Animal Weapons

by Douglas J. Emlen

  Lanchester’s models describing ancient warfare are called his “linear laws,” because the equations used to determine the rate of force loss are linear. He designed these first types of models so that he could contrast them with his second type, the models he was convinced mattered in the modern age. Lanchester recognized that long-range weapons would alleviate the “available space” constraint of hand-to-hand combat. Guns could be fired from a distance, and this meant that multiple soldiers could concentrate all their fire on the same target. If one army was larger than the other, the extras didn’t have to sit idly by, waiting in the wings. Their effectiveness could be brought to full force immediately because they could all fire at the enemy at once. Now, when he calculated the rate of attrition of opposing armies, he found that the effects of force strength—numbers of soldiers—mattered a lot more than they had before. The rate of loss of an army equaled the effectiveness of each soldier times the number of soldiers squared (these models, not surprisingly, are called his “square laws”).8 To put this difference into perspective, if one army in ancient times was five times the size of another, as the French were at Agincourt, it would have been five times as powerful. Now, because of the multiplicative effects of troop number, this same army would be twenty-five times as powerful as its opponent.9

  The genius of Lanchester’s models was his realization that the capacity to concentrate fire—to have multiple soldiers confronting a rival simultaneously—utterly changed the formula for victory. Based on his models, military strategists quickly recognized that investing substantially in the training and arming of individuals was not cost-effective, because the fighting effectiveness of each man was less likely to determine the outcome of battle than the numbers of men brought to the fight. Obviously, some training and equipment were needed, since soldiers armed more poorly than their opponents would be less effective. But given the choice of allocating resources to weapons and training, on the one hand, or to adding more soldiers on the other, the winning strategy was clear. Add more soldiers.

  Since Lanchester’s pioneering work, his square law of combat has been applied to post hoc analyses of thousands of battles, ranging from Ardennes to Iwo Jima, and it has formed the foundation and inspiration for countless models of military force allocation, military strategy, and military spending.10 Lessons from these original models, such as the near-sacred maxim of never dividing one’s forces on the field of battle, still permeate the military mindset today.11

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  Lanchester designed his square law for modern warfare and, without question, the vast majority of interest has focused on that category of equations. But it’s his linear law that is most relevant to the evolution of extreme weapons. By contrasting ancient and modern warfare, Lanchester helped define the circumstances in which large weapons will, and will not, be cost-effective. When opponents can concentrate their fire—gang up on an opponent simultaneously—investing in large weapons is probably a mistake. On the other hand, when soldiers duel each other at close range, face-to-face and one-on-one, then the better fighter is likely to win. Because fighting ability often depends on the size of a weapon, duels can lead to situations where bigger and bigger weapons prevail.12

  Duels matter for animal weapons, too, for essentially the same reason. Animal fights occur in all sorts of crazy places, from steep rocky cliffs to canopies of tropical trees and thermal vents on the ocean floor, and the details of these contests vary no less spectacularly. Some of these battles lend themselves to the use of weapons while others do not.

  With the exception of social insects such as ants and termites, most animals do not fight with armies.13 Males fighting for opportunities to mate do so as individuals. They fight for themselves. But this does not mean that all males confront each other one-on-one. In fact, many encounters are far wilder than this, involving chaotic scrambles as piles of rivals all attack at once. Just as Lanchester contrasted ancient and modern soldiers so, too, we can contrast animal battles that occur as duels with those that occur as scrambles.

  When males attack each other face-to-face, the battles tend to be stereotyped and repeatable—weapons lock, opponents strain against each other, and push or pull or twist, depending on the species. Battles become reliable tests of relative strength, and the best quality male generally wins. When rivals scramble together in chaotic tumbles, the outcome of fights is less predictable and the value of weapons diminishes.

