Craig Packer was behind the wheel when we came across the massive cat slumped in the shade beneath a spiny tree. It was a dark-maned male, elaborately sprawled, as if it had fallen from a great height. Its sides heaved with shallow pants. Packer, a University of Minnesota ecologist and the world’s leading lion expert, spun the wheel of the Land Rover and drove straight toward the animal. He pointed out the lion’s scraped elbow and a nasty puncture wound on its side. Its mane was full of leaves. From a distance it looked like a deposed lord, grand and pitiable.

Since arriving in Tanzania’s Serengeti National Park only that morning, I’d gaped at wildebeests on parade, dawdling baboons, gazelles rocketing by, oxpecker birds hitching rides atop Cape buffaloes, hippos with bubblegum-colored underbellies. The Serengeti usually dazzles first-time visitors, Packer had warned, making us giddy with an abundance of idyllic wildlife straight out of a Disney song-and-dance number.

The sublime brute only 15 feet away was my first wild Panthera leo. Male African lions can be ten feet long and weigh 400 pounds or more, and this one appeared to be pushing the limits of its species. I was glad to be inside a truck.

Packer, though, opened the door and hopped out. He snatched a stone and tossed it in the big male’s direction.

The lion raised its head. Its handsome face was raked with claw marks.

Packer threw another stone. Unimpressed, the lion briefly turned its back, showing hindquarters as smooth as cast bronze. The beast yawned and, nestling its tremendous head on its paws, shifted its gaze to us for the first time. Its eyes were yellow and cold like new doubloons.

This was one of The Killers.

Packer, 59, is tall, skinny and sharply angular, like a Serengeti thorn tree. He has spent a good chunk of his life at the park’s Lion House, a concrete, fortress-like structure that includes an office, kitchen and three bedrooms. It is furnished with a faux leopard-skin couch and supplied only sporadically with electricity (the researchers turn it off during the day to save energy) and fresh water (elephants dug up the pipelines years ago). Packer has been running the Serengeti Lion Project for 31 of its 43 years. It is the most extensive carnivore study ever conducted.

He has persisted through cholera outbreaks, bouts of malaria and a 1994 canine distemper epidemic that killed off a third of the 300 lions he’d been following. He has collected lion blood, milk, feces and semen. He has honed his distressed wildebeest calf call to get his subjects’ attention. He has learned to lob a defrosted ox heart full of medicine toward a hungry lion for a study of intestinal parasites. And he has braved the boredom of studying a creature that slumbers roughly 20 hours a day and has a face as inscrutable as a sphinx’s.

Packer’s reward has been an epic kind of science, a detailed chronicle of the lives and doings of generations of prides: the Plains Pride, the Lost Girls 2, the Transect Truants. Over the decades there have been plagues, births, invasions, feuds and dynasties. When the lions went to war, as they are inclined to do, he was their Homer.

“The scale of the lion study and Craig Packer’s vigor as a scientist are pretty unparalleled,” says Laurence Frank, of the University of California at Berkeley, who studies African lions and hyenas.

One of Packer’s more sensational experiments took aim at a longstanding mystery. A male lion is the only cat with a mane; some scientists believed its function was to protect an animal’s neck during fights. But because lions are the only social felines, Packer thought manes were more likely a message or a status symbol. He asked a Dutch toy company to craft four plush, life-size lions with light and dark manes of different lengths. He named them Lothario, Fabio, Romeo and Julio (as in Iglesias—this was the late 1990s). He attracted lions to the dolls using calls of scavenging hyenas. When they encountered the dummies, female lions almost invariably attempted to seduce the dark-maned ones, while males avoided them, preferring to attack the blonds, particularly those with shorter manes. (Stuffing still protrudes from the haunches of Fabio, a focal point of Lion House décor.)

Consulting their field data, Packer and his colleagues noticed that many males with short manes had suffered from injury or sickness. By contrast, dark-maned males tended to be older than the others, have higher testosterone levels, heal well after wounding and sire more surviving cubs—all of which made them more desirable mates and formidable foes. A mane, it seems, signals vital information about a male’s fighting ability and health to mates and rivals. Newspapers across the globe picked up the finding. “Manely, lady lions look for dark color,” one headline said. “Blonds have less fun in the lion world,” read another.

