. Paleontologists can tell from the shapes of fossil pelvic bones that all of the extinct elephant relatives did as well.38 This means that the crucial backdrop for intense male-male competition in African elephants, a highly skewed ratio of available males to available females, almost certainly existed in all of the other species in this clade, too. It’s probably no coincidence that so many relatives of elephants embarked on paths of rapid and extreme weapon evolution.

  This is why we often find not just a single species with huge weapons but entire clades packed with species after species all armed to the teeth. Inherited characteristics like asymmetrical parenting tip the balance in favor of arms races for all descendant members of the clade. All that is needed is for the remaining two pieces to fall into place. If many of these species also share another one of the ingredients, say an inclination for using habitats such as burrows that result in defensible resources or choke points, the balance is tipped still further. The result can be explosions of animal diversity, as species after species within these clades launch onto trajectories of rapid weapon evolution.

  There are more than one thousand species of stag beetles worldwide, and almost all of them have extreme male weapons.39 Stag beetles form a separate branch on the beetle evolutionary “tree” from the dung beetles and rhinoceros beetles. Instead of horns, they produce a pair of toothed mandibles so large that in some cases they are longer than the male himself. Stag beetles descend from an ancestor that likely experienced strong sexual selection and defended sap sites on the sides of trees, as almost all living stag beetles still do today. This shared suite of characteristics appears to have predisposed stag beetle populations to escalated evolution of large male weapons. Extreme mandible sizes evolved at least twice early in the history of this group.40 These large-mandibled species subsequently radiated into literally hundreds of daughter species who continue to this day to experience intense sexual selection, with males fighting in duels over localized oozes of sap.

  The same insight helps explain extreme weapons in flies. The vinegar flies, or Drosophilidae, contain more than three thousand species, and the vast majority of them lack extravagant male weapons. However, at least eleven different times in the history of this group, males evolved extensions to their heads that functioned in fights with rival males. (Both the stalk-eyed flies and antlered flies are clades of “big-headed” flies within the larger vinegar fly family; that is, they make up two of the eleven evolutionary origins of extreme male weapons.) When David Grimaldi, a curator at the American Museum of Natural History, looked closely at the biology of the flies, he concluded that all of these exceptional species stood out from the pack in the same three ways. Unlike other Drosophilid species, males with big weapons all displayed unusual levels of aggressive competition, guarded localized resources, and faced each other head-on in fights that were either described as “head butting” or “jousting.”41

  After the extinction of the dinosaurs (about sixty-five million years ago), mammals dominated terrestrial landscapes, and the ungulates, in particular, thrived. These hoofed plant eaters diversified in droves, as group after group expanded, diverged into clades of related lineages, and eventually disappeared. Punctuating this history was an exuberance of clades bursting with extreme weapons.42

  Arsinothere and Synthetoceras, early ungulates with unusual weapons

  Brontotheres began no larger than modern coyotes, but they rapidly evolved into giants standing eight feet high at the shoulder and weighing twenty thousand pounds. Early brontotheres had no weapons; later species bore broad, flat, bony plates on their noses that could extend more than two feet. Rhinoceroses began small, too—dog-sized—and hornless, diversifying later into enormous animals weighing up to thirty thousand pounds and wielding dramatic weapons. The woolly rhinoceros, for example, had a horn more than six feet long. At their apex, the rhinoceroses comprised more than fifty species worldwide, but most have disappeared, and only four species still survive today.

  Around the same time, the snouted ungulates diversified. Beginning with small, unarmed, early elephants, they radiated into more than 150 species with weapons like the “shovel-tusks,” with three-foot lower incisors jutting forward as flat blades from the lower jaw; the “hoe tusks,” with downward-curving tusks curling beneath the lower jaw; the “upper tusks,” as in mastodons and modern elephants; and even the “four tusks,” with two upper and two lower tusks.

