Animal Weapons

by Douglas J. Emlen

  From fossil skeletons it’s possible to determine the body sizes and proportions of these giant deer, and Moen, Pastor, and Cohen fit these values into their model to estimate how much the animals must have paid for the growth of their incredible weapons. Not surprisingly, Irish elk antlers appear to have been impressively costly, half again as much as the antlers of moose or caribou, and demanding almost two and a half times the basal metabolic energy requirements each day to grow. The calcium and phosphorus demands were severe, and seasonal osteoporosis was likely especially dire in this species. The time when Irish elk disappear coincides precisely with a period of rapid climate change called “the Younger Dryas.” This would have lowered the quality of available food and made it even more difficult for males to replenish the calcium and phosphorus cost of their weapons.26

  “Irish elk” had the largest antlers of any deer, shown here alongside a fallow deer buck.

  During the Allerød period, Irish elk lived in tall willow-spruce forests where high-quality forage was relatively abundant. However, pollen records show a radical shift in plant species composition at the end of this period, as temperatures plummeted during the mini ice age of the Younger Dryas. Relatively suddenly, the elk populations would have found themselves in tundra habitats with far worse forage conditions. It’s possible the sudden decline in available diet exacerbated the material costs of male weapons by making it much more difficult, if not impossible, for males to replenish the calcium and phosphorus they borrowed from their skeletons each year to make the antlers. If true, then the extreme cost of male weapons may have contributed to the decline and eventual extinction of this species.

  In the end, only the largest, fittest, best-armed males prevail in the competition for reproduction. For the fallow deer in Phoenix Park, one male in ten managed to mate at all, and the vast majority of copulations (73 percent) went to just 3 percent of the bucks. Such extremes in reproductive success—90 percent failure rates and extraordinary success by just a very few individuals—lead to intense sexual selection, and much of this is directed toward bulk, stamina, and big weapons. For the very best males, the reproductive benefits of investing in elaborate weaponry more than offset all of the costs combined. For all of the rest of the males, however, such armament extremes can be cost-prohibitive.

  8. Reliable Signals

  Appreciating when and why arms races erupt helps us explain why some species have big weapons and others don’t—the “big-picture” patterns of animal diversity. But the science behind these races also provides insight into what happens within each of these species. Arms races unfold in the same basic way, proceeding through the same sequences of stages, in all species with extreme weapons. Parallel processes lead to shared properties, so much so that I could take information gleaned from weapons studied in one species, say a beetle, and use it to predict with frightful accuracy how weapons work in other species. Antlers in flies, forelegs in harlequin beetles, tusks in narwhals and elephants—these structures are alike in far more ways than simply being large. But to see this requires a shift in focus. Instead of considering variation in weapon size across species, we need to turn our focus inward, and look at variation between individuals in a single species.

  Pick any one of these heavily armed species, and look closely at the weapons as they are expressed from male to male. Here, within populations, lurks another pattern: not all males produce extreme weapons. Measure a sample of one hundred males, and you’ll find that most of the weapons aren’t all that big. Sure, some males strut with monstrous racks. Organizations such as the Boone and Crockett Club keep meticulous records of these super-stud bulls and bucks. But they do this precisely because such magnificent specimens are rare. The majority of bulls are not Boone and Crockett caliber. They produce weapons, but they’re middling.

  Even though a history of sexual selection has led to evolution of extravagant weapons in the species, only a very few individuals actually achieve full weapon splendor, and lots of the males produce pathetic renditions of the structure. If males with the biggest weapons win in every sense of the word—they win the fights, they get the girls, and they sire the offspring—then why don’t all of the males produce full-sized weapons? The answer is simple. They cannot afford them.

