The cell phone alert was designed to wake anyone from a deep sleep. “MORTALITY EVENT DETECTED,” the text message read, accompanied by a cacophony of drums and bells blaring from the phone’s speaker at top volume. It was near midnight on May 22nd, but David Pauly wasn’t asleep; he knew this call was coming. Already he had received five alarms like it over the past month, announcing that a female moose wearing a GPS tracking collar and ear tag #075 hadn’t moved for at least six hours.

Usually, that’s enough to indicate that a moose is dead. But #075 was a survivor. Twice, her collar returned to normal status within a few hours of sending the mortality alert. The other three times, Pauly sent a field team into Minnesota’s North Woods in search of her carcass only to find the moose had rallied and moved on. But this time seemed different. Pauly had spent the previous few days examining movement data for #075 and monitoring her through a secure tracking site that showed hourly locations for GPS-collared moose in northeastern Minnesota; #075 had not moved much during that time—finally succumbing, it seemed, to illness or injury. She was among thousands within the region’s moose population stricken in the past few years, a calamity that has pushed the Minnesota Department of Natural Resources to initiate the largest GPS study ever undertaken of moose deaths in the world.

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Still, Pauly didn’t want to rouse other members of his moose retrieval team, who were based several hours away, for what could be yet another false alarm. So as the moon set, he and his elderly Springer spaniel, Butternut, climbed into Pauly’s pickup and drove deep into Superior National Forest, a three million-acre wilderness in the arrowhead-shaped northeastern region of the state. An hour later, Pauly parked in the darkness and left Butternut curled up in the passenger seat. With a VHF radio antenna in one hand and receiver in the other, Pauly—whom colleagues jokingly call “Griz,” for his resemblance to the famed mountain man Grizzly Adams—walked up a track used by snowmobiles during winter, listening for the beeps emitted on #075’s channel. They grew stronger and faster about a quarter mile up the track, pulling him off into a patch of conifers and up a steep and rocky ridge, frosted leaves crunching underfoot.

Pauly found #075 at the top of the ridge just as dawn broke. She was still alive, lying on her stomach with her legs under her. The quarter-ton moose flattened her ears as he approached, warning him away. She tried to rise, bracing her front hooves and heaving her chest off the ground, but her hind legs were useless.

I had also been waiting, almost 10 days now, for the alert from #075’s collar, ready to hop in my rental car with a daypack of warm layers, rain gear, water, and beef jerky. When Pauly called his team at 7:15 a.m., I joined them—a young biologist named Erik Hildebrand and a couple of on-call wildlife specialists from the Department of Natural Resources—at the snowmobile trailhead and hiked in through softening mud.

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I had come to Minnesota on the trail of a mystery, the same one that drew Pauly and his team into the woods. Alces alces andersoni, the Western moose, is an icon of the North Woods, beloved of hikers and hunters, and an important driver of tourism in the state. More than 300 businesses are named for the monarch of the forest (just in my adopted home base of Ely, I could spend my time waiting for a mortality call by wandering into The Chocolate Moose, Moose Track Adventures, Mostly Moose and More …), and the animal’s image adorns everything from T-shirts to cupcakes.

Yet the moose of northeastern Minnesota are dying at an alarming rate. The population has nosedived in recent years, dropping to about a third of what it was in 2009. In the past year alone, their numbers plummeted 35 percent, leaving only about 2,700 moose. That’s a mortality rate unseen anywhere else in North America—in fact, in other parts of the continent, moose are thriving. But something, or a combination of somethings, is threatening to wipe out moose from the North Woods in less than a decade, if the current decline continues unabated. Scientists suspect that everything from blood-draining ticks to brain-tunneling parasites to stress from higher temperatures—or a combination of all those factors and more—could be to blame. But to solve the mystery, they need to study dead moose, something that’s far easier said than done.

Moose usually live and die deep in the forest, where decay and scavengers claim their carcasses before scientists can reach them. The solution: GPS collars like the one worn by #075, which tell researchers when the moose have stopped moving and where to find them for quick retrieval.

