1. Bull to Bull and Cow to Bull

  The sky really is bigger in Montana—a colossal inverted bowl of vivid blue. In late July and early August, plumes of dust, rising with earth-warmed air from the brown grass and rolling rangeland, ascend into that bowl. The dust makers, a herd of bison on the National Bison Range, are going about their business—breeding; and I am going about mine—observing, recording, and trying to understand their behavior.

Most of the dust comes from wallows, shallow pits where the bison have torn away the sod with their horns and where the subsoil, dried by the sun and stirred by hooves and horns, turns to a flourlike dust. Some of the plumes start when threatening bulls paw and roll in these wallows; but most occur when fighting bulls plow the soil with their hooves, or when they slam their heads together and the shock explodes dust from their bodies.

Now an old bull bellows. His back arches, his belly lifts, his neck extends, and a sound that seems equal parts lion's roar and thunderclap booms across the grass. An eighteen-inch scar runs up his ribs. His horn tips, shattered in other battles, are blunted and worn. Fifty yards away his opponent, a six-year-old bull in his prime, bellows back, glances at the cow he is tending, then urinates into the dust of a wallow and rolls in it, slamming his 2,000 pounds sideways into the dust. It spurts from beneath him, filling the air around him like the burst of smoke a stage magician vanishes into.

The prime bull emerges from this cloud, headed toward the old bull in a menacing walk. His forefeet stamp with each step, making the hair pantaloons on his legs dance and exploding little puffs of dust from his coat. As each front foot stamps, the bull snorts. His tail stands up like a living question mark. It's an impressive display, and from where I sit, in an ancient jeep, an intimidating one. But the old bull is not intimidated. He too has wallowed and now advances, matching stamp with stamp, snort with snort. As they grow closer their bellows intensify; they seem to signify pure fury.

Most such challenges seem to be elaborate tests of the opponent's determination and end without a fight. Most fights involve a cautious locking of horns or hooking uppercuts or shoving head to head, ended when one animal signals submission and the winner lets him go. But this is not a test of determination and it's a different kind of fight—one of those in which the bulls hurl themselves at each other, elongating their bulky bodies into animated battering rams as they launch themselves for the first blow. Their heads come together with a terrific shock. It ripples through their bodies in a visible wave. I once saw a bull somersaulted backward by such a charge: 2,000 pounds of bull flipped upside down like a lawn chair in a gust of wind.

Both these bulls hold position after the first shock and dig in for a serious fight. They slam their heads together again. Clumps of hair the size of a fist are caught between their short, heavy, curved horns, then sheared off and tossed into the air. The animals circle, each trying to reach the other's flank with a hooking horn. Both pivot around their forefeet with the speed of featherweight boxers, and each parries the other's seeking horns with his own while their powerful necks absorb some of the force of the impact. Their hair absorbs some too. By the time a bull is six years old, a mat of hair several inches thick extends from the top of his head down across his forehead, thinning gradually until it stops just above his muzzle. His eyes peer from shallow wells, his ears flick out from deep recesses, and the space between his horns is completely filled with this luxuriant growth. Beneath this natural shock absorber, a thick layer of tough hide covers a rock-solid skull.

Now the bulls lock horns and push hard, their hooves plowing soil as each tries to drive his opponent back. The old bull is pushed back and a little sideways, dust spurting from beneath his skidding feet. Suddenly a foot catches on a rock and he trips and falls onto his side.

It is rare for a helpless bull to be attacked by the winner, but this time it happens. The younger bull strikes down and forward with his horns, slamming them into the old bull's flank and hooking right and left. The curves of his horns make most of the contact and deliver bruising, possibly rib-breaking, but not fatal blows. Then the tip of one horn plunges through skin and muscles and into his opponent's abdomen. Only one horn penetrates, and it penetrates only once, but the wound will be mortal.

The younger bull ends his attack and returns to his cow. In a few minutes the old bull will rise to walk away. He will graze again, drink again, sleep again. But an infection will send matter oozing down his ribs in a few days, and in a few weeks it will kill him.