  * * *

  I could buy a forty-foot yacht if I wanted to. Okay, maybe I couldn’t, but I like to think I could come close. An Azimut 40S has elegant lines and a stylish profile; two 480 horsepower engines; a spacious living room, master bedroom, guest room, and galley; and, of course, the latest navigation equipment and software. But it also costs $400,000. That’s more than the value of my house and the fourteen acres of land it sits on. In principle, however, if I really wanted that boat, I could leverage my house against a loan, and for monthly payments twice my mortgage payments—basically, if I spent all of the income I had to live on and some of my retirement—I might be able to swing it. I wouldn’t be able to feed my kids for the next decade, or take my dogs to the vet, or go to the movies, or do anything other than make payments on my shiny new boat. I wouldn’t even be able to afford the gasoline I would need to drive it or a slip to park it in at the marina on Flathead Lake. But I’d have that boat, sparkling on a trailer in my driveway for all the neighbors to see.

  Ted Turner, founder of CNN, lives just two counties over. I haven’t met him yet, but I’d like to. I’m told he’s the second largest landowner in the United States, and I know for a fact he’s done incredible work restoring grasslands along the Rocky Mountain front, building a substantial herd of bison. Ted owns his whole county, and he could walk into a shop today and buy a forty-foot yacht in cash. He could easily buy two or three. But he’s unusual. The mean income in this part of Montana is only $37,000 per year. For most of us, the cost of an Azimut 40S is prohibitive.

  This may seem like Economics 101, but it illustrates a crucial point. Costs are not the same for everybody. Some of us pay a much steeper price for our toys than others. Technically, a 40S is a 40S. Ted Turner and I would each hand the dealer $400,000 for that yacht—exactly the same dollar amount. But the absolute value of the boat isn’t the only thing that matters here.

  Ted Turner and I aren’t starting with the same amount of resources, and the cost of a 40S depletes a much bigger chunk from my resources than it does from his. Relative to what we each have available to us, that boat costs me a whole lot more than it does him. The bottom line: those of us with fewer resources pay a steeper price for luxury items.

  When weapons evolve to extreme sizes, they get extremely expensive. They become big-ticket items, the yachts and Lamborghinis of the animal world. As a general rule, males produce the largest weapons that they can afford. But males differ in the relative sizes of the “pools” of resources they have at their disposal, and limited resources force the majority of males to produce substandard structures.

  Of course, humans, too, have varying access to resources. A few among us are born to money, with family wealth passed from parent to child. Groomed in private schools with access to the best tutors and doctors, these kids grow up with contacts in the job market, fast-tracking them to lucrative careers in the best firms and companies. Others are born to hard times, living in low-income housing or run-down apartments. These kids may be forced to work early so that they miss a college education. As a result, they’ll end up in low-paying jobs with little opportunity for advancement. Most of us fall somewhere in between, but as a population we differ widely in how much we have to spend.

  A rich person can buy a house outright, without having to borrow. The rest of us must take out big loans, paying tens or even hundreds of thousands of dollars in interest to the bank, adding substantially to the cost of our houses. Banks charge higher interest rates for people with low income or poor credit scores, so the poorest among us actually pay the most interest, raising the cost of their houses still more. The sticker price for the 40S might not be different, but if Ted Turner pays cash and I have to borrow, I’ll end up payin
g a much higher total for the yacht than he will. For example, if I borrowed at a rate of 5 percent interest, my total for a thirty-year loan would be $750,000, or $350,000 more than the list price of the boat! All of these factors exacerbate the differences among us, expanding the rift between haves and have-nots.

  Animals are born with different resource pools, too. Elk calves born to the biggest, best-fed parents start life with an edge. They weigh more at birth, and have more stored nutrients and stronger immune systems. They have access to the best environments, including the safest, least-stressful places and the best food. Other calves begin life under duress, with poor-quality parents in poor physiological condition. They’re born smaller and weaker, in substandard habitats. They grow more slowly and are quickly outpaced by the others. Because of their small stature they lose contests over food, weakening them still further, and the stress they experience increases the risk of infection. All of these experiences reinforce the differences in size that they started with, magnifying the gap between dominant and subordinate, large and small. Even subtle differences early in life compound as the animals grow, and by the time these calves reach adulthood they’ll differ hugely in available resources. Only a very few will be able to produce the biggest, most extravagant weapons.