Lately, Packer’s research has taken on a new dimension. Long a dispassionate student of lion behavior and biology, he has become a champion for the species’ survival. In Tanzania, home to as many as half of all the wild lions on earth, the population is in free fall, having dropped by half since the mid-1990s, to fewer than 10,000. Across Africa, up to one-quarter of the world’s wild lions have vanished in little more than a decade.

The reason for the decline of the king of beasts can be summed up in one word: people. As more Tanzanians take up farming and ranching, they push farther into lion country. Now and then a lion kills a person or livestock; villagers—who once shot only nuisance lions—have started using poisons to wipe out whole prides. It is not a new problem, this interspecies competition for an increasingly scarce resource, but neither is it a simple one. Among other things, Packer and his students are studying how Tanzanians can change their animal husbandry and farming practices to ward off ravenous felines.

Scientists used to believe that prides—groups of a few to more than a dozen related females typically guarded by two or more males—were organized for hunting. Other aspects of the communal lifestyle—the animals’ affinity for napping in giant piles and even nursing each others’ young—were idealized as poignant examples of animal-kingdom altruism. But Packer and his collaborators have found that a pride isn’t formed primarily for catching dinner or sharing parenting chores or cuddling. The lions’ natural world—their behavior, their complex communities, their evolution—is shaped by one brutal, overarching force, what Packer calls “the dreadful enemy.”

Other lions.

The Jua Kali pride lives far out on the Serengeti plains, where the land is the dull color of burlap, and termite mounds rise like small volcanoes. It’s marginal habitat at best, without much shade or cover of any kind. (Jua kali is Swahili for “fierce sun.”) Water holes look more like wallows, prey is scarce and, especially in the dry season, life is not easy for the pride’s four females and two resident males, Hildur and C-Boy.

Early one morning last August, Serengeti Lion Project researchers found Hildur, a Herculean male with a blond mane, limping around near a grassy ditch. He was sticking close to one of the pride’s four females, whose newborn cubs were hidden in a nearby stand of reeds. He was roaring softly, possibly in an effort to contact his darker-maned co-leader. But C-Boy, the researchers saw, had been cornered on the crest of a nearby hill by a fearsome trio of snarling males whom Packer and colleagues call The Killers.

The whole scene looked like a “takeover,” a brief, devastating clash in which a coalition of males tries to seize control of a pride. Resident males may be mortally wounded in the fighting. If the invaders are victorious, they kill all the young cubs to bring the pride’s females into heat again. Females sometimes die fighting to defend their cubs.

The researchers suspected that The Killers, who normally live near a river 12 miles away, had already dispatched two females from a different pride—thus The Killers earned their names.

C-Boy, surrounded, gave a strangled growl. The Killers fell on him, first two, then all three, slashing and biting as he swerved, their blows falling on his vulnerable hindquarters. The violence lasted less than a minute, but C-Boy’s flanks looked as if they’d been flayed with whips. Apparently satisfied their opponent was crippled, The Killers turned and trotted off toward the marsh, almost in lock step, as Hildur’s female companion crept toward a stand of reeds.

None of the Jua Kali lions had been spotted since the fight, but we kept riding out to their territory to look for them. We didn’t know if C-Boy had survived or if the cubs had made it. Finally, one afternoon we found JKM, the mother of the Jua Kali litter, lolling atop a termite mound as large and intricate as a pipe organ.

“Hey there, sweetness,” Packer said to her as we pulled up. “Where are your cubs?”

JKM had her eye on a kongoni antelope a few miles away; unfortunately, it was watching her, too. She was also scanning the sky for vultures, perhaps in the hopes of scavenging a hyena kill. She stood up and ambled off into the hip-high grass. We could see dark circles around her nipples: she was still lactating. Against the odds, her cubs seemed to have survived.

Perhaps the apparent good fortune of the Jua Kali cubs was linked to another recent sighting, Packer speculated: a female from another nearby group, the Mukoma Hill pride, had been seen moving her own tiny bobble-headed cubs. The cubs were panting and mewling pitifully, clearly in distress; normally cubs stay in their den during the heat of the day. The Killers might have forsaken the Jua Kali females to take over the Mukoma Hill pride, which inhabits richer territory near river confluences to the north. The woodlands there, said Packer, were controlled by a series of “dinky little pairs of males”: elderly Fellow and Jell-O; Porkie and Pie; and Wallace, the Mukoma Hill leader, whose partner, William, had recently died.