Yet as one life starts to ebb, another begins. The victorious bull has mated with the cow he was defending and a calf has begun to form. Its birth is the renewal that has made North America bison country for thousands of generations. Siring as many as he can of that calf's generation is the bottom line for each bull, and it's the imperative that justifies their risking their lives in the battles their bodies are built for.

But while physical prowess is an essential tool in managing a relationship between two males, it can't be the only tool, and in fact it's one of the least-often used. Much more frequently they use communication, which evolved as a means not to transfer information but to do what attacks do—manage another individual's behavior to one's own benefit. In some relationships honesty is the best policy, and communicating animals transfer real information. A mother hen calling "food here" to her chicks is managing their behavior to their benefit as well as hers by telling the truth. They get food, and she gets her genes represented in the next generation. On the other hand, young men in a hormone-induced haze who exclaim, "Of course I love you!" while fumbling with a woman's bra are often lying through their teeth. But both the hen and the men are using signals in fundamentally the same way—to modify another's behavior for their own benefit.

It starts with anticipation. A bull's defenses work only if he knows when and where to deploy them—he must anticipate attacks. So the bull must be a seeker—actively scanning or even probing his environment for clues about whether or not he is likely to be attacked. Call it actively anticipating.

A territorial animal can predict attack pretty successfully by knowing territorial boundaries. The territory owner usually challenges all competitors within a given space and keeps up the pressure with threats and attacks until they leave. But bull bison aren't territorial. They are roamers, drifting singly or in small, temporary groups. Because they cannot use their location in space to predict whether or not another animal will attack them, they read the animals around them, detecting and responding to behavior that consistently precedes an attack. Reading it accurately is a second tool for managing relationships.

It's clearly to the advantage of an animal about to be attacked to become canny in judging his enemy's behavior. Generally the task is made easier by the opponent, who, instead of disguising the coming attack, often amplifies preattack behaviors, draws attention to them, and in every way makes it easy to see what he is about to do. A bull doesn't just walk toward his opponent: he stamps with each step, setting his foreleg pantaloons dancing, and grunting with each stamp. If forewarned is forearmed, why not attack first and give indication later? The reason, of course, is that it may not be necessary to attack at all. Forewarned may be foredefeated—at least often enough to make the warning worthwhile. Call these forewarnings threats. A fight avoided is also risk and energy expenditure avoided. Fighting is an occasionally necessary grand spectacle, but the real biological drama lies in the complex, drawn-out, and frequently subtle ways in which most conflicts are settled by communication.

Bulls do most of their communicating during the breeding season—the only time during the year that mature bulls and cows are together for any length of time. The bulls have been alone or in small, temporary groups. Now they join the cows, which have been living in larger groups with the calves and young bulls. The bulls seek out cows about to breed and stay with them (they "tend" them), keeping other bulls away by threatening and fighting.

But threatening and fighting are also common between bulls that are not defending cows. Since receptive cows are the only scarce resource in the bull's economy, this seems surprising at first: one wonders what the nontending bulls are fighting about. But a rival dominated now will probably give way later without a contest, saving a tending bull time and energy when he has none to spare. Not that the bull works it all out in this fashion. He simply has a powerful urge to dominate other bulls, and following this impulse works to his advantage. The drive to dominate is so powerful that it occasionally interferes with his real business and its ultimate function—bulls will sometimes leave a receptive cow to threaten a distant bull.

Virtually all of us air-breathing vertebrates have found ways to turn our exhalations into communications, and most interactions between bulls start with a sound. On a still day a bull's bellow carries for miles. It's a sort of roaring rumble, and if you can't see the bull or don't recognize the sound you may guess it's a thunderstorm. If the competition presses, the bellowing becomes louder, and a quality that is hard to define but somehow easy to recognize—a quality of fury—begins to grow in it. Often one or both bulls will interrupt their bellowing to paw the ground or wallow.