  * * *

  The real reason I couldn’t afford that Azimut 40S is that not all of my assets are available for me to spend. Unless I’m grossly irresponsible to my family, I cannot go spending the equity in my house or my retirement account. Nor can I burn the cash I need to make my monthly mortgage and car payments, pay my taxes, or buy the food we need. Lots of my net worth is spoken for, and it’s really only the income I receive above and beyond my mandatory expenses—my discretionary funds—that is up for grabs. In principle, I can spend this extra money however I choose. The problem is that once my fixed expenses are accounted for, there isn’t very much left over, and my discretionary pool is far too paltry to spring for something as expensive as a forty-foot yacht. Discretionary pools differ from person to person far more radically than total resource pools do.

  Animals work the same way. They have mandatory expenses that have to be covered first. Energetic demands for basal metabolic functions need to be met: things such as keeping the heart pumping, muscles contracting, digestive tract digesting, and brain thinking. All of these core functions burn calories and use nutrients, exacting a price from the animal, and these expenses are nonnegotiable. Default on them, and the animal dies. Only surpluses of resources—the biological equivalent of discretionary spending—are available for running and fighting, or building big weapons. This is why weapons begin growing so much later than other body parts. Horns, antlers, claws, and tusks all stay small during the course of development, and start getting really big only as males near adulthood—after they’ve already built their bodies—ensuring that mandatory expenses get paid first.1 Only what’s left over, if there is anything left over, gets shunted into weapons.

  Weapons only get big after the rest of the body has grown.

  Weapons are discretionary in another respect, too. They are not necessary for survival. Females, for example, fare just fine without them in many species, as do small males.2 In stark contrast with the rest of the body, which must be built regardless, no weapon growth need occur at all. This means that the sizes of weapons should be far more sensitive to the availability of resources than other, mandatory body parts. My colleagues and I tested this a few years ago by perturbing the amount of food available to growing rhinoceros beetle larvae. By restricting access to nutrients, we experimentally altered the sizes of the resource pools available to developing males, allowing us to measure just how sensitive the different body parts were.

  Rhinoceros beetles feed on decomposing logs. We made an artificial diet for them by fermenting sawdust in giant composters, mixing in healthy doses of year-old maple leaves. After about a month, the substrate turned chocolate brown and smelled like a wooded streamside on a rainy day, just the way the beetles like it. These are big beetles—a typical larva is the size of a mouse—and for this experiment we placed half of the larvae in pint-sized jars filled with substrate. We placed the rest of the larvae in gallon-sized jars fully filled with substrate. The only difference between them was the amount of food to which each larva had access. When adult beetles emerged from the jars several months later, we collected and measured them, comparing males from the two diet treatments.

  Not surprisingly, nutrition had a pronounced impact on beetle growth. Genitalia in poorly fed males were 7 percent shorter than in well-fed males. Wings and legs each were about 20 percent smaller. Horns, however, differed by almost 60 percent, meaning horn growth was three times as sensitive to nutrition as wings and legs, and almost nine times as sensitive as genitalia.3

  All big weapons are exquisitely sensitive to nutrition. Like lottery winners upgrading to bigger houses, male beetles fed artificially supplemented diets grow into adults with bigger bodies and much longer horns. Remove the food, and you find the reverse. Well-fed male earwigs grow longer forceps than poorly fed males do,4 and well-fed flies grow longer eyestalks.5 The same holds true for deer antlers,6 elk antlers,7 and ibex horns.8 Food is like income to an animal, filling its coffers so that it can later spend. Males able to sequester surpluses of nutrient reserves have large discretionary pools, and they can afford to produce big weapons. Other males have fewer resources to start with. Mandatory expenses may claim everything these males have, leaving nothing left to spend on weapons.

  Same-age male elk and beetles differ in the relative sizes of their weapons, providing honest signals of fighting ability.