  Male cicada-killer wasps pounce on each other in vicious midair tangles, twisting and writhing and biting, often crashing to the ground in the process.14 Fights occur in the open, above hard-packed soil containing preadult females that have yet to emerge, and it’s not uncommon to encounter three or four males locked in combat at the same time. Females complete their development underground, metamorphosing from larva to pupa to adult in the relative safety of the soil. They lie in clusters, climbing up to the surface unmated and reproductively receptive. Males can smell where these females lurk, and dozens may arrive to fight for possession of the prized real estate hiding them. Victorious males grab females and mate with them as soon as they appear—sometimes even helping them along by digging them out of the ground. Cicada-killer wasps experience intense male competition for localized resources (buried females), but they lack elaborate weapons.15

  Horseshoe crabs swim ashore under full and new moon high tides in fantastic swarms. Hundreds of thousands of individuals clamber out of the sea to mate in the moonlight, blanketing beaches in a deep carpet of white foamy sperm. Here, as with so many animals, reproductive females are few and far between. By the time a gravid female swims ashore, she will already have a male latched onto her back. But he’ll have to hold on tightly to have a shot at fertilizing her eggs, since males challenge him from all sides.16 It’s not uncommon for females to have four or five males piled onto their backs, pushing and scrambling and jockeying for position. Yet these males, too, lack significant weapons. Both cicada-killer wasps and horseshoe crabs face intense competition for females who are localized and defensible, the first two conditions for an arms race. But fights unfold as scrambles rather than duels. They lack the condition of Lanchester’s linear law, and large weapons are not cost-effective.

  Although the concept of “fair” is a human construct, it reflects, in a way, the predictability and consistency of outcome. In a fair fight, the best fighter should win (any upset to this outcome would be perceived as cheating), and the fairest fights are always duels. Since the beginning of recorded history, in military traditions ranging from Homer’s Greeks to medieval knights, samurai, and gunslingers in the American Wild West, the only form of confrontation that has ever been acceptable for establishing honor, status, or rank was the duel.17

  In animal duels, too, the best fighters usually win, whereas in scrambles they may not. Head-on encounters are relatively predictable and straightforward, without surprises. In these fights it is harder for a poor quality male to usurp a bigger, better male. As with ancient warriors, strength, stamina, and weapon size prevail.

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  All else being equal, we expect species where males fight each other one-on-one to be more likely to evolve extreme weapons than species where males fight less predictably. But what habitats are conducive to animal duels? It turns out that many of the same ecological situations that cause resources to be economically defensible also align, or otherwise constrain, male contests so that they tend to occur as duels. By acting as choke points, these special situations become cauldrons for the evolution of extreme weapons.

  Burrows are probably the most widespread ecological situation leading to the evolution of extreme weapons. They are localized and readily defendable. Males can guard the entrances to tunnels where females reside, and in so doing, block rival males from approaching the females. But tunnels also physically restrict access in a way that aligns the interactions of opponents. A rival male dung beetle has to enter the tunnel before he can challenge the guarding male. Ten males couldn’t attack a
t once even if they wanted to, because there isn’t space for more than one rival to enter at a time. The restricted confines of tunnels align battles so that they necessarily occur as a series of successive duels. Ball-rolling species, on the other hand, face no such restricted access. Males can challenge from all angles at once, and very often battles among ball rollers entail chaotic scrambles between three or four males. In dung beetles, species that fight one-on-one often have elaborate horns; species that fight in chaotic scrambles do not.

  Shrimp and crabs with huge claws battle over burrows.18 Wasps with long tusks fight over the tubular entrances to mud-pot nests—burrows—that they cement onto the undersides of leaves.19 Many species of rhinoceros beetles fight over burrows, either tunnels in the soil or hollowed-out stems of plants such as sugarcane.20 There are even a few species of unusual Asian frogs that fight over burrows, and these same species are unique among frogs in also bearing male fangs and spurs.21 And there is evidence that an extinct and giant species of horned gopher fought over burrows.22 Although by no means a guarantee of weapon evolution, burrowing behavior provides two of the three critical prerequisites for an arms race, and this appears to have tipped the balance in species after species.

