Too small for heat or even much light, our base camp's tiny fire served only as a human gathering point in the midst of the vast moonlit bowl, 11,000 feet up in the John Muir Wilderness of the eastern Sierra Nevada. It was mid-October, on the verge of winter, and I was more exhausted than I cared to admit. I was among four volunteer porters carrying gear for Frank Green, a filmmaker who was trying to capture footage of the elusive Sierra Nevada bighorn sheep before the last one was killed and eaten. Whatever romance Sherpadom may have held faded within minutes of hoisting my 70-pound pack and starting the steep climb from the trailhead at Rock Creek Lake. No worse for wear, how-ever, was our ginger-bearded guide, John Wehausen, who lounged by the fire in perfect comfort. A biologist with the University of California's White Mountain Research Station and the preeminent expert on Sierra bighorn, Wehausen spends as many as 100 nights a year in the field. The day's hike hadn't much tired him. It turned out that he thought nothing of ascending 7,000 feet in seven hours on a single bottle of water. After studying bighorn sheep for 25 years, Wehausen was taking on the hardy qualities of his subjects.

They don't come much tougher than the Ovis canadensis californiana, a subspecies similar in appearance but genetically distinct from the bighorn sheep that range the mountain West from the Canadian Rockies to Baja California to West Texas. Sierra bighorn inhabit the highest, craggiest recesses of the Sierra Nevada, rarely descending below 10,000 feet except in the harshest winter months. Short and stocky-a big male weighs about 220 pounds-they are built for agility, not for speed. When danger threatens-most often in the form of a mountain lion-they run to the cliffs and move up as surely as if they had wings. John Muir called the bighorn "the bravest of all Sierra mountaineers."

Recently declared an endangered species, Sierra bighorn may never have been very numerous. Before they started appearing on Gold Rush menus, historical accounts suggest there were only about a thousand of them. Even deadlier than hungry miners were the domestic sheep that were brought to graze the high Sierra pastures-the "hooved locusts" against which Muir railed-and which passed on deadly respiratory bacteria to their country cousins. (In 1988, a thriving herd of 65 bighorn in the Warner Mountains in northeastern California was wiped out by contact with a single domestic sheep.) Though bighorn hunting was banned in 1878, by the end of the century Muir wrote that "few wild sheep, I fear, are left hereabouts."

In 1940, the Sierra Club proposed the creation of a bighorn sanctuary-a proposal rejected by the U.S. Forest Service, which feared that it would attract poachers more than protect the wild sheep. In 1971, sanctuaries were finally established on the spiny ridge of the eastern Sierra at Mt. Williamson and Mt. Baxter for the only two remaining populations (out of nine at the beginning of the century). By 1978, these herds numbered 250 animals.

For the next decade, working in part from data that Wehausen collected as a graduate student, sheep captured from the larger Mt. Baxter herd were used to restock three historic bighorn ranges, at Mt. Langley just north of Mt. Whitney, in Lee Vining Canyon (where they sometimes graze only feet away from the cars and trucks whizzing out of Yosemite National Park on Highway 120), and on Wheeler Ridge-beneath which we had pitched our tents.

The next morning, what looked from the bottom like a tough scramble up the ridge proved to be that and more, all loose scree and tippy, lurching boulders and dicey maneuvers it was just as well my wife didn't hear about. From the pass on top of the ridge the Owens Valley and White Mountains spread out to the east. We crouched low behind boulders, striving for inconspicuousness while Wehausen unpacked his telemetry equipment to see if any radio-collared sheep were in the vicinity. He picked up some weak readings, so we headed down the far slope, working southwest ridge by ridge toward a drainage known for its many avalanches as Barf Canyon. Resting atop the last ridge, we were set to scree-slide half a mile down to the next bench when Mike, one of the Sherpa crew, announced the presence of four ewes directly beneath us.

As quietly as possible in such rocky terrain, we moved downslope so as to avoid appearing above the sheep-a sure way to spook an animal whose survival depends on climbing higher than any pursuer. From an unthreatening vantage point we watched a valleyful of sheep-a total of 19, all ewes and lambs and yearling males. (Except for mating season, bighorn segregate by sex. During the rut, mature rams wander from one group of ewes to another.) Two young males worked their way up the vertical cliff across the cirque with effortless, economical grace: a leap, a bound, a pause, a pose at the ridgeline, and then they were gone. A ewe, two lambs, and a young ram picked their way up a talus slope until they disappeared by blending perfectly into the surrounding rocks. The lambs were nice and fat; Wehausen predicted that the female would be lambing by the next year. "Too bad she'll never get a chance to enjoy herself," lamented porter Dennis.