Packer recalled a similar pattern of invasion in the early 1980s by the Seven Samurai, a coalition of males, several with spectacular black manes, who had once brought down two adult, 1,000-pound Cape buffaloes and a calf in a single day. After storming the north they’d sired hundreds of cubs and ruled the savanna for a dozen years.

It took a while for Packer to tune into such dramas. When he first visited the Serengeti lions in 1974, he concluded that “lions were really boring.” The laziest of all the cats, they were usually collapsed in a stupor, as if they had just run a marathon, when in reality they hadn’t moved a muscle in 12 hours. Packer had been working under Jane Goodall in Tanzania’s Gombe Stream National Park, observing baboons. He slept in a metal structure called The Cage to be closer to the animals. In 1978, when Packer’s plan to study Japanese monkeys fell through, he and a fellow primatologist, Anne Pusey, to whom he was married at the time, volunteered to take over the Lion Project, begun 12 years earlier by the American naturalist George Schaller.

By the time Packer and Pusey installed themselves in the Lion House, scientists were well aware that lions are ambush predators with little stamina and that they gorge at a kill, each one downing up to 70 pounds in a sitting. (Lions eat, in addition to antelope and wildebeest, crocodiles, pythons, fur seals, baboons, hippopotamuses, porcupines and ostrich eggs.) Lion territories are quite large—15 square miles on the low end, ranging up to nearly 400—and are passed down through generations of females. Lions are vigorous when it comes to reproduction; Schaller observed one male mate 157 times in 55 hours.

Packer and Pusey set out not just to document lion behavior but to explain how it had evolved. “What we wanted to do was figure out why they did some of these things,” Packer says. “Why did they raise their cubs together? Did they really hunt cooperatively?”

They kept tabs on two dozen prides in minute detail, photographing each animal and naming new cubs. They noted where the lions congregated, who was eating how much of what, who had mated, who was wounded, who survived and who died. They described interactions at kills. It was slow going, even after they put radio collars on several lions in 1984. Packer was always more troubled by the lions’ sloth than their slavering jaws. Following prides at night—the animals are largely nocturnal—he sometimes thought he would go mad. “I read Tolstoy, I read Proust,” he says. “All the Russians.” Packer and Pusey wrote in one article that “to the list of inert noble gases, including krypton, argon and neon, we would add lion.”

Still, they began to see how prides functioned. Members of a large pride didn’t get any more to eat than a lone hunter, mostly because a solitary animal got the proverbial lion’s share. Yet lions band together without fail to confront and sometimes kill intruders. Larger groups thus monopolize the premier savanna real estate—usually around the confluence of rivers, where prey animals come to drink—while smaller prides are pushed to the margins.

Even the crèche, or communal nursery that is the social core of every pride, is shaped by violence, Packer says. He and Pusey realized this after scrutinizing groups of nursing mothers for countless hours. A lactating female nursed another’s young rarely, usually after an unrelated cub sneaked onto her nipple. An alert lioness reserves her milk for her own offspring. In contrast to the widespread belief that crèches were maternal utopias, Packer and Pusey found that nursing mothers stick together chiefly for defense. During takeovers by outside males, solitary females lost litter after litter, while cooperating lionesses stood a better chance of protecting their cubs and fending off males, which can outweigh females by as much as 50 percent.

Surviving cubs go on to perpetuate the bloody cycle. Juvenile females often join forces with their mother’s pride to defend the home turf. Males reared together typically form a coalition around age 2 or 3 and set out to conquer prides of their own. (Hard-living males rarely live past age 12; females can reach their late teens.) A lone male without a brother or cousin will often team up with another singleton; if he doesn’t, he is doomed to an isolated life. A group of lions will count its neighbors’ roars at night to estimate their numbers and determine if the time is right for an attack. The central insight of Packer’s career is this: lions evolved to dominate the savanna, not to share it.