  Branches work the same way. In essence, they are “inverse tunnels” since a branch, like a tunnel, is a linear substrate that can be blocked and along which a rival must pass. Like trolls guarding bridges in fairy tales, males can plant themselves as gatekeepers. To get to a female on the other side, a rival must challenge the guarding male, and because the branch is long and narrow, only one male can approach and fight at a time. Animals as diverse as rhinoceros beetles,23 leaf-footed bugs,24 and horned chameleons25 defend branches, blocking the passage of rival males and gaining access to females in the process. Males in many of these species invest in elaborate weapons.

  Even exposed locations can align male interactions if the critical resource is stationary and small enough for a male to stand over it and guard. Males plant themselves atop the resource and swivel as needed to face each attacker, like an ice fisherman protecting the hole he’s drilled in a lake. Stag beetles battle over sap oozing from nicks on the sides of standing trees, much like the harlequin beetles. Males grasp each other head-on with their mandibles and strain, lifting with their bodies and legs, trying to rip their opponents free from the tree trunk and fling them to the ground below. Females visit these sap sites to feed before they fly off to lay their eggs, and victorious males mate with them while they are feeding.26

  New Guinean antlered flies fight over tiny holes in the bark of fallen trees. Females must get through the bark to lay their eggs, and they can do this only using an existing hole (the flies are not robust enough to drill holes of their own). Males capitalize on this situation and stand guard over the holes, diligently moistening and marking them to attract females and fighting with all trespasser males who come by.27 Because males can stand over the resource, they can turn to face each rival head-on.

  In all of these examples, males compete for females, and they control access to females by fighting to guard something that is economically defensible. And, in each of these species, the very feature of the habitat that rendered resources defensible structured male contests so that they occurred as duels, rather than scrambles.

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  Eyes in male stalk-eyed flies sprout like lollipops from the sides of their heads, giving these flies an uncanny resemblance to miniature sets of barbells. Males in some species have absurdly long eyestalks, while males in very closely related species do not. As with dung beetles, we can explain this variation by looking closely at the natural history of each stalk-eyed species.

  Ingrid de la Motte and Dietrich Burkhardt studied natural populations of five species with long eyestalks, and several additional species without them.28 What they found exactly fits our prediction. Species such as Teleopsis whitei and T. dalmanni, each with males with huge eyestalks, spend their daytimes walking along the ground or on low vegetation near forest streams, feeding on fungi, molds, and yeast from decaying leaf litter or dead animals. During the day they forage alone, and they are aggressive toward other flies—male or female—who approach.

  Male stalk-eyed fly

  Each night, these same flies cluster together to roost in dense aggregations on dangling perches in the forest. In sheltered alcoves beneath the undercut banks of small streams, tiny rootlets hang down like threads. Some rootlets are longer than others, and long threads can hold more flies than small threads. Females often crowd onto these threads in groups of as many as twenty or thirty, forming a linear hanging harem.

  From a male fly’s perspective, rootlets are critical resources routinely used by females. For the few males able to do it, guarding a thread means securing disproportionate access to females, which, given the numbers of females per thread, translates into a huge reproductive benefit. A number of biologists have now hiked up tropical streams in Africa and Asia to study these flies at their nighttime roosts. Two major research groups, one headed by Gerald Wilkinson and including John Swallow and Patrick Lorch,29 and the other including Andrew Pomiankowski, Kevin Fowler, and Sam Cotton,30 have all spent long nights with headlamps watching males fight to guard their hanging territories.

  The dominant male perches near the top of the thread, sometimes swaying his eyestalks back and forth in a rocking motion that twists the thread in gentle, sinuous ripples. Flies may be able to assess the relative size of a male from afar by observing the amplitude of these undulations. When a rival male approaches, he will hover in front of the resident male, eyestalk to eyestalk. If the newcomer is smaller than the resident, he generally departs without incident. But if he is equal in size or larger, then a battle ensues.