Threatening bulls usually do something we don't understand at all: they urinate into a wallow, then roll in the dampened soil. Do they get enough urine on themselves to send a signal? If so, what could they be signaling? Could they be exposing the opposition to an index of their testosterone level as salient to a bull bison's keen nose as their bulk presented broadside is to his eyes? Or is there another chemical billboard being displayed? As a bull uses up his physical resources during the rut, he eventually begins to metabolize his muscle. The metabolites in his urine will report that chemistry to a sophisticated nose, and the same nose will know when the bull is still burning fat and his muscle is intact. This would be an honest signal of physical condition. The cost of signaling weakness when you are weak would be compensated for by the benefit of signaling strength when you are strong. An intriguing story, but so far 100 percent speculation. Maybe someday we'll know.

If the challenge does not end at the wallowing or bellowing stage, the bulls draw closer to each other and begin to posture. There seem to be two distinct postures. In the "head-on threat," which is simply the posture and movement that precedes a charge, the bull moves toward his opponent with his head held slightly to one side. The more slowly the challengers are moving, the farther to the side their heads are held. When they approach nearly straight on, either one bull submits by turning away or they bang heads. But when they approach slowly with their heads well to one side, they often stop close to, but not quite touching, each other and "nod-threat."

Nod-threatening bulls stand close enough to reach one another; their bodies may form a single straight line or an angle of up to ninety degrees, but in either case they turn their heads aside. From this position they can attack suddenly by hooking a horn into the opponent's head. The hook always starts when the head is close to the ground, the muzzle tucked back. But in the threat itself, the head-low, muzzle-back position is only a brief interruption of a head-high stance: the bulls' heads drop in a matched movement, then swing back up again, still to one side. A hooking attack may start at the bottom of any one of the down swings, but the opponent never seems to be caught off guard. After a series of such nods one animal may suddenly submit, ending the clash.

Nod-threatening takes place most often between bulls that are not tending cows, as does the "broadside threat." A bull in this posture keeps himself broadside to his opponent with his head held a little higher than normal. Usually his back is arched and he is bellowing. If he moves, he does so slowly, in short, stiff steps that keep him broadside to his opponent. Often two bulls will threaten by standing parallel to each other just a few feet apart. Only rarely does this threat lead to a fight. The encounter may be long as threats go, lasting up to a minute or more, but one of the animals almost always submits.

The broadside threat and the nod threat emphasize the degree to which the bulls forewarn their opponents. This forewarning is so elaborate that it has become a force in its own right. It goes beyond permitting the prediction of attack: by substituting for attack, it often overpowers the opponent.

That function may account for some puzzling aspects of these postures. Why, for example, do the bulls threaten by turning broadside? When turned this way, a bull seems very vulnerable to attack, particularly if the bull he is threatening is facing him. Since all his protection is concentrated at his head, the usual point of attack, a bison bull is easily wounded by a horn thrust from the side. (This danger, by the way, is more apparent than real. In watching thousands of such threats I have seen only one attack on a bull turned broadside.) Perhaps the function of the broadside threat is to display the full size and power of the bull, as well as to forewarn the opponent. If the threatening bull makes a big enough impression, he may save himself a fight.

The one recurrent note in all these descriptions of fighting and threatening is that they go on until one animal submits. Submission signals have two functions: they enable a bull to withdraw from an encounter without getting into a fight, and they enable a losing bull to end a fight without retreating a long way. There are two questions to ask about bison communicating submission: How do they signal it? Why do the winners accept it?

All bison submission signals are variations on a theme: the submitting bull turns away. Sometimes it's a 180-degree turn followed by a galloping retreat. At other times it's an abbreviated swing of the head and neck to one side. When it involves a 90-degree turn, the submitting animal ends up in the same general position as one who is threatening broadside. But it's easy to tell the difference. In submission the bull's head is usually low, muzzle extended as if to graze—and sometimes he does graze—and the bull is silent. Whatever form the submission signal takes, it almost always stops the threats or attack immediately.

One day two bulls dramatically demonstrated the power of this signal. They were fighting in a swale below me. The low ground was moist, so the grass was green even in early August. The spurting dust raised by most fights was missing, and the rich contrast between the warm brown of the bulls' coats and the green grass gave the scene a certain tranquillity. But the bulls were fighting in earnest. They slammed their heads together, stepped back a few feet, then drove their foreheads together again so hard that the shock of the impact seemed to pass visibly through their bodies. After three or four such blows, just as they had drawn back and were poised to plunge together again, one of the bulls simply stood in his tracks and swung his head ninety degrees to the right. The winner had already started his forward lunge. His front feet plowed sudden dark furrows in the green grass as he skidded to a stop. His horns could not have been more than eighteen inches from the loser's neck. The two animals stood immobile for a few seconds; then both walked quietly away.