As we crossed the plains one morning, the Land Rover—broken speedometer, no seat belts, cracked side mirrors, a fire extinguisher and a roll of toilet paper on the dashboard—creaked like an aged vessel in high seas. We plowed through oceans of grasses, mostly brown but also mint green, salmon pink and, in the distance, lavender; the lions we hunted were a liquid flicker, a current within a current. The landscape on this day did not look inviting. Sections of the giant sky were shaded with rain. Zebra jaws and picked-clean impala skulls littered the ground. Bones don’t last long here, though; hyenas eat them.

Packer and a research assistant, Ingela Jansson, were listening through headphones for the ping-ping-ping radio signal of collared lions. Jansson, driving, spotted a pride on the other side of a dry gully: six or seven lions sitting slack-jawed in the shade. Neither she nor Packer recognized them. Jansson had a feeling they might be a new group. “They may never have seen a car before,” she whispered.

The sides of the ditch looked unpromising, but Packer and Jansson couldn’t resist. Jansson found what seemed to be a decent crossing spot, by Serengeti standards, and angled the truck down. We roared across the bed and began churning up the other side. Packer, who is originally from Texas, let out a whoop of triumph just before we lurched to a halt and began to slide helplessly backward.

We came to rest at the bottom, snarled in reeds, with only three wheels on the ground, wedged between the riverbanks as tightly as a filling in a dental cavity. The ditch was 15 feet deep, so we could no longer see the pride, but as we’d slipped downward, a row of black-tipped ears had cocked inquisitively in our direction.

Jansson stepped out of the truck, long blond ponytail whipping around, dug at the wheels with a shovel and spade, and then hacked down reeds with a panga, or straight-blade machete. Earlier I had asked what kind of anti-lion gear the researchers carried. “An umbrella,” Jansson said. Apparently, lions don’t like umbrellas, particularly if they’re painted with large pairs of eyes.

Packer is not afraid of lions, especially Serengeti lions, which he says have few encounters with people or livestock and have plenty of other things to eat. To figure out if a sedated lion is truly down for the count, he’ll get out of the truck to tickle its ear. He says he once ditched a mired Land Rover within ten feet of a big pride and marched in the opposite direction, his 3-year-old daughter on his shoulders, singing nursery school songs all the way back to the Lion House. (His daughter, Catherine, 25, is a student at the Johns Hopkins Bloomberg School of Public Health. Packer never tried such a stunt with son Jonathan, now 22, although Jonathan was once bitten by a baboon. Packer and Pusey divorced in 1997; she returned to studying chimpanzees.)

Not being handy with a panga, I was sent a short distance down the riverbed to gather stones to wedge under the wheels. Packer’s nonchalance was not contagious. I could not decide whether I should creep or sprint. Every time I glanced at the grassy riverbanks above I was sure that I would find myself the object of some blond monster’s greedy regard. As I bent to claw stones out of the ground, I knew suddenly, with complete, visceral certainty, why Tanzanian villagers might rather be rid of these animals.

I’d already taken stock of their carving-knife incisors and Cleopatra eyes, observed their low, rolling, hoodlum swaggers, heard their idling growls and nocturnal bellows. If you live in a mud hut protected by a bramble fence, if your cows are your bank account and your 7-year-old son is a shepherd who sleeps in the paddock with his goats, wouldn’t you want to eliminate every last lion on earth?

“People hate lions,” Packer had told me. “The people who live with them, anyway.”

After more than an hour of reed-whacking, stone-wedging and wrestling with mud ladders placed under the tires to provide traction, the vehicle finally surged onto the far side of the ditch. Incredibly, the lions remained precisely where we’d seen them last: sitting with Zen-like equanimity on their little doily of shade.

Jansson looked through binoculars, taking note of their whisker patterns and a discolored iris here and a missing tooth there. She determined this was the seldom-seen Turner Springs pride. Some of the sun-dazed lions had bloodstains on their milky chins. Though they hadn’t displayed the slightest interest in us, I uttered a silent prayer to go home.

“Let’s go closer,” Packer said.

The first true lion probably padded over the earth about 600,000 years ago, and its descendants eventually ruled a greater range than any other wild land mammal. They penetrated all of Africa, except for the deepest rain forests of the Congo Basin and driest parts of the Sahara, and every continent save Australia and Antarctica. There were lions in Great Britain, Russia and Peru; they were plentiful in Alaska and the habitat known today as downtown Los Angeles.

