To understand why males have such long forelegs, the Zehs needed to observe who won these fights and determine whether winning a fight brought with it opportunities to mate. Their problem was that most of the fighting and mating happens at night. In order to watch the beetles, the Zehs had to hike into the rainforest in the dark.
Nightfall happens suddenly in the rainforest, and getting caught after sundown without a light can be a harrowing experience. The understory is dim even at midday; at night it becomes profoundly dark—with the eerie, disorienting blackness of a cave or a closet where you can’t make out even your own fingers. I know people who’ve had to spend the night on the trail, stuck where darkness caught them without a flashlight. Worse, they had to stand up all night. You can’t sit or lie down because the leaf litter comes alive with stinging bullet ants, scorpions, tarantulas, and bushmaster and fer-de-lance snakes. Bullet ants, so named because their sting is as painful as a gunshot, climb tree trunks to forage in the canopy, so you can’t lean on a tree, either. Balance gets tricky because you have no visual sense of up or down—no rising bubbles such as divers use to orient themselves—and it becomes hard to tell if you are tipping or not. After a few hours you begin to hallucinate and see light—or so you think—but it is actually the barely discernible glow of bioluminescent fungi and the occasional glowing click beetle waddling through the litter.
When equipped with proper lights, however, the forest at night is exhilarating and wild, full of noise and wonder. To watch the harlequin beetles, David and Jeanne Zeh hiked in to their treefalls at dusk and then put red acetate filters over their headlamps. Because beetle eyes cannot detect red light, the modified lamps did not disturb them, and the red glow was enough for the Zehs to see what happened on the upside-down stage. By painting numbers on the backs of the beetles, they could keep track of each individual and watch many different encounters. They staged fights between males as well. The Zehs observed that males with the longest forelegs almost invariably win, and males who win are the ones who mate with the females7—exactly what we expect when sexual selection favors the evolution of increased fighting ability and ever-larger weapons. In short, the extreme paucity of suitable egg-laying locations, combined with the restricted size and ease of defense of these resources, creates in this species an ecological situation where the benefits of fighting and possessing large weapons are enormous.
* * *
The best part of the harlequin beetle story is actually not the beetles. It involves an even stranger animal that hitches onto the beetles for a ride. The Zehs noticed that nestled on the abdomens and tucked beneath the wing covers of their beetles clung tiny little arthropods called “false scorpions,” or “pseudoscorpions.” This particular species of pseudoscorpion feeds on fallen fig trees, and their nymphs develop inside the rotting wood. Like their beetle counterparts, male pseudoscorpions have weapons—a large pair of clasping appendages called “pedipalps”—and they use these claspers in fights with rival males over opportunities to mate with females. Their weapons, too, are impressive in proportion: much longer in males than in females, and extreme in the largest of males. But in absolute terms these animals are minuscule, and as a result, the context of their battles is different.
Pseudoscorpions battle on the backs of harlequin beetles.
To a giant beetle with a reach of sixteen inches, a wound on the belly of a fallen fig tree is both small and amenable to vigorous guarding. To a male pseudoscorpion, whose reach in the best of circumstances may span just a quarter of an inch, the same wound on that tree is enormous. An aggressive, territorial male may keep rivals away from one side but, while he does this, ten other males can approach females from other sides. Like trying to defend a lake from a spot on the shoreline, the effort is futile; investing in weapons for such fights would be a waste.
However, female pseudoscorpions depend on something else as well, and this resource makes a much more effective choke point, or bottleneck. Fallen fig trees can be miles apart, and these little arthropods lack wings. To travel from one tree to another, they clamber onto the backs of the harlequin beetles and ride from tree to tree. Many pseudoscorpions hitch rides on other insects by grabbing a leg with their pincers and dangling. The harlequin beetle–riding species has a relatively comfortable ride since they have the entire back of a beetle to cling to. They nip the rear of the beetle with their claws and, when the beetle squirms in response, hop aboard. They even spin silk nets to keep from falling while the beetle flies.
It turns out that the back of a beetle makes a perfect mobile mating territory if you happen to be a pseudoscorpion, and males fight with one another to guard this resource. Males use their clawlike weapons in these fights and, as in the case with beetles, males with the biggest weapons win. These fights were understandably harder for the Zehs to observe, so they used a method of DNA fingerprinting to identify which male pseudoscorpions fathered the nymphs produced by riding females. They found that males with the largest weapons were the ones most likely to be on the backs of beetles. They also showed that not all beetle backs were equal. More females could ride on the largest beetles, and pseudoscorpion males with the largest weapons guarded these bigger beetles. A successful male may be able to mate with two dozen or more females before the beetle lands. Even better, once they land, the mated females hop off, and a new round of females hop on.8
Harem of beetle-riding pseudoscorpions
* * *
Harlequin beetles and pseudoscorpions each have something of critical importance to females that is rare and localized, and therefore economically defensible, though the specific resource in each case is different. Males of both species who are successful at guarding the resource are able to mate with many different females. That is, they translate their success in fighting into success in reproduction. And, in both cases, this blend of ecological circumstances has led to a history of strong sexual selection for large male weapons.