Fights usually stop just that abruptly with the loser just that vulnerable. One more step, one more lunge by the winner, and the loser would be out. But that step is almost never taken.

Why does an act of submission change the winner's behavior so profoundly? One is tempted to explain it by analogy to human institutions, to say that bison operate by a set of "rules." Thus the loser is kept from harm by the winner's acquiescence to rules, just as a football player who has been knocked off his feet is protected from further assault by the rules of football. But this kind of analogy between social behavior and social convention obscures rather than clarifies. The rules of football are a social invention based on enlightened self-interest and reciprocity. Players agree to be restrained from some destructive acts, provided that the opposition is similarly restrained. Kicking an opponent in the head when he is down is so dangerous that everyone agrees that it should draw a penalty that gives the other team some advantage in play. This penalty imposed by a specialized group of rule enforcers is the mechanism through which rules control the football players' behavior.

There are, however, no reciprocal agreements and no rule enforcers among bison. Each bull's behavior meets his own needs and no other bull's. The only penalties for any action are those assessed by the action itself. At first this seems wrong. How would a winning bull penalize himself by polishing off a loser? In fact, he would be deprived of two precious commodities: time and energy. The breeding season, when most conflicts take place, is limited, and time spent fighting, even a mop-up operation, is time lost from breeding.

Fights to the finish would take even more energy, and that's in shorter supply than you might imagine. When you see bulls in the middle of summer, in the midst of tall grass and warm sunshine, their good health and nutrition seem assured. But bison are northern animals, one of the most northern of the cattle family. They have adapted to a climate where food is scarce through long winter months. Bulls die during the winter if fall catches them without enough energy stored as body fat. As it is, the breeding season takes a lot of energy. Mature males lose an average of 200 pounds between June and October. If every fight were long and rough and ended in a cross-country chase, bison bulls, winners and losers alike, might well die before spring renewed the plains.

Pursuing and destroying the loser would eliminate the need ever to face him again, of course, but that would accomplish little. Bison breed in large groups, with the males moving constantly from one group to another. There are always many more challengers where the last came from. In the final analysis, the winner does not spare his defeated rivals, he spares himself.

The prolonged forewarnings, the reluctance to fight, and the generosity to losers are neither the last noble vestiges of chivalry in our time nor nature's way of exhorting humans to live on a higher ethical plane. Rather, they are carefully balanced behavioral adjustments to the social and ecological circumstances in which the competition between bulls evolved.

  Older and Bolder

Picture this: two men, strangers, both sober, are quarreling about which will buy a drink for a young woman who sits on a stool watching. One is old enough to be subjecting strangers to baby pictures of his first grandchild; the other is just old enough to drink legally. One of the men is pushy—escalating from words toward violence; the other is cautious and restrained. Now, if you can, picture this—it's the old guy who is escalating—shoulders hunched, on the balls of his feet, fists clenched, movements abrupt, voice loud and angry, while the young guy is back on his heels, hands held up and open before him, trying to avoid a fight.

There's a lot wrong with this picture. It's a rare young man who gets bolder as he gets older. But the ecologists Chris Maher and John Byers have shown this to be the rule in bison. The older they get, the more likely they are to take the risks of combat in order to tend a breedable cow—not because older mature bulls are more likely to win than younger mature bulls, but because they have less to lose. Thereby hangs an intriguing budgeting issue, which might be titled "How to spend your life the way that buys the most descendants." It's an investment program in which your life is your capital and the return is your offspring's offspring. For male bison, producing offspring usually involves conflict. Each such conflict puts their lives at risk. A bull bison's optimal investment strategy weighs possible losses against possible gains and decides how much risk is prudent. He's balancing making a killing against getting killed. Now, a male is not going to live forever, so an optimal strategy must be age sensitive. The younger he is, the more time and future opportunities he stands to lose if he dies. So he should invest cautiously when he's young and has more to lose, but more and more boldly as he gets older and has less to lose. Bison behavior tracks this straightforward logic—the old bulls are the bold bulls.