Some sheep we saw right away; others had to reveal themselves. "My job," said Wehausen, binoculars permanently glued to face, "is finding needles in haystacks-a rock with legs in the midst of a bunch of other rocks." It took the rifle-crack of a head-butt to alert us to two young rams facing off on a flat rock. Even so, I had trouble locating them. "The gray rock," said Dennis, trying to be helpful. "Oh. The gray one." (The only thing harder than finding a rock with legs is finding a rock without them.)

Autumn sun fading, we had a challenging climb back to the pass, up a slope a bighorn could have covered in a few minutes. With little energy left at the top, we still had to descend, by the light of headlamps, 1,400 feet of a boulder field that had yet to settle on its angle of repose. It was all worth it, though, for 30 seconds of satisfaction on a perfect scree field, striding straight down the mountain atop my own little landslide.

In camp, Wehausen sorted plastic bags of sheep scat. His aim was to collect samples from every last Sierra bighorn for genetic analysis (with declining numbers, inbreeding is a real concern). Despite the fine show we had that day, Wehausen was far from sanguine about the bighorn's prospects. In fact, at that time-October 1999-the Sierra bighorn seemed to be sliding down their own scree slope toward extinction.

Initially, the reintroduced herds drawn from Mt. Baxter had thrived. By 1985, there were an estimated 310 sheep in five locations. But then the numbers plunged, and by February of 1999, barely 100 adult animals remained. According to a petition for an emergency endangered-species listing filed by a variety of conservation organizations that month, "if current trends in sheep numbers and behavior continue, populations of these sheep could begin to disappear from the face of the earth within a few years."

When things get that bad, the harsh rules of population dynamics magnify every problem. The fewer the animals in a population, the greater the effect of acts of God: a broken leg, a harsh winter, even unfortunate distribution of the sexes. Following the severe winter of 1995, for example, the Mt. Langley herd was left with 4 ewes and 11 rams. That same year, an avalanche wiped out a dozen members of the Wheeler Ridge herd, reducing it by almost half.

What is causing the decline? Hunting is no longer an issue, and there has been no documented contact between domestic sheep and Sierra bighorn for 25 years. (This has been largely due to luck: In 1995, 23 domestic sheep escaped from a Forest Service grazing allotment adjacent to the Lee Vining herd's summer range, and were discovered within sight of the bighorn in Yosemite National Park-fortuitously, before they were able to mingle.) Rather than domestic sheep or sharpshooters, all evidence points to a natural cause for the bighorn's recent decline: Puma concolor, the mountain lion.

Shy and reclusive, mountain lions share the tawny color of their African cousins, but bear a longer, heavier tail. They are extraordinarily adaptable, ranging from Canada to South America with the largest distribution of any non-anthropoid in the New World. In California, their prey of choice is mule deer, although a few have developed a taste for mutton. And therein lies the problem: When an expanding cougar population in the 1980s turned its attention to the Sierra Nevada bighorn, it drove the sheep to the brink of extinction.

In the past, mountain lions have themselves been victims of ruthless predators. California paid bounties for dead lions until 1963, and trophy hunting wasn't banned until 1972. In response to repeated attempts by the California Department of Fish and Game to reinstitute hunting, voters in 1990 passed Proposition 117 (strongly backed by the Sierra Club), declaring lions "specially protected" and prohibiting all hunting or control except in cases of predation on humans, pets, or livestock. Six years later, sport hunters and conservative legislators sought to overturn 117 with another statewide proposition, which was summarily rejected by the voters. The message was clear: Even though mountain lions are neither endangered nor even threatened, Californians didn't want to see them killed.

No one knows how many lions there were in California before European settlement or how many there are now. (Among the unknowable variables is the number of lions formerly killed by grizzly bears, now extinct in California.) Even so, most available signs point to an increase in their numbers following the end of bounty- and sport-hunting, which killed up to 400 cats a year. Reports of predation on livestock went from virtually zero in the early 1970s to over 300 in 1995. Attacks on humans occurred almost annually. Among radio-collared deer, lions were the primary cause of death. Meticulous records kept by employees of the Los Angeles Department of Water and Power of everything drowned in the L.A. aqueduct list no lions until 1988, but a dozen in the five years after that. And Wehausen was recording sheep after sheep dispatched by cats in the dwindling herds of the eastern Sierra.

What had been a successful bighorn reintroduction effort began to falter. Lee Vining Canyon lost a number of sheep, including one of its most productive ewes. Between 1976 and 1988, Wehausen documented 49 mountain lion kills in the winter range of the Mt. Baxter herd alone.