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When Pauly’s team and I met him on the snowmobile track, we followed him into the dark shadows of a stand of white pine, diving through low branches while long needles grazed our cheeks. We emerged into sunlight blinking at leafless oaks, maples, and birches stretching to the horizon in every direction. Hildebrand, the young biologist, scanned the contours of the land; the retrieval team would need to drag #075’s body across a dry streambed and down several steep scrambles bristling with tree trunks. Hildebrand looked at Pauly. “I thought you said this would be the easiest mortality call ever!” Another team member started laughing, and Hildebrand shook his head. “We might as well bring the gear up now,” he said, and they turned back to retrieve an industrial-rubber sled and chainsaw modified with a cable winch. Until now, all of the mortality calls had happened in the snow, a more cooperative surface for towing sleds. This, Hildebrand feared, was going to be a challenge.

Pauly, who has been with the Department of Natural Resources for his entire career, starting as a seasonal technician during college in the 1970s, slowed as we drew near where #075 lay, moving quietly. We gave her a wide berth, setting down packs 30 feet away. Pauly pulled out a video camera; the researchers would want video of #075 while she was still alive, if possible, as a record of her symptoms. As Pauly moved around her, #075’s ears swiveled like satellite dishes, following him. Her long face was a coppery brown, and she tested our scent with flared nostrils. The point of her withers, three feet off the ground even as she lay, still bore tufts of coarse hair, but her chest and ribcage were completely bare where she had rubbed her coat off to rid herself of hordes of winter ticks feeding on her blood.

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She had deteriorated since the early morning when Pauly first found her. When she tried to push herself up, she couldn’t even get her front hooves underneath. Pauly was crouched on her far side, looking pained, and rubbed his hand over his face. I caught the scent of rotting flesh; part of the moose was already dead. Pauly carefully picked up his rifle, knowing he needed to euthanize her. Other members of the team moved in front of #075 to draw her attention. She followed them with her eyes, then lay her head down on the ground, spent. One sharp crack split the air and her great body unfolded onto the ground. Her muscles convulsed and then were still. Beside me, Pauly slowly shook his head. “No matter how many times I have to do that, it never gets any easier.”

* * *

The current North Woods moose crisis echoes an earlier collapse on the opposite side of the state, in the northwestern corner, where a healthy population thrived until the mid-1980s. Over the course of the next 20 years, their numbers crashed. Scientists attributed the vast majority of the deaths to parasites and malnutrition, both of which they believe were related to warming temperatures. From a population of 4,000, fewer than 100 moose remain in the northwest part of the state, and any rebound is almost certainly impossible.

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“I was raised in northwest Minnesota, in a little town called Fertile,” recalls Mark Johnson, executive director of the Minnesota Deer Hunters Association. “As a kid in the ’60s and ’70s, we always saw moose. I remember a young bull standing in my mom’s garden, eating her peas.”

At a conference on large mammals in 2007, Johnson and his association’s board of directors were stunned by the data presented about the die-off of the northwestern moose, and by the fact that no organization was actively pushing the state legislature to allocate more resources for moose research and conservation. There were no aerial surveys of the northwestern population between 2003 and 2007, Johnson says, and there was no dedicated moose biologist at the Department of Natural Resources. The association began lobbying state lawmakers to create a moose advisory board and adopt a new management plan to replace the last one, written in 1990—before the collapse.

Across the state from the decimated northwestern population, the northeastern Minnesota moose appeared stable through the mid-2000s as aerial surveys continued to turn up roughly 8,000. But alarmed by the northwestern die-off, the department initiated a radio-collar mortality study in the northeast in 2002 to learn more about the population. Despite the relatively high survey numbers, researchers found that among the moose in their study, survival rates of both calves and adults were lower than in other healthy populations in North America, meaning their numbers weren’t likely to last long.

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In 2007, Erika Butler, then a recent graduate of the University of Minnesota veterinary school newly hired by the department, decided to use outdoorsmen to study the still-sizeable northeastern moose population. When moose hunters arrived at their mandatory licensing orientation, she was ready with an educational slideshow, a sampling kit, and a request: she asked the hunters to collect biological samples from any moose they killed and bring those samples back for testing. The idea was to help the agency establish baseline values for the health of the moose. In addition to the hundreds of pounds of meat and antlers the hunters were packing out, Butler requested they add blood, feces, hair, teeth, ticks, livers, and lungs, plus brains and lymph nodes if possible.