But a man finds such budgeting more complicated. For the bull, getting his sperm on the way to the egg is the end of his investment. For a man, it's the beginning. Until the offspring reproduce, the father's investment has not paid off. Human babies don't stand on their own two feet within ten minutes of being born, don't run in two hours, aren't weaned at six months. Children must be fed and protected for years, and fathers can help, thus investing more in each child. As the investment in children grows, so does their value. Once he has children, staying alive to support their development and eventual reproduction and maybe even that of their children's children is a good alternative to risking his life to produce another child he may not live long enough to rear.

Small wonder grandfathers would rather dote than fight, and are more likely to risk their lives to preserve their grandchildren than to win wooing rights to a woman, no matter how fecund. This attention to one's young is called nepotism. Bull bison never engage in it, but human males, whose genetic survival such lofty detachment would threaten, engage in it all the time—occasionally despite a whole body of civil service law invented to thwart their urge to do so.

Not only is life a series of trade-offs, but life itself is one of the things that gets traded off. The trade-offs that natural selection selects usually bring all the costs and benefits to bear and therefore favor very different strategies in different kinds of animals. Different strokes work better for different folks.

  Cow-Bull Relationships

Bison bulls, and many bison watchers, are preoccupied with bulls' relationships to other bulls and with the tools—spectacular and subtle—they use to manage their relationships. But such tools are just the means to a more important end, an essential and showy precursor to what matters most: making calves.

Many living things reproduce with neither preliminaries nor fanfare. They just split in two, grow a bud that becomes a new entity, send up a shoot from a root whenever and wherever. They don't need any help, or even cooperation, from another individual. But apart from a few parthenogenic species of fish and lizards, we vertebrates are stuck with sex. That means, at a minimum, two members of the species being at the same place at the same time so eggs can attract sperm and get fertilized in a fertile egg-friendly environment. For some fish the water around the coral reef where they live is plenty good enough. One to hundreds of females get close to one to hundreds of males, each one of each sex releases his or her gametes at the same time, and eggs and sperm find each other and fuse.

But for mammals the equivalent of warm seawater is in a female's uterus. The eggs stay there and the sperm have to journey to join them. This journey requires some seriously specialized equipment and some seriously intimate contact. Where this contact occurs isn't very important, but when and with whom matter a lot. Cows, unlike coral reef fish, release one egg at a time. To get that egg fertilized right they need to attract a suitable bull and become willing to mate with him. Neither of these things is everyday activity for a cow, so she requires substantial changes—changes that make her more attractive to bulls and, eventually, changes that make her willing to breed with one of them.

Back to bulls for a moment. The way to a bull's enthusiastic attendance is through his vomernasal organ, a sensory organ found in many mammals; it has an opening in the roof of the mouth. Cows' urine is full of facts about how near ovulation is. The bull's vomernasal organ seems specialized for an analysis of female urine chemistry that provides information on when a female will be ready to breed. But while bulls will joust seriously to get some female urine on their vomernasal organ, I've never noticed that females go out of their way to present it. However, I have many times seen bulls during the rut bring a resting cow to her feet by prodding her belly gently but firmly with a horn. The cows arise, with what I take to be resignation, and often urinate in a minute or two. The bull thrusts his muzzle into the stream of urine, then elevates his head, upper lip curled, tongue fluttering inside his mouth, his whole demeanor suggesting a gourmet's appreciation of a fine wine. If he goes from lip-curling to tending, the chances are good that the cow will breed sometime that day.

