Climate Change Enters Its Blood-Sucking Phase
As winters grow warmer in North America, thirsty ticks are on the move.
We found the moose calf half an hour in. He lay atop thin snow on a gentle slope sheltered by the boughs of a big, black spruce, curled up as a dog would on a couch. He had turned his long, gaunt head to rest against his side and closed his eyes. He might have been sleeping. The day before, April 17, 2018, when the GPS tracker on the moose’s collar stopped moving for six hours, this stillness had caused both an email and a text to alert Jake Debow, a Vermont state field biologist who stood next to me now with Josh Blouin, another state biologist, that moose No. 75 had either shucked his collar or died.
“You want pictures before we start?” Debow asked me. He’s the senior of the two young biologists, both still in grad school, both in their late 20s, young and strong and funny, from families long in the north country, both drawn to the job by a love of hunting and being outside. Debow had always wanted to be a game warden; in college, he “fell in love with the science.” His Vermont roots go back 10 generations. “Jake Debow,” Josh told me, “is about as Vermont as you can get.” It was Debow’s second season on the moose project, and Blouin’s first. This was the sixth calf, of 30 collared, that they’d found sucked to death by ticks this season. They were here to necropsy the carcass, send the tissues to a veterinary pathology lab in New Hampshire, and try to figure out as much as possible about how and why these calves were dying.
First, they weighed him. To the trunk of the big spruce they strapped a custom-made scale—a steel ell with three pulleys and a thick rope to which they hooked a spring scale. They wrestled the moose onto a heavy net, collected the net’s four corners, and with the triple-pulley system and considerable effort, hoisted him off the ground. “Any guesses?” Debow asked after they’d secured the rope. The moose swung slowly just above the snow. I asked what this ten-month-old calf would have weighed if healthy: about 400 pounds. Blouin guessed 286. Debow said 312. I said 299. Debow looked at the scale. “Two-seventy. Lightest one yet.” The ticks had taken a third of this animal’s weight.
They gently lowered the calf to the ground and pulled him from the net. Debow took a six-inch steel ruler from his jacket pocket, kneeled behind the moose’s shoulder, and, with his hands, parted the fur and held it down, as one might hold a stiff-spined book to spread its pages, to expose a narrow, tick-width channel of skin some six inches long. Rather, he exposed not the moose’s skin, but some 50 ticks that completely obscured it. A few were male moose ticks, which sport a jagged fan pattern of tan and brown. Most were females, which have tan heads and tan legs that are conspicuous next to their dark, raisin-colored bodies. Beginning early in April, pregnant females living on a moose—and most females are pregnant—take a “blood meal,” sucking themselves full for days in anticipation of dropping to the ground to lay their hundreds of eggs. About half the females on this moose were in that engorged state. Swollen to 10 times their normal size, these ticks were stretched until they turned pale, like raisins morphed into cadaver-toned grapes. When a moose so infested rises from sleep, the imprint left behind in the snow is dotted and smudged with spots of blood.
Debow, wielding his ruler, called out square-centimeter tick counts to Blouin: nine ticks in the first square centimeter, seven engorged; eight in the second, none engorged; 28, four engorged; eight, none. This count would later produce an estimated infestation total of just under 14,000 ticks. This was actually far fewer than they often found, but it was enough to render this calf chronically anemic from January through March and then acutely, fatally anemic in the last couple of weeks of his life. In April, when the gravid females start taking their blood meals, the blood loss over their last two to four weeks aboard the moose “can equate to a calf’s total blood volume,” according to one recent paper—some three and a half gallons. As Inga Sidor, the New Hampshire state veterinary pathologist who processed the tissue samples Debow and Blouin took that day, later told me, the ticks “literally bleed the moose to death.”