  Weapon growth is extrasensitive to illness, too, for exactly the same reason. Infections drain resources from the surplus pool. Males fighting infections as they develop cannot afford to pour as much into weapon growth. Parasites gnaw away at tissues, pathogens battle the immune system, and all of this gobbles up stored reserves. Weapons and other discretionary structures absorb the brunt of these losses. Antlers grow much smaller in sick males than they do in healthy ones,9 for example, as do Cape buffalo horns10 and fiddler crab claws.11

  Everything about weapons is expensive, from the resources pulled from the pool to permit their excessive growth, to the constant drain required to keep them, carry them, and use them in battle. Which is why weapon size is extrasensitive to the vagaries of life.

  * * *

  The biggest and best human weapons have always been exorbitantly pricey, unattainable to all but the richest few. During the Middle Ages, for example, the cost of a knight’s armory was extraordinary.12 The biggest cost of all was the opportunity cost. A knight had to be wealthy enough to never have to work. From the time a potential knight became a teenager, training to fight was a full-time occupation, often spanning a dozen or more years. The freedom to pursue this path simply wasn’t available to the overwhelming majority of young men in Europe at the time, since they were indebted tenants of local lords. Even among the aristocracy who could swing it, not all apprenticeships were equal: some could afford to train with better masters than others.13

  In combat a knight wore many layers of garments, each of them elaborate and expensive. Padded protection from shock was provided by an aketon, thick quilts of fibrous cloth filled with linen and horsehair. On top of this was a full jacket of mail, linked iron rings densely arrayed in fine overlapping rows designed to dull the slash of a blade. The best mail was tailored to the individual knight, so that it fit snugly around all joints without impeding movement. On top of this sat the armor proper, hinged plates hammered by metalsmiths into shapes that wrapped around shoulders, elbows, arms, and legs, as well as the chest and head.14

  The quality of armor varied tremendously. The wealthiest knights had their own armorers who custom-shaped plates of the highest quality so that they fit perfectly. Others resorted to purchasing armor from local businesses. These suits were less expensive, but they were mass produced and “one size fits all.” The poor fi
ts of cheaper suits meant that they chafed when knights marched or rode into battle, and they restricted movement.15 Finally, on top of the armor, knights wore elaborate and colorful tunics emblazoned with coats of arms or other identifying symbols, and the best of these were beautifully tailored and pricey.

  Knights needed to purchase lances—lots of lances, since they shattered on impact during jousts—as well as pikes, swords, daggers, maces, and shields.16 They also needed horses. A knight’s warhorse was the most important, and by far the most expensive, tool of the lot. The best warhorses were bred for the task, tall, strong, fast, and reliable. The rarest and most highly prized were called “destriers.”17 These animals were trained from an early age to respond instantly to the slightest commands, and to walk in a perfectly straight line. They had to hold course despite extraordinary distractions: screams and howls of battle and foot soldiers rushing them swinging axes and clubs. A horse that balked at the moment of battle was a dangerous liability. Breeding and training the best horses cost a fortune, and the wealthiest knights had three or four in their retinue.

  Warhorses had to be equipped with armor, which cost even more than the knight’s armor did, since there was so much more of it. Padded underlayers, mail, plate armor, and extravagant outer tunics were the norm. Here, again, the best armor was custom-built for each animal, so that it didn’t chafe or impede movement.

  Knights spent months of the year away on campaigns, and they brought with them fancy tents, trunks with clothing and gear, carpets, kitchens, cookware, and furniture. Strings of packhorses carted the freight, and additional horses carried squires, apprentices, servants, and cooks.18 Everything from the breed of the horses to the styles and sizes of tents, clothing, armor, and entourages separated the best from everyone else. Like the horns of beetles and the antlers of elk, the quality and extravagance of a knight’s shining armor revealed his status and wealth, and, because training, ease of movement, and protection all matched the price of his armor, a knight’s appearance also revealed his fighting strength.