  The intruding male lands on the thread and walks up to the guarding male. Forelegs outstretched, the males butt heads and grapple for control of the thread and, in virtually every instance, the male with the longer eyestalks wins. The winner then mates with each of the roosting females during the night. For these species, the benefits of territory defense appear to vastly outweigh the costs of producing and bearing a weapon, even a truly enormous and awkward one.

  Male stalkie guarding a harem

  Remarkably, when people studied related flies that lacked large eyestalks, they found that the single biggest difference in their behavior was that these flies did not roost communally at night. Flies like Teleopsis quinqueguttata, which have only rudimentary eyestalks in males and females, never grouped into defensible clusters. Like their relatives, they fed in isolation during the daytime on molds and fungi. But at night they roosted alone dispersed in vegetation. All mating occurred in the daytime, in happenstance and brief encounters between the sexes. No duels, no weapons.

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  When Lanchester modeled duels in his simulated ancient battles, he pictured soldiers grappling with other soldiers, but his logic works just as well for confrontations between larger entities. Ships attack other ships, fighter planes attack fighter planes, and nation-states attack rival nation-states. For these confrontations, too, the nature of the interaction matters, and opponents lining up one-on-one can spark an arms race. For example, for almost 1,500 years oared galleys churned through Mediterranean waters as Egyptians, Phoenicians, Carthaginians, and Greeks all battled for supremacy.31 For most of this period (1800–750 BCE) ship design remained largely unchanged. Fleets of long, canoe-like ships shuttled soldiers back and forth to battle, powered by sail when the winds cooperated and at all other times by sweat and muscle, as rows of oarsmen spaced along the length of each side pulled long oars in unison to propel the vessel. But somewhere around 750–700 BCE everything changed. A new weapon was added to the galleys: the battering ram.

  Cast from the finest quality bronze forged in the best kilns of the day, rams permitted well-manned ships to smash into other ships, shattering their hulls and sinking them along with their crews. Because of rams, naval ships could be more than simple vessels. They could be wea
pons. Ships suddenly acted as units—individuals—and they confronted each other up close and one-on-one. Maritime battles began to resemble the clashes of ancient infantry, with ships lined up abreast in long rows, crashing into opposing lines of ships from another fleet.32 The battering ram caused naval warfare to fulfill the conditions of Lanchester’s linear law: ships fought other ships in close range duels. From this point onward, bigger was better and navies with the largest ships won.33

  What unfolded was one of the grandest naval arms races of all time, as shipwrights struggled to add speed and power to their vessels, and each new innovation from one side was instantly copied and then bested by the other. Early galleys such as the penteconter had roughly 25 oarsmen per side, and the first attempts to add speed and power involved lengthening the hull to add more oars. But ships rapidly reached a maximum at roughly 130 feet, beyond which the hulls buckled in rough seas.34 By around 600 BCE modifications to the hull permitted greater height, and a second tier of rowers was added above the first, doubling the power. These new ships could pack their power into shorter hulls, which were stronger and more maneuverable than longer ships.35 The bireme had a wooden hull just 80 feet long and 10 feet wide, but it now housed 50 oarsmen per side, for a total of 100 oars. Triremes soon followed, with the addition of still greater height and a third tier of oars. Triremes grew to 130 feet long and 20 feet wide, propelled by 180 oars. The trireme reached a maximum, however, since increases in ship length meant buckling, and increases in ship height (for additional rows of oars) meant tipping.

  For almost two hundred years the trireme prevailed as the dominant ship of the line for naval fleets, until yet another modification to the hull opened the way for even greater increases in size. Thus far, each galleon had housed just one man per oar. Much like the trunk of a centipede, which has a string of repeated units sprouting a leg from each side, the ancient galley comprised a string of sections, with oars extending from port and starboard. Penteconters were called “ones” because one man pulled the one oar projecting from each side of every segment. Biremes were called “twos” because two men sat on each side of every segment, one above the other, each pulling one of the two oars. Triremes were “threes” for the same reason. But by the fourth century BCE shipwrights had discovered that they could add power, and with it speed, by cramming additional men into these confined spaces.36