Almost all of these attacks took place while the bighorn grazed their winter ranges. The result was disastrous: The wild sheep stopped coming down from their high-mountain refuges, even in the howling blizzards of midwinter. In addition to putting themselves at increased risk of avalanches, they also lost access to the grasses that constitute an essential source of nutrients. Ewes that avoided the winter range, Wehausen found, lambed a month later than those that did not, and the snows and freezing temperatures took a heavy toll on lambs forced to overwinter on the mountaintops. The Mt. Langley herd, which numbered 42 in 1990, fell to 15 after the harsh winter of 1995. Lee Vining dropped from 85 to 29, recovered slightly over the next two years, but spiraled down to 17 after another hard winter in 1998. What had once been seen as a potential new translocation source became a basket case.

As we huddled around our little fire that night below Wheeler Ridge, Wehausen spun stories of a lifetime counting sheep, like the rare "play behavior" of well-fed bighorn in their winter range when the sheep get up on their hind legs and dance. And while head-butting generally occurs only between equal-age males, in this playful mood yearlings and aged rams would engage in mock combat: rearing up, charging, then pulling up short at the last moment, gently tapping their horns, and rearing up again to dance.

As the fire guttered, Wehausen told of lions, too, including the time he was pounced upon by a cougar protecting a fresh kill. Oddly, he recalled, he wasn't frightened: Even in mid-leap, something in the lion's demeanor telegraphed to him that it only meant to scare him off, not kill him. He did recommend the use of a hiking staff or similar long stick, which, he said, could keep a troublesome feline at bay indefinitely. I kept my own stick close by for the duration of the trip.

Since the sheep on Wheeler Ridge seemed to have moved south, our next approach was an 11-hour flanking dayhike from that direction. From Mosquito Flat, we quick marched past a lovely lake chain, over Morgan Pass, then north up a road that once serviced a large tungsten mine, where the gigantic pit mouths still gape amidst a postindustrial sci-fi wasteland of avalanche rock and twisted, rusting metal. Wehausen had other business below, so guiding us was Karl Chang, another indomitable eastside sheep researcher. Green and the other equipment-Sherpas trailing behind, Chang and I climbed a series of steep switchbacks to the bottom of the canyon, where we knelt behind a boulder to glass the surrounding slopes. Not 40 yards away stood a beautiful dark ram, which moved slowly away with a dignified air, pausing every 10 yards or so to turn his profile, finally disappearing around the corner.

The others arrived with the heavy camera equipment and asked if we had seen anything. There was a ewe far up on the eastern cliff, Chang said, but it had moved off over the mountain. "Well, there was that ram on the hill right there," I interjected. A moment's silence. "You're joking, right?" Chang said, dismayed. Rocks with legs reveal themselves selectively.

After lunch we ascended the canyon, 1,000 feet up a 35-degree slope until we reached the remnants of an old tramway that looked like an abandoned ski jump. Coming out on the northern peak at the old Adamson mine, we found ourselves with a sheep's-eye view of our first campsite. But, no sheep-at least until we turned back, and saw my dark ram grazing in our lunchtime meadow. We bushwhacked and scree-slid back down, but the bighorn faded back into the rocks. Once again, we finished our day by headlamp. Green, dispirited, walked back in the dark.

We returned to the nearby town of Bishop to clean up, and eat at a Chinese restaurant. Across from Wehausen was a large, garish painting of Chinese mountain sheep posed in a snowy forest. The biologist frowned. "Wrong habitat," he declared. "They prefer open country, like our sheep, and I'm not sure what species that is anyway . . ."

Cougars may have been the proximate cause of the bighorn's decline, but California's political process shares the blame. While Prop 117 allowed the state to relocate or kill a mountain lion that ate your heifer or poodle, it had no provisions for culling lions that lunched on vanishing species. Bighorn advocates and the Department of Fish and Game sought a legislative exemption, arguing that failure to control lions in the eastern Sierra would result in the disappearance of an animal endemic to the area since the last Ice Age.

In this effort they ran smack into the Mountain Lion Foundation-an organization fiercely protective of its namesake cat-which suspected Fish and Game of looking for a new excuse to kill big cats. An ugly brawl ensued, with mutual accusations of faulty science and "deceitful" arguments. Wehausen called the foundation "an animal-rights group, not an environmental group." For its part, the foundation sought (and still seeks) to minimize the role of mountain lion predation in the decline of the bighorn: "The historic cause of bighorn decline, and continued threat to survival," it argues, "is 130 years of human indifference and hostility to the bighorn's habitat needs. The facts are simple: predators kill bighorn one at a time; habitat loss and contact with domestic sheep decimates entire populations."

In its endangered-species listing, however, the U.S. Fish and Wildlife Service noted that "habitat throughout the historic range of Sierra Nevada bighorn sheep remains essentially intact . . . neither fragmented nor degraded." Nor has domestic-sheep disease been a problem in the recent past. Rather, wrote Wehausen, what "caused these sheep to decline over the past 15 years was apparently an unacceptably high level of mountain lion predation that developed in the 1980s, and nothing else."