Johnson’s association raffled off a gun or muzzle loader as an incentive for the hunters’ participation, “but I’m not sure it was absolutely necessary,” he says. “The hunters were really engaged and wanted to be part of it.” Over the next six years, until the hunt was suspended indefinitely in 2013, more than 90 percent of hunters complied, bringing Butler samples from 634 moose.

What the researchers discovered in the samples—from relatively healthy moose—was disturbing. Their findings encompassed a dizzying array of parasites and pathogens, including one that scientists have yet to identify. Four percent of the moose brains had healed-over tracks from brainworm, which can cause death if the worm burrows through critical tissues of the brain. Another ten percent had brain lesions of unidentifiable origins. Thirty percent had cysts in their livers from Fascioloides magna, giant liver flukes that can grow up to three inches long and migrate elsewhere in the body.

More than 30 percent tested positive for the mosquito-borne West Nile virus, which biologists already knew was present in the population. But they were surprised to find what Butler calls West Nile’s “evil twin,” Eastern Equine Encephalitis. EEE is far more lethal than West Nile, killing 70 to 90 percent of the horses that contract it within days (and, worryingly, 33 percent of humans, leaving the survivors severely impaired). EEE had never been detected in Minnesota, in any animal, and its lethality in moose remains unknown.

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In addition to the diseases and parasites afflicting the moose, the Minnesota researchers have also found that rising temperatures correlate to higher mortality rates in both the state’s northwestern and northeastern moose. Average temperatures have been rising in Minnesota over the past 50 years, and the rate is accelerating. The relationship between higher temperatures and moose survival isn’t yet clear, but some researchers think that moose become physically stressed by the effort to keep cool.

Although temperatures are also rising elsewhere in North America, the problem seems to be one of thresholds. Moose are adapted for extreme cold, and Minnesota’s North Woods are at the southern boundary of their range on the continent. Researchers in one small study observed moose beginning to pant in order to cool themselves when the temperature rose above just 23 degrees F in winter, when they are carrying a heavy double-layered coat. In summer, temperatures as low as the 60s can cause moose to seek shade. At higher temperatures, their need to keep cool can override their pursuit of the 50 to 70 pounds of food they need to eat each day. The warming trend in Minnesota is bringing an increasing number of days above those thresholds for the moose. There are currently no data to support a direct causal relationship, but some moose biologists suspect stress at critical times of the year may put them at greater risk of dying from predators, parasites, and diseases they might otherwise have been able to fight off.

Warmer temperatures are also beneficial to some of the parasites that plague the moose. Brainworm and liver flukes are spread by white-tailed deer, which pass them on to the moose but are, themselves, mostly untroubled by the freeloaders. Since a round of severe winters in the mid-90s, deer density has steadily increased in northern Minnesota. Winter ticks are also proliferating as the climate warms and can be found in the tens of thousands—sometimes even more than a hundred thousand—on a single moose, taking blood meals en masse that can weaken and kill both calves and adults. Infested moose trying to rid themselves of the ticks will rub their coat off down to the skin, risking hypothermia in winter and secondary infections from abrasions.

The state researchers are trying to work out how much danger each factor poses by itself and how they’re compounded by the others—or whether there might be another threat they haven’t yet recognized. To solve the mystery, the researchers needed to study as many dead moose as possible. So in 2011, Butler and Glenn DelGuidice, the state’s lead moose biologist, proposed the ambitious GPS-collaring project. Knowing taxpayer money was scarce, they arranged to pay for the $1.2 million study through a combination of private donations, in-kind personnel and lab support, and the state’s Environmental and Natural Resources Trust Fund, which is funded by profits from the state lottery and covered half of the study’s costs.

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For two weeks in January of this year, the researchers captured more than 100 adult moose using helicopters and tranquilizer guns. Butler, Pauly, and their crew were dropped into chest-deep snow in temperatures well into the negative double digits to fit the moose with GPS-enabled collars. (Fortunately for me, reporters weren’t allowed.) Each time the team captured a moose, it took measurements of the animal’s size and fat layer, collecting blood to test for nutrients, diseases, and hormones. The brick-sized battery hanging at the bottom of the collar can last for six years and Butler is anticipating the other components will last at least four; if a moose dies during that time, and if the study is extended, the researchers will have a record of its basic health markers and an hour-by-hour record of the animal’s life post-capture. Data transmitted from the GPS sensor fastened at the top of the collar can show them where the animal ate and slept and where it sought shelter on warm days.