There are two phases to a cow's getting-pregnant physiology: she must ovulate and she must become willing to breed—enter estrus. Each phase is the end point of a complex sequence of hormonal events, and chemical traces of these events appear in the cow's urine. Since each sequence takes several days, chemical signposts show up in the cow's urine days in advance. We call the several hours before the cow breeds pre-estrus, and bulls that test the urine of pre-estrous cows are likely to try to spend the next few hours with her. That's not easy to do. Pre-estrous cows become restless—breaking away from a tending bull and running through the herd. A running cow attracts bulls, and a string of them are soon following her just as a tail follows its comet. When she stops they gather and quickly sort out who among the present company gets to stand by his cow. The cow's best shot at having many grandchildren is to have sons that can claim a cow just as this bull claimed her. If we assume "like father, like son," he is the best candidate in the immediate circle—but the cow may well make him prove it again with another run through the herd.

I always interpreted cows' runs as simply what they did at a particular stage of estrus. The zoologist Jerry Wolff thought otherwise, and he gathered data showing that the lower the tending bull's rank, the more likely it was that the tended cow would run. In addition, cows that ran usually ended up with a bull that ranked higher than the one they ran from.

It's just possible the urine left by wallowing bulls plays a part in the cows' physiological progress toward ovulation. All bison wallow and frequently sniff at a wallow before rolling in it. The smell of male urine could stimulate ovulation and a breeding frame of mind, as it may do in moose. Of course a cow can ovulate on her own, but not all ovulations are equal. If you're a calf born on an open grassland roamed by predators, the best cover you have is other newborn calves, who may fill the predators' bellies before they get to you. The biologist Richard Estes has shown that wildebeest calves born at the height of the calving season, when the African savanna's predators are relatively satiated, are the least likely to be eaten. So using a clue, any clue, that other cows are relying on to time ovulation can improve a particular cow's chances of having grandchildren.

We don't know if bull urine helps synchronize cow ovulation but we do know that cow urine launches bull-cow relationships. A pre-estrous cow's chemistry attracts one-year-old bulls as powerfully as the ten-year-olds, and they all vie to tend a pre-estrous cow.

Tending pairs are unveiled as the movement of a grazing herd leaves them behind. Study a pair and you will see the cow grazing a bit, looking fretfully toward the increasingly distant herd. A big bull stands beside her, moving to block her when she sets off after the departing herd. He moves like a basketball player staying between a dribbling point guard and the goal. Sometimes she allows him to hold her in place, but "allows" is the operative word. His moves to block her are quick and graceful, but she is still quicker and more graceful. If she stays it's because she has chosen—or at least settled for—him, not because he has chosen her.

She may head straight for another tending bull. When Jerry Wolff compared the ranks of the bulls left to those approached, he found the cows were usually approaching a higher ranking bull. That's one of the forms of choice a cow has, and it makes sense for her to be as choosy as she can manage to be. She will have just one descendant in the next year, and only half its genes will be hers; she should be as fussy as possible about where the others come from. But the bulls usually severely limit the cows' choice. On any given day there are fewer ready-to-breed cows than eager-to-breed bulls in a herd with a natural sex ratio—that is, nearly as many bulls as cows. The bulls use their bull-bull relationships and social tools to allocate this scarce resource. But the cows are not simply passive. Cows seem to be more receptive to older bulls, and that makes sense; winters, battles, disease, and predators have tested them.

We don't yet know what cues besides age the cow may use in making her choice. Could all that bellowing make a difference as to which bull is standing beside her when she stops running? Could it work like some birdsongs or frog croaks—a clue to the female about who might be a better mate? The bulls don't seem to be sending a signal—they appear only to bellow to other bulls. They seldom bellow unless they already have a cow, are trying to displace a bull that has one, or are in the midst of a dominance contest.

Even so, the cows can't help but hear them, and I have seen them react. One day I watched a closely tended but resistant cow standing quietly beside the tending bull. Her tail was clamped firmly over her vulva, she chewed her cud, her ears lay passively back, and she jumped away from the tending male's attempts to mount. He was bellowing and glowering at a half dozen bulls standing in a semicircle around the tending pair. It was an all-around stalemate. Then another bull broke it. He walked in from directly behind the tending pair, and when he had closed to forty feet he bellowed. The effect was electric. The tending bull left without a backward glance, hurrying to join the semicircle, and the cow, without a backward glance, lifted her head and tail and flicked her ears forward.

