Freshwater and marine sticklebacks differ in the length of their spines and the number of bony armor plates.
Today, sticklebacks in many lakes lack defensive weapons. Dolph Schluter and his students at the University of British Columbia found that lake habitats have far fewer predators than marine habitats, and this appears to relax the pattern of natural selection for larger plates and longer spines.19 With fewer predators, lake fish benefit less from large weapons than marine fish. Armor also costs more in lakes than it does in the ocean. Low freshwater concentrations of the ions necessary for bone growth mean that fish pay a higher price for mineralizing bony plates in lakes. Unarmed sticklebacks are larger as juveniles and begin breeding sooner than their armed counterparts. In freshwater, it appears, the costs of long spines and large plates are steeper than the benefits they provide.
Of course, every story has exceptions. But with sticklebacks, the exception proves the rule. Dan Bolnick has been studying sticklebacks in Lake Washington, where fish have much bigger weapons than in other lakes. Bolnick found that this shift in armor happened very recently—fish samples collected prior to the 1960s had reduced armor typical of other lake sticklebacks. Efforts to stem pollution in this lake resulted in a dramatic improvement in the transparency of the water, and in these especially clear waters, introduced trout began to feed in earnest on sticklebacks. More predators translated almost immediately into bigger weapons.20
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Since the beginnings of recorded history, soldiers have sought protection from enemy weapons. Their armor mirrors the armor of stickleback fish and other animals, evolving for similar reasons and in surprisingly similar directions.
The earliest forms of body armor were the shield—made originally from animal hide and later from leather stretched over wood—and protective clothing made from leather, padded fabric, wicker, or wood.21 Over time, as the technology of weapons changed, so did the shapes and styles of protective body armor. The first manufactured weapons were sharp, fire-hardened pointed sticks and spears tipped with flint blades. Knapped flint could slice flesh, but blades shattered easily—stiffened leather provided adequate protection against flint for thousands of years.22 Metallurgy brought with it tougher weapons—first bronze, which was soft and dulled quickly, and later iron—and against these weapons, leather armor was not as effective. Armorers began attaching metal rings or scales to the outsides of leather garments to block against the stabbing or slicing blows of metal-tipped pikes and swords. Ancient Greek soldiers wore a leather cuirass covered with front and back plates of hammered bronze, and Roman legionaries wore leather cuirasses stitched with metal plates arranged in overlapping tiers much like the scales of a fish (these warriors also wore helmets and carried bronze shields).23
Roman legionary wearing a leather cuirass with attached metal plates.
By the time of the Crusades (1100–1300 CE), protective shirts of mail—intricately linked chains of iron rings—had become the standard attire for battle in Europe. Mail could block penetration from most strikes by metal blades, but shock from the impact was still severe. Soldiers often wore thick clothing of padded fabric or leather underneath mail. On top, they added scaled metal and leather cuirasses and helmets. Soon, plates of iron were added to vulnerable areas, such as the elbows, shoulders, and legs. By the end of the fourteenth century, full suits of plate armor had replaced mail—think “knights in shining armor.”24 Plate armor predominated until the sixteenth century, when its use in warfare was rendered obsolete by gunpowder and firearms.25
From the beginning, the evolution of protective manufactured armor was shaped by a balance between benefits and costs. As weapons became more dangerous, the thickness and toughness of armor increased, but so did its bulk and weight. On the one hand, armor could protect a soldier, but on the other it restricted his movements and slowed him down. A suit of chain mail weighed up to fifty pounds, not counting the heavy leather beneath it. A helmet alone could weigh twenty pounds, and helmets were so hot and suffocating that knights generally carried them on the pommel of their saddles until the last minutes before battle.26 Plate armor was a huge burden; getting knocked over or unhorsed could mean death for a knight, since he couldn’t get back up without assistance.27 By the end of the sixteenth century, crossbows and longbows had already called the efficacy of armor into question (arrows striking straight-on could penetrate), and the spread of gunpowder sealed its fate.28 As with the bony plates of lake-bound sticklebacks, once the benefits disappeared, armor was no longer worth the price. Although plate armor thick enough to stop a bullet was possible, nobody could wear it because the plates were too ungainly and heavy.29 So body armor all but disappeared from the battlefield for four hundred years, until a new invention: Kevlar.30
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In armor we see all of the processes that matter for the evolution of extreme weapons: individuals vary in the extent of their armament; these differences in weapon size affect the performance of their bearers (survival, growth, and reproduction in sticklebacks, and survival in soldiers); and, as a result, the sizes and shapes of these weapons evolve rapidly and dramatically over time. Weapons such as armor come at a price, and sometimes, when this cost is high enough, individuals with small weapons fare better than those with big ones. Indeed, most of the time, for most weapons, bigger is not better.