In the Grotte Chauvet, the cave in France whose 32,000-year-old paintings are considered among the oldest art in the world, there are more than 70 renderings of lions. Sketched in charcoal and ocher, these European cave lions—maneless and, according to fossil evidence, 25 percent bigger than African lions—prance alongside other now-extinct creatures: mammoths, Irish elk, woolly rhino. Some lions, drawn in the deepest part of the cave, are oddly colored and abstract, with hooves instead of paws; archaeologists believe these may be shamans.

The French government invited Packer to tour the cave in 1999. “It was one of the most profound experiences of my life,” Packer says. But the dream-like quality of the images wasn’t what excited him; it was their zoological accuracy. By the light of a miner’s lamp, he discerned pairs, lions moving in large groups and even submissive behavior, depicted down to the tilt of the subordinate’s ears. The artist, Packer says, “doesn’t exaggerate their teeth, he doesn’t make them seem more formidable than I would. This was somebody who was viewing them in a very cool and detached way. This was somebody who was studying lions.”

The lions’ decline began about 12,000 years ago. Prehistoric human beings, with their improving hunting technologies, probably competed with lions for prey, and lion subspecies in Europe and the Americas went extinct. Other subspecies were common in India and Africa until the 1800s, when European colonists began killing lions on safaris and clearing the land. In 1920, a hunter shot the last known member of the North African subspecies in Morocco. Today, the only wild lions outside Africa belong to a small group of fewer than 400 Asiatic lions in the Gir Forest of India.

Lions persist in a handful of countries across southeastern Africa, including Botswana, South Africa and Kenya, but Tanzania’s population is by far the largest. Though devastatingly poor, the nation is a reasonably stable democracy with huge tracts of protected land.

Serengeti National Park—at 5,700 square miles, about the size of Connecticut—is perhaps the world’s greatest lion sanctuary, with some 3,000 lions. In Packer’s study area, comprising the territories of 23 prides near the park’s center, the number of lions is stable or even rising. But the Serengeti is the exception.

Part of the blame for Tanzania’s crashing lion population belongs to the trophy-hunting industry: the government allows the harvest of some 240 wild lions a year from game reserves and other unprotected areas, the highest take in Africa. Safaris charge a trophy fee of as little as $6,000 for a lion; animals are shot while feasting on baits, and many of the coveted “trophy males” have peach fuzz manes and haven’t even left their mother’s pride yet. The use of lion parts in folk medicines is another concern; as wild tigers disappear from Asia, scientists have noticed increasing demand for leonine substitutes.

The central issue, though, is the growing human population. Tanzania has three times as many residents now—some 42 million—as when Packer began working there. The country has lost more than 37 percent of its woodlands since 1990. Disease has spread from village animals to the lions’ prey animals, and, in the case of the 1994 distemper outbreak that started in domestic dogs, to the lions themselves. The lions’ prey animals are also popular in the burgeoning—and illicit—market for bush meat.

And then there is the understandable ill will that people bear lions, which loiter on front porches, bust through thatched roofs, snatch cattle, rip children from their mother’s arms, haul the elderly out of bed and seize women on the way to latrines. In the 1990s, as Tanzanians plowed large swaths of lion territory into fields, lion attacks on people and livestock rose dramatically.

Bernard Kissui, a Tanzanian lion scientist with the African Wildlife Foundation and one of Packer’s former graduate students, met Packer and me in Manyara, a bustling district southeast of Serengeti National Park. Kissui said five lions nearby had recently died after eating a giraffe carcass laced with tick poison.

“Is that one of your study prides?” Packer asked.

“I’m suspecting so,” said Kissui, who works in the nearby Tangire National Park. He wasn’t sure who had poisoned the lions or what had provoked the killings. A month earlier, lions had killed three boys, ages 4, 10 and 14, herding livestock, but that was in a village 40 miles away.

“Africa is not Africa without lions,” Kissui told me, but “human needs precede the wildlife’s. As the number of people increases, we take the land that would have been available to the wildlife and use it for ourselves. Africa has one billion people now. Think about what that one billion implies in terms of the future of lions. We are heading into a very complicated world.”