The new bull continued past the cow, stalking stiffly around the semicircle while the bulls forming it ducked their heads down and away as he passed each of them. As he presented his right side to the semicircle of bulls the cow pressed against his left side, half mounting him every few steps. When he stopped after passing the last deferent bull the cow stepped directly in front of him, lifted her tail and braced herself. He mounted immediately and four or five seconds later her breeding season was successfully concluded.

However important bellows may be to a resistant cow, her priorities and behavior change as ovulation approaches. The time of choosing is a period of conflict between the cow and the many courting bulls. The more competitors, the better for her but the worse for him. His earlier behavior minimized the number, hers maximized it. But now their interests coincide, and they must cooperate and collaborate.

Often the cow redirects the relationship. Her physiology is changing fast, altering her behavior along with it. Now, instead of breaking into a gallop every time the bull is distracted by a challenger, she follows him, and when he has disposed of the distraction she stands close, perhaps even positions herself in front of him. If he continues to glower round at the competition instead of mounting her, she may announce her readiness to breed by licking him or by sticking a horn in his ribs and prying upward—extracting from the bull a grunt, a tuft of hair, and more attention. She may even mount him, and when he begins to mount her she no longer squirts forward like a stepped-on bar of wet soap, but plants her feet and moves her tail to one side. Even so he may half mount, then drop off several times before he catches on and copulates.

Bison sex does not involve a lingering mingling of mucous membranes. He clamps his forelegs around her ribs and penetrates with a lunge. The bull almost always ejaculates within five seconds of intromission (I timed it from movies of the event). His last pelvic thrust is driven home by a contraction of his abdominal muscles so strong that it jerks his hind feet forward and completely clear of the ground, making the 1,100-pound cow's hindquarters suddenly support an extra ton of buffalo. Brief though the encounter is, it's usually enough for the cow. She staggers under his weight, not infrequently limps for a while afterward, and four times out of five rejects further attempts by this or any other bull to mount her again for the coming twelve months.

What a difference five seconds can make! In five seconds the cow is transformed from eager to unavailable. In just five seconds she has gotten everything of value the bull has to offer for the coming year. Only one cow in five—almost always a cow that bred near the end of the breeding season—will stand for another mounting during one breeding season. In fact one is enough. In a well-nourished herd, 85 to 90 percent of the mature cows will bear a calf in the spring.

Though she moves a bit gingerly—back arched, tail extended, sometimes limping—she is soon grazing again. Her breeding program is completed, but not the bull's. His season is in stride, and its success depends on two things—being the only bull to fertilize each of the cows he couples with and coupling with as many cows as possible. Just now a dilemma has him on its horns. This cow could be one of the 20 percent that accept another mount. If she is and he has left her to search elsewhere for a fertile field to plow, her second coupling will be with another bull whose sperm will compete with his for the one egg headed for the cow's uterus. If he stays with her, he will be the bull at the second standing and all the sperm will be his. He can ensure this by staying with her (or, as behavioral ecologists say, "sequestering" her). But the longer he stays, the more likely the other cows' dance cards will already be full when he comes courting. Breeding season moves at a spanking pace. I've seen half the cows breed in the peak four days. To be here or to be there, that is the question. The answer is, play the odds. The odds are that a cow that breeds a second time will do so within forty-five minutes of the first encounter. Most bulls play the odds, sequestering the cow for about forty-five minutes, then moving on, looking for a new relationship.

It's enormously taxing. While breeding, bulls lose 10 to 15 percent of their body weight—mostly fat they will dearly miss in the coming winter. But the potential rewards are also enormous. The winning bulls win big. One year I saw one bull breed five cows while others bred none—one-third of the bulls sired two-thirds of the coming spring's calves. Over three years Jerry Wolff saw one bull breed sixteen cows, while another never bred. The biologists Joel Berger and Carol Cunningham followed a herd for four years and saw one bull breed twenty-eight times while others never bred.

Breeding season ends with neither a bang nor a whimper, just a fairly rapid decline in the number of tending pairs. And more and more of the bulls drift away to concentrate on providing their complicated digestive system with the fodder it will convert to fat to carry them through the winter.