2. Teeth and Claws
Mountain lions are still common where I live in Montana, and for many Montanans these cats are part of why we choose to live here. There’s a tiny rush you feel every time you step out into the wilderness, a reminder that in some places we’re actually not the top of the food chain. I had my first face-to-face encounter with a mountain lion this past December, hiking on the ridge behind my house. I’d known they were there, having seen paw prints in fresh snow on countless winter mornings and from stumbling upon buried kills in the forest nearby (lions cover a carcass with pine needles and branches so they can come back to it later). I also knew they were there because I was actively looking for them; a few years before, I’d placed a motion-activated trail camera next to a little spring in a ravine behind my house. Each week I hike up the mountain to the spring to swap out the memory stick and then sort through the thousands of pictures of magpies, deer, skunks, bears, eagles, and, of course, mountain lions.
That morning last December, I was coming over the hill and starting down to the spring when my dog tore ahead of me. The cat he was chasing leapt to the nearest pine, shot up into thick branches, and disappeared. For a stunned moment I was impressed with my dog—a flat-coated retriever; a family dog, not actually a mountain dog—and then it was time to assess. No bear spray that day, no camera, no knife, not even a leash for the dog. Basically, I was utterly unprepared, and the cat looked small enough that I knew it might not be alone. A mother lion could spring from the bushes behind me and, unless I saw it coming, I wouldn’t stand a chance. So I took my dog by the collar and we backtracked over the hill and down to my house. When, a half hour later, I hiked back up better prepared, I found no trace of the cat whatsoever. But when I retrieved the memory stick from the camera and opened the files, there were pictures of two lions that morning, not one. Big cats rarely choose to hunt humans, but they can be devastatingly efficient when they do. I was lucky I’d retreated.
Silent, fast, and deadly, cats are quintessential mammalian predators, yet their weapons are relatively small. To appreciate why, consider what it is these animals do. Canada lynx, for example, stalk alone through vast boreal forests, soundlessly sweeping back and forth across the snow in search of their preferred prey, snowshoe hares. Overtaking a hare is no easy feat. Finding them is difficult because their fur blends seamlessly with their backgrounds. Hares molt from brown to white during the winter when their landscape becomes snow covered—their solution to the problem of camouflage on more than one background.
Once found, hares have to be caught. Massive hind feet give them a tremendous advantage, including an almost unmatched potenti
al for acceleration. Galloping hares can top forty-five miles per hour, making them second only to pronghorn antelope as the fastest land mammals in North America. On top of that, the power in their long hind legs lets hares change directions erratically, without sacrificing acceleration or speed.1 There’s a reason it’s the hare in Aesop’s fable “The Tortoise and the Hare.”
Given the hare’s speed and agility, it’s no surprise that lynx very often fail to catch them. Footprints in fresh snow tell the stories of these encounters, revealing where each hare was flushed, how far it ran, and who won the race. In one study following hundreds of miles of lynx tracks over five years, the cats caught their prey in only one out of every four chases; in a similar study lynx captured a hare only once every four or five days—barely enough to sustain their body mass.2 Even in good years most lynx hunts end in vain. In bad years, lynx fare much worse.