Armed with this insight, we can return to the problem of horned and hornless dung beetles. Why do some species produce horns while others feeding on the same resources in the same habitats do not? The answer, I have come to suspect, has nothing to do with the dung and everything to do with what these animals do after they arrive at the dung. The world of the dung beetle is a world filled with intense competition (think of the numbers of beetles that flew into just one pile of elephant dung). Dung is actually a very valuable resource, if you happen to be a beetle or a fly. Rich in nitrogen and other nutrients, it’s ambrosia for larvae, and adult insects compete vigorously to get it for their offspring. Beetles must find the dung fast, and then they must contend with the hordes of other insects that also have found it and are attempting to steal it for themselves.
Most dung beetles are either “rollers” or “tunnelers.” Rollers are what everybody thinks of when they think of dung beetles, if they think about dung beetles. These conspicuous scarabs push their spherical globes of sculpted manure over the ground and tussle with one another along the way. When dung falls onto hard-packed, uncluttered soil or baked clay, they can roll their balls surprisingly fast, and many will travel tens of yards before they stop.
Ball rolling is a superb strategy for pushing food away from rivals. Carve a hunk from the pile, sculpt it into a smooth-sided ball, and roll it away from everybody else. These tasks can be accomplished in minutes, and, by pushing their dung ball several yards away, rollers can escape most of the competition. Ball rolling is most often performed by males, but females will join males as they leave the main dung pad and either cling to the ball and somersault along for the ride or simply follow the male until the pair reaches an adequate patch of soft or moist soil. Here they stop and cooperate to bury the ball, laying eggs beside or on top of the buried dung, depending on the species.9
Females are not the only ones to approach males as they roll their balls away. Rival males constantly challenge one another over ball ownership, and vigorous battles are commonplace. But these fights occur
out in the open on the exposed surface of the soil. Furthermore, the objects of these fights—balls of dung—are themselves mobile and malleable. Balls are pushed, pulled, even torn in half during fights as the males tussle around and around, clinging to and scrambling over the rolling balls. (These battles are great fun to watch, incidentally. At the Barro Colorado Field Research Station we’d paint numbers on the backs of rival males and place them on a dung pile centered in the bull’s-eye of a horizontal dartboard, betting on the winner and cheering them on as they fought to roll their marble-sized balls out of the ring.) Despite their pugnacity, not one of the thousands of ball-rolling dung beetle species has horns.
A second strategy adopted by many dung beetles is tunneling. Females of these species fly into dung and immediately begin to excavate burrows into the soil below. Once they’ve dug sufficiently deeply—a foot to a yard, depending on the species—they begin pulling pieces of dung down into the tunnels to stash them away from the other dung-feeding insects above. Females may make fifty or more trips to bury sufficient dung to provision just a single egg, and they’ll repeat the process for a string of successive eggs. While females are working on this arduous task, male beetles fight among themselves for tunnel ownership. A victorious male will guard the entrance to a tunnel—not so much to keep other species away from the food as to keep rival males of the same species away from the female. While in residence, a male will mate repeatedly with the female, but he will often get kicked out of the tunnel by an intruding, larger male. Males of tunneling species often have horns.10
Ball-rolling dung beetles fight in scrambles, rather than duels.
Tunnels are localized and confined—exactly the sort of fixed, economically defensible substrate in which we would expect to find a performance advantage to large weapons. Males stab their horns into the sides of tunnel walls to block entry of intruder males, or use them to twist or pry opponents loose before pushing them up and out of the hole. In these fights, males can lock themselves into position by bracing against the tunnel walls with barbs, teeth, and thick spines on their legs. Anchored, they can bring the leverage of their weapons to full effect, and males with longer horns win these contests.11 Because mating almost always occurs inside tunnels, winning fights is a critical prerequisite for reproduction.
Ball rollers have no such leverage. Their fights occur out in the open and over a mobile resource. Although males fight briskly with tumbling and pushing and scrambling, they cannot brace themselves into position like the tunnelers can. As a result there appears to be no leverage or other performance advantage to having large weapons and, without the performance advantage, weapons are not cost-effective. A simple change in the way these beetles hid their food resource had profound implications for the evolution of their weapons.
We’ve identified two of the three critical ingredients for arms races: intense competition, generally arising among males as they battle for access to females, and ecological situations that cause resources to be localized and economically defensible. There’s one final ingredient, and it involves the details of the fights themselves—the way that males face each other in battle. Males must face each other one on one, rather than all together in a scramble. For bigger weapons to perform better than smaller ones, the battles must be matched and “symmetrical,” with comparably armed contestants challenging each other face-to-face. Oddly, this last ingredient has been almost entirely overlooked by biologists. To appreciate its significance, we must turn instead to attrition models of military forces, and to the century-old insights of an eclectic automotive and aeronautical engineer.