Already this spring and early summer, 15 of the adult moose have died. Sometimes they’re too distant or difficult to bring back to the lab, and Pauly and his field crews conduct necropsies on the spot. But whenever possible, they use their rubber sleds and chainsaw winches to heave the carcass out of the forest for a full examination.

* * *

It was a 250-mile drive from the spot where moose #075 died to the D-Lab, the veterinary diagnostic lab at the University of Minnesota in St Paul. Hildebrand and I arrived at half-past midnight and were met by a sleepy on-call student, rubbing her eyes and pulling on rubber boots over her sweatpants. Hildebrand backed the truck into the loading bay, and the student wrapped chains around the legs of the carcass, hauling it up with a hydraulic winch and steering it along heavy ceiling tracks into a huge refrigerator where wildlife, livestock, and pets awaited investigations of their mysterious deaths.

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I showered off the blood and mud of the 16-hour day and managed a few hours of sleep at a nearby motel. The next morning, revived by several cups of coffee, I donned safety glasses, an apron, and plastic boot covers before entering the floor of the D-Lab, hoping this necropsy might yet uncover the smoking gun responsible for the decline of the moose population.

Arno Wünschmann, one of the veterinary pathologists for the moose mortality project, stood out against the backdrop of stainless steel. He wore all white—scrubs, rubber apron, and boots—amid butchering implements and decomposing carcasses.

“I think we’ve got all our brightest people working on getting the weight now!” he announced to me, teasing a pair of students who were struggling with the dangling carcass of #075. Wuenschmann was sharpening his knives with a rhythmic swick-swick-swick. “Poor thing,” he trailed off as #075 was lowered onto a stainless steel wheeled table with a drain hole. Wuenschmann has necropsied some 50 moose—radio-collared moose, road-killed moose, and moose found dead by civilians—since coming to work at the university from his native Germany in 2000, and he’s not sure how many more to expect over the next few years as Butler’s study proceeds. Even if the recovery teams only get 10 percent of the moose out of the forest, the field necropsies should provide the blood and organs he needs to evaluate the factors in each animal’s death. As an expert in infectious disease, with a special interest in West Nile Virus, Wuenschmann will be watching carefully for the appearance of deadly viruses as mosquito season gets underway.

He noted the extent of #075’s hair loss, plucking off a few ticks for testing, then began methodically deconstructing the carcass with a sound like a seam ripper as he split the hide. Once Wuenschmann had opened the moose’s rib cage with the assistance of branch cutters, an assistant removed her heart and lungs, and dissected her carotid arteries, which were clear of parasitic worms. Wuenschmann removed her platter-sized liver, pronouncing it “pretty” and free of any obvious liver flukes. There were several visibly infected sores on her tongue, and abscesses in her lungs, and her lymph nodes were inflamed. As he moved toward her haunches, Wuenschmann navigated carefully with his knife. The entire pelvis was a mass of infected tissue the consistency of dried leather covered in grainy mud. This, he was fairly certain, was the cause of death: a systemic infection throughout her bloodstream, stemming from an injury between her legs and leading to the lesions he had observed on her tongue and lungs.

The degree of tissue death led him to believe the infection was probably a few weeks old, which gave the researchers a good guess at the source of the injury. When her collar had triggered a the first mortality alert a month earlier, the crew that went to retrieve her found instead evidence of a standoff: a trampled disc of snow with wolf prints arrayed around it. Nearby they saw a few drops of blood, and a small piece of tissue. It seemed almost certain now that #075 was the source of the blood, that the wolves had made contact and landed at least one bite; bacteria did the rest, taking down the enormous animal that the wolves could not.

I turned at the sound of a hack saw to see one of the assistants struggling to remove the top of the skull, finally revealing the brain, pale and pink, and not much bigger than a mango. It would be preserved for inspection under a microscope, to look for evidence of brainworm infection.