Snowshoe hare populations fluctuate wildly in number, with over forty times more animals in “boom” years than in “bust” ones. These population cycles mean that every eight to ten years lynx experience drastic shortages in their food supply and, in these lean years, starvation is rampant. Kitten survival plummets from 75 percent in years with abundant hares to 0 percent when hares are scarce.3 The difficulty of catching prey and the periodic shortages in abundance of prey together result in intense selection for improvements in the lynx’s hunting performance, placing a premium on the weapons necessary for successful hunts.
In fact, much of the diversity of predators can be understood from an examination of their weapons and the ways in which these structures have adapted to different types of habitats and prey. The history of the mammalian order Carnivora, for example, is a story filled with successes and failures all defined by the evolution of their weapons: forelimbs, claws, jaws, and, especially, teeth.
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The earliest meat-eating mammals appeared not long after the disappearance of the dinosaurs, roughly sixty-three million years ago. These carnivores were scrawny semi-predators who ate a mixed diet, and their teeth were correspondingly all-purpose. Vulpavus, for example, was a small, ferret-sized animal with a lanky body and slender tail that probably fed on insects, spiders, lizards, birds, and small mammals such as shrews.4 The principal instruments in this early carnivore’s dental toolkit were canines, incisors, and a row of premolars and molars along each jaw. Already, by the appearance of the first carnivorous fossils, subsets of teeth had begun to take on specialized functions. Canines were longer than the other teeth, and effective at apprehending and killing prey. The pointed premolars could grip and hold prey, and the molars could slice and crush a carcass during feeding.
Subsets of carnivore teeth became increasingly specialized for specific tasks, such as piercing, slicing, or crushing.
Over time, these subsets of teeth evolved to become increasingly efficient for their specific tasks. At the same time, the nature of these tasks began to change as the number of carnivore species multiplied. Many species began to specialize on narrower and narrower subsets of prey, and the demands on their teeth started to differ from species to species. Carnivore teeth evolved in new and different directions, depending on the particular diet and hunting habits of each species. Although some species retained relatively basic tooth shapes suitable for omnivorous diets, many, including wolves, hyenas, cats, and sabertooths, diversified into efficient “hyper-carnivore” predators specialized for diets that consisted of meat only.5
Wolves are the “jacks-of-all-trades” of the hyper-carnivores. Their long, slender jaws snap shut with amazing speed, and their sturdy canines grip flanks or legs of large prey as they wrestle them to the ground. Wolves hunt in packs and, by pulling from several directions at once, they can topple animals far larger than themselves. After the kill, wolves tear into carcasses using dual-purpose molars. Sharp outer edges work like shears to slice through sinew and flesh, but these teeth are still broad enough to crush small bones.6
Hyenas also hunt in packs, but their jaws are very different from those of wolves. Hyena canines are relatively short, and their molars have lost the “dual-purpose” functionality of their ancestors. The slicing edges of hyena molars are gone. Hyenas are bone crushers who feed primarily on marrow, and their teeth are wide and strong with rounded, dome-shaped caps. Their faces and jaws are squat, giving their teeth a huge mechanical advantage. It’s basic physics: the nearer an applied force is to the joint of a lever, the stronger it will be. Teeth on short jaws aren’t anchored very far from their hinges, resulting in slow speeds but powerful closing forces (contrast this with wolves, where canines positioned at the ends of long jaws snap shut faster, but without as much force). In hyenas, jaw-closing speed appears to have been traded for increased closing force. They have a tremendously powerful bite, and because of the shapes of their teeth, they use this bite for cracking bone rather than puncturing or slicing flesh.7
Wolves, hyenas, cats, and sabertooths differ in the relative sizes and shapes of their teeth.