  But the ungulates were only just getting started. A clade of pigs diversified into species with a unicorn-like head horn and species with long, curling tusks. An offshoot of the camels exploded into crazy armed forms such as Synthetoceras, which sprouted a
pair of horns from the back of the head along with a huge, forked horn protruding up from the snout, to Kyptoceras, which had two long horns curving forward from the back of the head and a sideways pair of horns arcing like pincers above the nose. A clade of pronghorn antelope gave rise to dozens of species with elaborate horns, and the giraffes radiated into at least ten species with bizarre and diverse horns. Last, but not least, the deer began as tiny fanged animals like the modern-day Chinese water deer, but rapidly diversified into almost a hundred species with bony antlers famous for their size and complexity.

  What these broad patterns reveal is a simple and surprisingly universal rule: after the final ingredient for arms races falls into place, entire clades of descendant species can all experience rapid evolution of extreme male weapons. Mastodons and flies could not be more different from each other. They lived at different times, in different habitats, and fed on different foods. One was more than 120 million times the size of the other. One had enlarged teeth, and the other had chitin protrusions from the forehead. Yet, the same three ingredients triggered evolution of extreme weapons in both cases. So it is with wasps, beetles, crabs, earwigs, elephants, and antelope. Despite extraordinary differences among these species, arms races are arms races, and the circumstances leading to big weapons are the same.

  Weapon diversity in deer

  PART III

  RUNNING ITS COURSE

  After a race is triggered, weapons begin getting really big, and several things happen along the way. Appreciating the stages of a race reveals striking similarities across species—a surprising suite of characteristics shared by all of the most extreme weapons, including our own.

  7. Costs

  Far below, the waters of Gatun Lake lay dotted with green and red as channel markers of the Panama Canal blinked in the moonlight. It was five a.m. and I was sitting in bed staring out at the shadowy branches of a rainforest canopy. My room was on the upstairs floor of a small laboratory surrounded by lush tropical forest, at the top of a very steep slope. The lab was a simple wooden structure with four dorm rooms in a row. Mine was at the end, and three of its four walls were encased only in screening. The moist wind and spray of rain passed straight through the room, covering my face and the sheets of my bed. The forest teemed with whines and chucks of túngara frogs, eerie trills of cane toads, and, of course, the ever-present drip of rainwater from the leaves and eaves overhead.

  I’d awakened before dawn that morning as I did every day, to listen for a particular sound that would lead me to the beetles I’d come to the island to study. It was August 1991, well into the rainy season, and I was a doctoral student there for a stint on Barro Colorado Island to conduct field research into the function of horns in beetles. Although the beetles I was studying that year were abundant, they were tiny—about the size of an eraser on a new pencil—and they could be very difficult to locate. The trick to finding them was to find their food, which, unfortunately, happened to be howler monkey dung. So my task each morning was to locate a troop of howlers before they left their nighttime roosting tree. Dung materialized fairly predictably with monkey departure, so if I could find that spot fast enough, I could catch these elusive little beetles as they arrived.

  That morning it didn’t take long for me to hear what I was waiting for. Almost like clockwork, just before the first light of dawn, came the throaty roar of the howlers announcing their location to rivals in adjacent territories. When the monkeys were nearby, their howls were painfully loud. But those were good mornings because the monkeys and beetles could be located quickly. On other mornings the dawn roar was just barely discernible above the din of the forest night, and I might have a mile or more to trek before I found them. My routine was simple: take a compass bearing and then go back to sleep. An hour later, when it was actually starting to get light inside the forest, I’d head into the understory mist to track down that day’s troop.

  Sun streaked through cracks in the canopy, trapping clouds of steam in angled slabs of light as I jogged toward the now-silent monkeys, guided only by my compass. I pushed forward through the branches, stepping over roots, listening for movement in the canopy above. A branch rattled, and I’d found them. Ten faces glared down at me, coal-black shadows against the leaves; one of them hurled a stick.