Their interactions change from constant confrontation to nearly invariable tolerance or passive avoidance. They're not looking for any trouble, and, in the two months following the rut, they come to look a lot less like trouble. The magnificent, menacing mass of hair on their forehead and between their horns, the flowing beard, and the dancing pantaloons that gave advancing bulls such presence are gone. Much of the hair between their horns was barbered away—caught between rubbing horns and sheared off during fights. But the rest of that hair, the beard and the pantaloons, simply falls out after the rut ends and before winter starts. Only mature bulls molt this way, not cows and not even young bulls.

Perhaps the hair loss is triggered by the stress of the rut—and perhaps instead, or in addition, it de-escalates the tension between the bulls: each benefits by looking less big-male threatening and thus less likely to provoke other mature males to challenge him, a management tool worth having when your goal is to graze as much and exercise as little as possible. Then, as winter wanes, the beard, the pantaloons, and the hair between the horns regrows and the bull again wears his special combination of parade ground display and battlefield combat dress. He will again be able to deliver or to survive a full gallop charge squarely on his forehead, but still won't if there is any other way to manage his fellow bulls.

  Bulls in Spring

The midsummer breeding season is full of sound, fury, dust, and danger. But it would be suicidal to try to keep that up all year round. Bull-bull relationships in the other seasons are comparatively understated. In spring the bulls are hurrying to get ready for summer and winter, but they hurry, in good part, by taking it easy.

Two old bulls are lying down, resting and ruminating, about thirty feet apart. The nearest other bison are at least a mile away. One bull rises, stretches, and stands, and in a few minutes the other does too. The first to rise approaches the second. At a distance of fifteen feet, a little tension develops. Their bodies stiffen slightly, and their tails, which had been discouraging flies by flicking from side to side, become still. The bulls look carefully at each other, then slowly relax. They lick their own noses, sending their tongues into first one broad nostril, then the other. They amble off companionably for a hundred yards, grazing as they go, then lie down again.

In the summer, in the breeding season, they will be competitors. They may be locked in combat, even mortal combat. But this is no time to quarrel. This is a time to put on fat against the demands of the breeding season in summer and of the cold winter that will soon follow. Not all animals, not even all hoofed animals, must obey this imperative to get fat once a year. Neither the African buffalo nor the Asian buffalo ever get very fat. The seventy species of African antelope don't either, but they don't have to contend with winter. Most animals that do, like the bison, live according to a "fat economy."

This strategy simply means that the animal builds up (saves) a store of fat when food is abundant, then lives off it when food is scarce. Both black and grizzly bears are archetypal fat economy animals; they pile up fat during a few frantic months of gluttony, then hibernate during the winter, resting cool and even comatose while life is sustained by a store of fat melting slowly like a candle's wax. Storing and using fat is somewhat inefficient. Converting digested food to fat uses some of the food's energy, and converting it back to usable energy requires still more. But what the process lacks in efficiency it makes up in reliability. It makes it possible to survive lean times like the hard winters that always lie ahead of bison.

Spring is the time of year for fat economy animals to eat. Not only are their fat stores depleted, but the chance of replacing them is best then. Fat is stored energy, and for most living things energy ultimately comes from the sun. Plants convert the sun's energy into a form plant eaters can use. Though plants are stationary, they too are in a race, a race to have the most surviving seeds or stolons. And so they grow aggressively, sometimes with their greater size depriving their neighbors of sun or water. To the herbivores these weapons are the means to get fat. In the spring plants are not only abundant but also especially nutritious. Growing grass and the flowers, seeds, and other reproductive organs of plants are rich in protein, a critical nutrient that is generally scarce in grass. So now, as the energy from the sun increases day by day, primary production surges and the bison can get fat.

And so today the two old bulls move on, grazing, resting, ruminating, and grazing again. Tomorrow both may be all alone and miles apart, or one or both may have joined three or four others; but wherever they are and whoever their companions, they will be grazing, resting, and ruminating: making fat while the sun shines. Their slow pace and laid-back social behavior are their way of hurrying, the fastest way to their goal—fat stores.