Cats also have snouts and jaws that are relatively short, favoring the mechanical power of jaw closure over speed. And, like hyenas, their molars have become specialized for just a single task. But this task is slicing, rather than crushing. Cat molars are narrow and sharp—useless for breaking limbs or bones, but ideal for cutting through flesh. And, while the primary weapons of hyenas are their molars, in cats the weapons are canines. Cats use their canines to puncture thick hides and sever the spinal cords of prey.8
Cats are specialized in another way. They can supinate their forelimbs—twist their wrists and pull the pads of their feet inward, facing their bodies. Flexible forelimbs let these animals cling to their prey and position themselves for the careful execution of their powerful, killing bite. Their canines are long and narrow, which makes them great for piercing but vulnerable to breaking if they get yanked to the side. A cat that can hang on to its prey in the throes of an attack can pierce its long teeth straight through the skin in a calculated stab. Failure to hold on results in torque during the bite, and may cause the canines to snap.9
Thanks to their flexible forelimbs, cats are unusually agile, able to pounce and, like the lion behind my house, climb trees.10 (The old adage that a cat will always land on its feet is more apt than most people realize.) But as deadly as cats may be, they pale beside their extinct relatives, the sabertooths. The canines of sabertooths were truly massive: ten-inch daggers that could sever the spine of a mammoth. Sabertooth teeth wouldn’t work without severe adjustments to jaw and skull shape, and to body posture. Over time, upper jaws became much shorter—even shorter than in other cats—producing powerful bite forces by bringing the canines ever closer to the hinge. Sabertooth jaws were thick, and the hinges could swing open to an unusually wide gape. Sabertooths had to pull their lower jaws all the way out of the way, as if releasing the bottom plate on a stapler, before they could sink their big teeth into prey. Finally, the shortened face and compressed skull tipped the head back, so the canines pointed forward during an attack.11 All of these modifications rendered these carnivores among the most deadly ever to live, but the postural and head-shape changes came at a steep cost. They made running—and basically all movement—cumbersome and awkward.
Extreme tooth size allowed sabertooths to kill larger and larger prey. In a time when titanotheres, giant sloths, and mastodons abounded, this advantage would have been considerable. Saber-toothed predator forms arose in at least four mammalian lineages, first within two now-extinct carnivore groups, the creodonts (Apataelurus sp.) and nimravids (Barbourofelis fricki), then within the cats (for example, the scimitar-toothed and dirk-toothed cats), and finally, within the marsupials (Thylacosmilus atrox). We associate most living marsupials with Australia, but these pouch-bearing mammals once ranged over much of the world, and Thylacosmilus actually lived in South America.
Well-preserved specimens of the dirk-toothed cat Smilodon fatalis from the La Brea tar pits indicate that it was smaller than a modern lion, but
more than twice as heavy (six hundred pounds), with a bobbed tail.12 These short, stout animals could never have chased down prey and almost certainly ambushed them from close range. Fossil bone beds suggest that sabertooths specialized on lumbering prey such as camels and young mammoths and mastodons, and the shapes of their forelimbs suggest they leapt onto the backs of these behemoths from trees.
Saber-toothed cats probably leapt from trees onto unsuspecting mastodon calves.
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Carnivore teeth aren’t small because they couldn’t or didn’t evolve. They’re small because individuals with unusually large teeth performed poorly when hunting their particular prey. Teeth and other major structures are almost always subject to trade-offs—a balance of opposing forces of selection. Bigger weapons may be better for killing prey, but they may also prevent an animal from catching prey in the first place. Individuals with unusually large weapons surely crop up from time to time in predator populations, but their hindered performance at critical tasks such as overtaking prey causes them to fare poorly and, over time, these extreme weapon forms are likely to disappear.
Sabertooths are a case in point. In each instance where canine evolution proceeded to this extreme, the enlargements to these teeth required dramatic adjustments to jaw and skull shape. Opening the jaws wide enough wasn’t possible without modifications to the hinges, and sinking teeth this long into the neck or throat of prey required a severe backward tilt of the head.13 Sabertooths could not run fast. They were simply too awkward. Such big weapons could never have arisen in carnivores who relied on speed to hunt down prey.
Animal Weapons Page 3