Mountain Lion Foundation executive director Lynn Sadler is unconvinced. "Given that the bighorn sheep population is already enormously stressed," she says, "now is not the time to experiment with the removal of predators in hopes that it will solve the problem." With fewer lions, she suggests, the deer population might explode, and "suddenly the bighorn sheep could be competing for forage." Sadler also warns that removing lions might result in "mesopredator release," a proliferation of lesser predators, like coyotes, which might, she suggests, kill as many sheep as the lions. "Coyote, bobcat, gray fox, golden eagle, and domestic dogs have been known to prey on bighorn sheep," says the foundation. In response, Wehausen points out that of 75 Sierra Nevada bighorn documented to have been killed by predators in recent decades, only 3 were not by lions.

Even when cornered, however, the Mountain Lion Foundation is still able to brandish the large stick of public opinion. "If the [U.S. Fish and Wildlife Service] decides to contravene the twice-affirmed will of the people of the state of California that mountain lions not be killed within thestate," it warns, "the Service should at least be reasonably certain that its efforts will actually benefit the Sierra Nevada bighorn sheep."

Yet with bighorns so few that the loss of any individual threatens the species' survival, selective control of depredating mountain lions can only help. Only a handful acquire a taste for mutton, and those that do can be tracked in winter snows from the scene of the crime. For many conservationists, it is a bitter draught to contemplate the killing of a noble creature they once fought to protect. Yet once we start playing God-by exterminating grizzlies, introducing exotic breeds of sheep, or granting special protection to mountain lions-we seem obliged to continue. To so alter the natural balance and then throw up our hands and say, "Let nature take its course," might erase the Sierra Nevada bighorn from the book of life.

More forays into the mountains came up empty. Frank Green, the filmmaker, grew increasingly anxious: After a week in the field, he had only about a minute of useable footage. So on the last day, when we were presented with a choice between an easy hike in Lee Vining Canyon with its tiny herd and a monster hike to Mt. Langley with the possibility of encountering a much larger group, there was no question which we would choose. We wound for most of the day through whitebark pine, lunched at Muir Lake, and then embarked on a post-prandial forced march up the mountains to the east. Halfway through the sandy slog Chang spotted five ewes trying to decide whether to bed down or run. We set up the camera and started filming, then moved forward 50 yards for a better shot. Incredibly, we came upon a large group of sheep actually moving toward us (the usual view of a bighorn is its retreating white rump) to a small meadow not 40 yards away, where they paused for a few valuable moments, Green's camera whirring, before turning to flee uphill. We watched for half an hour until they dissolved into the gray rocks.

The inability of California Fish and Game officials to deal with lions preying on bighorn was a major impetus for the April 1999 emergency listing of the sheep as a federal endangered species (federal law trumping state law). That September, the California legislature passed a bill, nearly unanimously, that allowed for the limited taking of lions to protect sheep populations, and provided funds for bighorn recovery efforts. In January 2000, the endangered species listing was made permanent. Among the happy consequences is that the Forest Service finally stopped renting out domestic sheep pasture butting up against bighorn territory and the Department of Water and Power ended sheep grazing in the Mono Basin, greatly reducing any threat of disease transmission.

As for the mountain lions, only two depredating animals have been killed in the last two years, and even the Mountain Lion Foundation's Sadler admits that "things seem to be under better control." Luckily for all, the last two winters in the Sierra Nevada were extraordinarily mild, allowing the sheep to stay safe and high and still find good forage. In March 2000, Wehausen says, he was at 10,000 feet on Wheeler Ridge, and there was green grass growing. Overall, he counted at least 34 new lambs and 25 yearlings. The Wheeler Ridge group has been increasing at the rate of 30 percent per year. "That population is our hope for the future," says Wehausen. The state Bighorn Recovery Team is already considering using individuals from Wheeler Ridge to rebuild the depleted Mt. Baxter herd-the original source of the Wheeler flock. While it is still too early to declare victory, bighorn advocates are planning for the future rather than preparing for the end.

In planning that future, we bear the burden of the sins of the past. If hunting and disease had not decimated the Sierra bighorn, they might have been able to weather an uptick in lion predation. Had it not been for the decades of slaughter of mountain lions, there might never have been such a steep rise in their numbers. The only way the public could assert its support for the right of large predators to exist was through the blunt instrument of a statewide ballot initiative, which proved clumsy when applied to the unexpected plight of the bighorn sheep. Surely the voters didn't intend to champion one magnificent species at the cost of wiping out another, but it took the real threat of extinction to bring the situation back in balance. Here we teeter, as ever, trying to restore the order we have disturbed, trying, when in danger, not to run away, but to move up.