Two months before, in mid-March, I had gotten my pre-fieldwork briefing from Vermont’s moose-team leader, Cedric Alexander, at his house in the snowy countryside in Cabot, Vermont. Smack in the middle of the northernmost quarter of the state, Cabot is an apt spot for Alexander. One of 11 children (four of them twins), he was born in the 1950s in the state’s wildest, emptiest, northeasternmost county, Essex, a place of mostly farms and timberland. He grew up fishing for trout and hunting deer, rabbit (with a beagle), and upland birds. When he was in middle school, his family moved across the state to the most citified, populous county, Chittenden, where he eventually earned a degree in wildlife biology at the University of Vermont. He has spent almost all of the 37 years since working as a state wildlife biologist, ranging around the physical and social landscape between Essex County and Burlington. He still believes in hunting—in its power to compel intimacy with a landscape, its honesty about where meat comes from, its world-erasing focus. He looks forward each fall to deer and moose season partly because he gets to staff the department’s weigh stations, most of them at the back of small-town general stores or gun shops—the Rack N’ Reel, Pauline’s Quick Stop, Mr. O’s Sporting Goods—where he queries the hunters who bring their take to be weighed and examined. Good stories, he says, and good intel.
Alexander is a tall man, lean and pleasant, relaxed at home. He was making coffee when I arrived, dressed in jeans and a green-and-black plaid shirt of heavy wool. After stirring us each a cup, he led me to a den beyond the living room. We sat on a big ruby-red modular sofa that framed a tinkling woodstove. Otherwise, it was dead quiet. Out the window, a sun-glinted snowfield sloped away under a brilliant blue sky.
Alexander got into wildlife because he loved birds, he said, and still does. But in a department organized along vernacular lines, he ended up on neither the feather nor fin team, but the fur. That was 1980. At the time, he said, “We didn’t have any moose.” Close; the official estimate that year, I later found, was 200. Moose, along with beavers, bears, fishers, eagles, and many other animals that were once abundant all over northern New England, had been killed off and driven from Vermont, New Hampshire, and most of Maine by the 1800s through uncontrolled hunting, trapping, water pollution, and logging. This environmental destruction, two centuries in the making, was the first of the ecological casualties of North America’s European immigration, and it remains one of the most astounding. When Europeans arrived, 80 percent of northern New England was forested. By 1850, using just handsaws and fire, they had stripped away three- quarters of that—almost 16 million acres—and left only 20 to 25 percent forested.
Another reversal was to come. Starting in the mid-1800s—at first accidentally, as post–Civil War farmers fled west for better soil, and then more and more intentionally—the people of New England righted these ratios; by 1990, the region was 85 percent forested. This recovery, the noted environmentalist Bill McKibben has written, is one of America’s greatest and perhaps least-recognized conservation victories. In the past 50 years, and especially the past 30, many of the species driven out centuries ago have returned. They came down from Canada or over from lands west to which they’d retreated: eagles, peregrine falcons, other birds of prey; pine siskin and black-throated blue warblers; salmon and brook trout; beavers, bears, bobcats, fishers; and moose.
The moose was the last to reappear. Perhaps because of this late return, and definitely because of their strange magnificence, the moose, Alces alces, holds a singular place in the hearts of New Englanders. In a land of straightforward, understated, and sometimes stubborn people, it’s a straightforward, understated, and sometimes stubborn beast.
Adult moose are huge, with females averaging more than 800 pounds and males about 1,100, and some weigh as much as 1,700 pounds. Yet this ponderous, overbuilt beast can run as fast as 35 miles an hour, which would pose some competition to most elite racehorses. See ya, Secretariat. Even belly-deep snow barely slows a full-grown moose. In water, the same moose could swim neck and neck with Michael Phelps at his fastest, about six miles an hour—and after a couple hundred meters, as Phelps fades, keep that pace for another two hours. Their long front legs let them easily clear fences or downed trees. YouTube has a clip of one jumping over the hood of a car as if it’s a puddle.
These and other charms made the returning moose an official mascot in Maine, an unofficial mascot in New Hampshire and Vermont, and a sort of sacred beast throughout New England. Three years ago, in my adopted hometown of Montpelier, Vermont, I emerged from the hardware store early one summer Sunday morning and watched as a moose wandered casually down the empty Main Street sidewalk before me—just me and her at the moment—gazing at store displays and peering into the local dive, Charlie-O’s (“Good Drinks and Bad Company Since the War Between the States”). We parted ways at Main and State. Later I learned that when someone called the police saying that a moose cow was looking confused on State Street, a cop drove over, flipped on the party lights but left the siren silent, and from a respectful distance, slowly escorted her past the golden-domed statehouse to greener spaces beyond.