Young men from pastoral tribes no longer care to tend cattle, Kissui says. “They want to go to Arusha and drive a car.” So their little brothers are sent into the bush instead. Packer and his students have shown that lions tend to target livestock tended by boys during the dry season.

Packer, Kissui and other scientists are experimenting with ways to keep people and lions safe. Special funds repay herders for lost livestock—if no lion is harmed. They have suggested that corn farmers in southern Tanzania hang chili peppers in their fields, which repel the bush pigs that lions relish, or dig ditches around their crops to keep the pigs out. And Packer is assisting Kissui with a program that subsidizes herdsmen who want to replace their bramble-enclosed paddocks with fences of metal and wood.

In Manyara we visited Sairey LoBoye, a study participant. He was attired in stunning blue blankets and talking on his cellphone. LoBoye is a member of the Maasai tribe, whose traditional culture centers on safeguarding cattle: teenagers spear lions as a rite of passage. LoBoye said he simply wanted lions to leave him alone. Two years ago lions devoured one of his precious bulls, but since installing a modern fence, he hasn’t had any problems and his cattle and children are safer. “Now I can sleep at night,” he said.

Packer argues that the Serengeti, like some South African parks, should be surrounded by an electric, elephant-proof, heavily patrolled fence that would encompass the whole wildebeest migration route and keep the lions in and the poachers out. The idea has little support, in part because of the tens of millions of dollars it would cost to erect the barrier.

Packer and Susan James, a former business executive he married in 1999, founded a nonprofit organization, Savannas Forever, which is based in Arusha and monitors the quality of rural village life. They’ve hired Tanzanians to measure how development aid affects such variables as children’s height and weight; they’ll spread the word about which approaches are most effective so other programs can replicate them. The hope is that improving the standard of living will bolster local conservation efforts and give lions a better shot at survival.

As hard as it is for Packer to imagine the prides he has followed for so long ending in oblivion in the next few decades, he says that’s the most likely outcome: “Why am I doing this? I feel like I owe this country something. So 100 years from now there will still be lions in Tanzania.”

Before I left the Serengeti, Packer took me to see a fig tree that had served for decades as a lion scratching post. As we drove across the savanna, graduate student Alexandra Swanson fiddled with a radio scanner, searching for signals from radio-collared lions, but we heard only static.

The tree was on a kopje, one of the isolated piles of rocks in the grasslands that are popular lion haunts. Packer wanted to climb up for a better look. Lulled, perhaps, by the silence on the scanner, I agreed to accompany him.

We’d climbed most of the way up the pile when Packer snapped his fingers and motioned for me to crouch down. The world seemed to zoom in and out, as if I was looking through a camera’s telephoto lens, and I imagined hot lion breath on my neck.

Packer, at the top of the kopje, was waving me closer.

“Do you see that lion?” he whispered. “No,” I whispered back.

He pointed at a shadowy crevice beneath the fig tree, about 20 feet away. “You don’t see that lion?”

“There is no lion,” I said, as if my words could make it so.

Then I saw one tiny, yellow, heart-shaped face, and then another, bright as dandelions against the gray rocks. Golden eyes blinked at us.

Mothers often leave their cubs for long stretches to hunt, but this was only the second time in Packer’s long career he’d found an unattended den. Young cubs are almost completely helpless and can starve or be eaten by hyenas if left alone too long. One of the cubs was clearly horrified by our presence and shrank behind its braver sibling, which arranged itself in a princely fashion on the rocks to enjoy these strange, spindly, cringing creatures. The other cub seemed to forget its fear and bit the bold one’s ear. They were perfect fleecy things. Their coats had a faint tiled pattern that would fade away with time.

That night we camped beside the kopje, Swanson and I in the bed of the Land Rover and Packer in a flimsy tent. It wasn’t the most restful evening of my life: in the lion’s last great stronghold, we were outside a mother’s very den.

I kept thinking of the cubs in the crevice. Their mother might return while we slept. I almost hoped she would.

Abigail Tucker, Smithsonian’s staff writer, has covered narwhals, salmon and the link between birds and horseshoe crabs.