  Once I found the monkeys it was a quick task to find the dung, and it never took long for the insects to arrive. Big, shiny, metallic beetles landed with a plunk and clambered over twigs and leaves. Tiny yellow and brown beetles perched on leaves with their antennae outstretched, reaching for scent. Within minutes they were everywhere, undulating back and forth in tiny sweeps as they maneuvered clumsily into the pieces of fallen dung. Soon still other hovering, pea-sized beetles zigzagged over to the dung fragments. Swirls of flies landed first on surrounding leaves before hopping onto the dung as well. In less than an hour the forest teemed with insects that had all converged on the dung to find food and mates.

  Onthophagus acuminatus, my Panama beetle

  The species I was after that day has no common name and is rarely noticed by anyone other than entomologists.1 The largest males wield a pair of horns, two cylindrical spikes rising side by side in a line between the eyes. (Smaller males, incidentally, do not have horns; they develop with only nubbins where the weapons would be.) My objective that year was to watch these horns evolve. I planned to accomplish this by applying selection pressure to the beetles myself in the dusty shed that served as my laboratory. I had purchased in bulk a ten-foot-tall bag of unlabeled plastic shampoo bottles from a company in Panama City. In the field station wood shop I cut the top off of each bottle with a band saw, so that they formed cylindrical tubes twelve inches deep and three inches in diameter. Almost a thousand of these tubes lined the counters of my little screened-in lab, each one filled with ten inches of packed, moist, sandy soil. An ice cream scoop’s worth of monkey dung sat at the top, and the whole thing was enclosed by screen mesh and a rubber band. A thousand furnished homes, if you happen to be a dung beetle.

  Each tube would house a single pair of beetles, going about their nuptial business of pulling pieces of dung into a tunnel and fashioning them into finger-sized compacted sausages called “brood balls.” On the tip of each ball sat an egg, perched on a tiny stalk and encased in a thin shell of soil and dung. Once the egg hatched, the larva would spend the entirety of its development inside the little dung ball, eating and growing in solitude until it was ready to crawl to the soil surface as an adult a month later. Each pair of beetles could provision roughly six to eight of these egg-containing sausages in a week and I kept them at it, giving them fresh dung every few days, until I had twenty or thirty offspring per pair.

  I started with one hundred wild-caught beetles, half males and half females. I measured the males under a microscope, and selected the five males with the longest horns to serve as breeders. Each chosen male was paired with two different females in succession. (I chose the females at random from within the lab population, since females don’t have horns.) The mated females were housed in separate shampoo bottle tubes, and from each I attempted to collect between twenty and thirty offspring. Ten reproductive females times thirty offspring each yields roughly three hundred progeny per generation. Males of this generation would again be measured and, as before, the five individual males with the longest horns relative to their body size would be selected as breeders. Each male would be paired with two females apiece, and their offspring would comprise the third generation of the experiment, and so on.

  The logic of artificial selection experiments is pretty straightforward. In my case, my breeding program selected for increased horn length in generation after generation of the population. The empirical question then concerned whether or not the population evolved in response to this selection. Did the horns of males get longer with each successive generation?

  Treatments in a scientific experiment need to be replicated, however, to minimize the possibility of arriving at results by chance. Popula
tions shift gradually from generation to generation simply as a result of serendipity. Picture a large jar full of fifty different colors of jelly beans, all mixed together. Reach in and scoop out a thousand and move them to a new jar. Chances are, you will happen to include most, if not all, of the fifty original types. Some might be a bit better represented in your scoopfuls than they had been before, but these changes in flavor frequency are likely to be minor. The jelly beans in the new jar should resemble the blend of flavors that were present in the old jar.

  If, instead, you scooped only five jelly beans from the original jar and these then populated your new jar, they almost certainly would not be representative. Most of the original fifty flavors would be lost. If those five jelly beans were then replicated until the new jar was full, the total number of jelly beans might be similar to what was present before, but the blend of flavors would be drastically different. The jelly bean “population” would have evolved simply due to chance.