Moose had been back for about a decade when Alexander and his moose team recognized that they’d soon have to allow hunting to keep the northernmost population in check. In Alexander’s old hunting grounds of Essex County, as in New Hampshire and Maine, the moose, with no predators and abundant browse, were eating so much low-growing greenery that they were reducing the forest’s value to not just landowners but other wildlife, from birds to bears, that counted on low growth for food and cover. Maine started hunting moose in 1980, New Hampshire in 1988, and Vermont in 1993. This provoked passionate objections. Moose had been left to thrive for so long, and could be approached so easily, that many people felt it unsporting to hunt them. A deer would generally bolt at first sight. A moose would stand and stare.
Even with the hunt, though, the moose population grew denser, especially in the north. By the mid-2000s, they were thick enough that Maine, New Hampshire, and Vermont together were having a thousand moose-vehicle collisions a year. Some of these collisions—usually a moose through the windshield, and in some cases a moose and a motorcycle—killed people. The moose had gone from something you had to take pains to find to something you worried about smacking into.
This was the case in Vermont’s northern region in the 2000s, Alexander told me. By then, Vermont had some 5,000 to 6,000 moose, with almost half in the state’s Northeast corner. The department decided to cut the overall population to about 3,000, primarily by increasing hunting permits in Essex and adjacent counties. For most of the decade, they increased the number of permits given each year, with a peak 1,251 distributed in both 2007 and 2008 after hunters bagged a record 648 moose in 2006. It seemed the only way to check the growth.
Even as Vermont ramped up hunting, however, state biologists to the east, in moose-heavy New Hampshire, saw signs that moose numbers there were leveling off. To find out why, a research team led by the wildlife biologist Pete Pekins of the University of New Hampshire put radio collars on 92 moose cows and calves each year from 2002 to 2005 and tracked them to measure survival rates and habitat use. Amid a stack of findings that seemed perfectly normal, two things stood out: In 2002, the study’s first year, fully half of the calves in the study died in the spring from heavy tick infestations, and the preceding winter had been mild and late in coming, leaving the forest floor free of the snow that usually arrived early in fall, when tick nymphs were looking to attach to wandering mammals. Those two factors seemed to explain New Hampshire’s slowdown in moose expansion. As far back as the early 1900s, moose biologists had noted that late-coming warm winters, such as that of 2002, sometimes led to tick irruptions that killed young moose. But such winters were rare. Like droughts, then, these weather-related tick irruptions fit into a pattern of rare stressors that had little long-term impact.
But was it weather related in this case—or climate related?
At the time, in the early 2000s, climate change was being heavily studied as a potential disruptor of New England wildlife, but the focus in current-day wildlife management was still on how to handle the expansion of formerly depleted species, including moose. In a 2002 paper titled “Wildlife Dynamics in the Changing New England Landscape,” for instance, a particularly prominent team of regional ecologists at the Harvard Forest research area concluded that the main challenge for managing the moose and other large mammals in the region was their continued expansion. No one was worried yet about climate dinging moose populations. As Pete Pekins would later note, “Moose managers concluded that the winter tick epizootic in  was an anomaly and that the moose population would make a strong comeback in subsequent years, compensating for the losses.”
It soon became apparent that they had badly underestimated both the possibility of rapid climate change and the explosive effect that change could have on both moose ticks and young moose.