6. Duels
By the end of the nineteenth century, Frederick William Lanchester had emerged as a brilliant designer and builder of cars. He invented one of the first self-starting devices for gasoline-powered engines, and he was among the first to incorporate carburetors. He’d built his first full car by 1895, and in 1899, he and his brother started the Lanchester Engine Company, one of the first factories in England to build and sell cars to the public. When his company went bankrupt a few years later due to the incompetence of its board, Lanchester shifted his focus to aircraft. He modeled lift and drag for a variety of wing designs, and his “circulation theory of lift” became the basis for modern airfoil theory.1
During the First World War, Lanchester started using mathematics to try to predict the outcome of battles. He was obsessed with aircraft, and convinced they could play a critical role on battlefields. In the process of writing his book Aircraft in Warfare: The Dawn of the Fourth Arm (1916), he derived a simple set of equations portraying the losses of forces under general circumstances of combat.2 These little equations, called “Lanchester’s laws,” sparked an explosion of research into the dynamics of military engagements. Books were written about the influence of these equations,3 and international conferences were convened to debate their applicability to battle.4 Although modern models of force depletion in warfare are vastly more complicated than Lanchester’s original equations, his models formed the backbone for literally hundreds of subsequent theoretical approaches, and are credited with spawning what would eventually become the multibillion-dollar, intellectually thriving field of operations research.5
The logic behind Lanchester’s equations was to find an explicit way to calculate how rapidly the forces of one army would be depleted by fire from another, and vice versa. Each army was assigned a force strength, the number of available troops, and a force effectiveness, which translated shots fired from one side into losses incurred by the other. Effectiveness could mean different things, depending on the nature of the engagement and the types of weapons, but it was essentially a measure of the fighting ability of each member of an army.
When armies faced each other in battle, the losses by one side could be calculated as the number of soldiers from the opposite side times the effectiveness of each of those soldiers—for example, the number of bullets fired multiplied by the probability that each bullet hit a soldier from the opposing army. More soldiers meant more rifles and therefore more shots fired (greater force strength); better training, aim, and more powerful guns meant a higher probability of incapacitating a target with each bullet (greater force effectiveness). The objective of the models was to simulate salvos of gunfire by simultaneously calculating losses for both armies with matching equations, adjusting force strength based on these losses, and then repeating the process with additional salvos. Iterated over a succession of salvos, Lanchester’s models elegantly revealed the rate of depletion (attrition) of troops and predicted both the duration of the battle and the eventual winner. Permutations to these simulations could then be run with all sorts of altered conditions, revealing the factors leading to victory in each case.
As Lanchester worked through his models, he discovered that out of all the countless advances in types and styles and sizes of weapons that punctuated the history of human warfare, one change altered the rules of engagement more dramatically than any other: the deployment of long-range weapons such as guns and artillery. The way that troops killed each other in the past, when soldiers faced up man-to-man and clashed in combat hand-to-hand, was fundamentally different from the way they killed each other post-firearms. To incorporate these differences into his equations, Lanchester derived two types of models.
The first was designed with ancient battles in mind. Lanchester recognized that when soldiers fought with close-range weapons, such as pikes, maces, and swords, there was little opportunity to concentrate attackers.6 One soldier might face ten on the field of battle, but the nature of hand-to-hand combat meant that he was unlikely to face all of his opponents at once. There simply wasn’t space enough for them all to approach him at the same time and, if they did, the swinging weapons of adjacent soldiers would get in the way. The reality was that most of the time, such engagements occurred in succession. Our lone warrior would face each of his opponents in turn, one after the other.
Armies of this age lined up against each other an
d clashed man-to-man in a long string of individual encounters. Reinforcements lurked in the wings, since there was no room for them on the line, and soldiers stepped up as needed to fill the gap when the man before them fell. At Agincourt (1415 CE), for example, 1,500 English infantry, knights, and men-at-arms suited in armor and carrying pikes and swords, faced off against 8,000 Frenchmen.7 The armies stood shoulder to shoulder in parallel lines that crossed the full extent of the meadow, and reinforcements pressed in rows behind their respective front lines. The French outnumbered the English by nearly five to one, but this only meant they were stacked twenty deep instead of four. The actual battle was fought man-to-man.
In conflicts such as this, Lanchester realized, victory hinged both on the number and effectiveness of soldiers. Specifically, the loss inflicted equaled the number of soldiers times the effectiveness of each soldier. Because the fights were face-to-face, the strength and training of each soldier, as well as the quality and often the size of his weapons, mattered a great deal. The best-armed soldier was the most likely to win in each of the individual engagements, causing the better quality army to lose forces at a slower rate. Soldier numbers mattered, too, of course, since a numerical advantage permitted an army to sustain prolonged engagements by cycling or replacing soldiers on the line, but the fighting ability of individual soldiers could tip the balance.
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