Five months into the mortality study, 92 of the 107 moose collared back in January remain. Wolves killed five, not including #075 and one other moose that died from an apparent attack and subsequent infections. (Researchers think that wolves cause fewer than 10 percent of adult moose deaths overall each year, but the predators have their greatest success in late winter, when moose are weakened by dwindling fat stores and hindered by deep or crust-covered snow.) Winter ticks drained the blood from three (which can cause anemia and fluid buildup around the heart), brainworm excavated the brain and eye of another, and as-yet-undetermined causes took another four. But none of those causes of death tell the full story. It will take several years of data collection for the researchers to tease out patterns of co-infection by parasites and disease and correlations with temperature and habitat changes due to climate change.

Moose are still plentiful elsewhere in the cooler climes of North America. Some 200,000 roam the Alaskan wilds. As many as a million inhabit the Canadian interior. In New England, the rebounding of moose populations over the past 30 years has been one of the great comeback stories of conservation. Heavily logged forests have regrown, spreading ideal moose habitat—a mix of pine and spruce with oak and birch and maple, not dissimilar from the forests of Minnesota—over the mountains of the northeast. An aerial survey in Maine last year turned up an estimated population of 76,000 animals. New Hampshire doesn’t conduct an aerial count, but by the state’s best estimate, it had 6,500 moose a few years ago, concentrated in the Great North Woods.

But New Hampshire moose hunters have had decreasing success over the past decade, indicating to biologists that the population might be declining. In 2012, managers cut the number of hunting licenses by two thirds, concerned there may now be only 4,500 moose left in the state, which, like Minnesota, is part of the ragged southern border of their range. The suspects are familiar: exploding tick populations, rising temperatures, shorter winters, and flourishing white-tailed deer and their parasites. Without an intensive mortality study of their own underway, New Hampshire’s wildlife managers are watching Minnesota with trepidation.

Signs of trouble are appearing elsewhere in the moose’s southern range. Wildlife managers in Montana and Wyoming are counting fewer adults and fewer surviving calf moose each year. Across the border from Minnesota, populations of moose in southern Manitoba have begun dropping too, prompting the government to cancel the annual hunt in several parts of the province.

“We know we might be too late to stop the decline,” Dave Pauly told me, “but the results of this research can guide our management strategies.” Reducing the deer population and actively managing forests through prescribed burning and logging to create better habitat for the moose could help them withstand the onslaught of parasites and disease.

If there’s any benefit to be gained from the rapid decline of the North Woods monarch, it’s that the moose of northeastern Minnesota have now become the most intensely studied moose population on the planet. What researchers learn from these animals will yield valuable knowledge about the changing ecology of the northern forests.

A week after #075’s death, Pauly already had his eye on another moose, observing movement data from the tracking satellites as her numbers shrank. He was fairly certain she was only hours from death, and he didn’t want to lose her to scavengers. There wasn’t enough time to muster a field team that day, but he and Butternut were only about an hour and a half drive from her location. So he decided to get in and out of the woods to check on her before darkness fell.

He parked in drizzling rain off a forest service road and picked up the radio signal from her collar right away. Picking his way through the mist, he was focused on the blips coming from the VHF receiver, trying to find a good angle down the slope, when he stumbled on a little clutch of bones directly in his path. It was just a few pieces, but he knew what he had found: hoof sheaths, pieces of ribs, and long bones from a very young calf—most likely the offspring of the moose he was tracking. Just a few feet away, he spotted a wolf scat.

Pauly collected the remains and trekked down the hill, flushing two startled ravens and a bald eagle up over the trees. He told himself they were probably scavenging on the rest of the calf parts, still holding a shred of hope that the mother might be alive as the signal from her collar grew more insistent. He saw no signs of a struggle as he approached, no broken branches, no gouges in the dirt, no tufts of hair, or blood, no evidence of any fight. At the edge of the bog, beneath undisturbed branches of alder and willow, he found the mother moose’s final bed. There wasn’t much left to unravel the story of her final days—just her skull, a few bits of tissue, and some gnawed-on bones. Pauly had gotten to her as fast as he could, but the wolves had found her first. Her death might offer no answers, but Pauly would keep looking.