The moose tick, a.k.a. the winter tick—or Dermacentor albipictus, to use its aptly sinister, Potteresque Latin name—feeds on almost every mammal across the reasonably wet parts of North America. But it has an outsize effect on moose. Its life cycle in the northern New England climate is fairly simple. In April, female ticks who have feasted on moose over the winter take their last blood meal and drop off into the leaf litter to deposit several thousand eggs apiece. In May, as the forest starts to leaf out, these eggs release tiny, six-legged larvae called seed ticks. Over the summer, they live on the nutrients from their mothers’ winter feast. Around September, these seed ticks start to form loose groups of up to 1,000, which then climb trees or shrubs up to heights of about four to six feet. There, these groups of poppy-seed-sized ticks, having linked their tiny limbs to form long, almost invisible chains, go out on a branch and, as tick biologists call it, “quest.” They simply wait, and when a big, tall, blood-filled mammal walks by and brushes the branch, one or more of the ticks grasps the animal’s fur and holds tight while the rest of the gang swings as a gossamer-thin thread onto the animal. Then they separate, spread out, follow fur to skin, and dig in.
Two main factors influence how many of these ticks a New England moose will pick up in any given year and area. The biggest factor is the weather from roughly October 1 to January 1, when the seed ticks are questing. A lot of cold and snow during that period, especially early on—normal weather for moose terrain—decimates questing ticks, so that wandering moose are apt to pick up only a few hundred.
The second crucial factor is moose density—that is, how many moose live in a given area and distribute in spring the pregnant tick females whose young will quest in the fall. More wandering moose per square mile increases not just the number of eggs laid, but the number of places they are laid—and thus the number of times that any given wandering moose will encounter questing ticks. When moose are many and winter late, young calves will meet far, far more strands of questers than usual—entire curtains of them—and end up carrying anywhere from 10,000 to 100,000 through the winter.
Tick loads so large are unique to moose, perhaps because moose live almost exclusively in places where warm winters are rare, and have developed no defense against such infestations. Unlike their white-tailed deer cousins and most other furry mammals (including humans) that range more widely, moose don’t groom one another, and they are not habitual or “obligate” groomers; they groom only themselves, and only when heavily infested. By that time, alas, the ticks are on to stay. In a warm fall, then, a dense moose population seems to prime tick populations for an explosion, and calves for a slow, sucking slaughter in the coming winter.
To the dismay of Cedric Alexander, Jake Debow, Pete Pekins, and many others, such winters became far more common just as moose were growing denser than ever through much of their upper New England range. In the mild, late-coming winters of 2008 and 2011, moose biologists in all three northern New England states, now on the alert, saw signs of more ticks and higher calf mortality. By 2013, New Hampshire and Maine had started an updated, five-year version of the earlier winter moose-collaring study, which Vermont joined in 2015. It is for this study that Jake Debow and Josh Blouin are doing field autopsies on dead, collared moose calves.
The study’s results so far are sobering. Before every one of the study’s first three winters (that is, the winters that carried into 2014, 2015, and 2016), the autumn was warm and short on snow; and at the end of every one of those winters, mainly in April, ticks sucked the life out of more than half of all collared calves in all three states. In 2016, when much of the study area had received very little snow before January, 80 percent of the collared calves succumbed.
The ticks didn’t just kill calves. Their depletions made cows smaller and less fertile. In the decade ending in 2005, when ticks started to take their toll, Vermont moose cows averaged about 575 pounds. By 2015, that dropped to 525—a loss of almost 10 percent. Their ovulation rate dropped some 25 percent, to only 0.67 ova per cow in 2015—the lowest rate ever recorded, and the second consecutive year it was below one. The cows also brought fewer pregnancies to term. Cows, in other words, were having fewer calves to start with, and fewer of those calves were surviving their first winter.
Would cold winters make a difference? They did. In the fall of 2016, the first sustained snows began in October, and calf mortality the following winter—2017—dropped to 30 percent.
But the larger pattern remained alarming. While no one systematically counted tick loads from 2008 to 2012, the fall and winter weather in those four years roughly matched the warm, tick-friendly weather seen in the winters of 2014, 2015, and 2016. This means that, as of October 2018, five of the 10 most recent winters had probably produced epizootic events that killed more than half the calves. Moose in the wild generally live a bit longer than a decade. An entire generation of New England moose might have a top-heavy age structure. And it could get worse, both in New England and in moose range elsewhere in North America.
What is to become of these animals?
With the ticks counted, Debow and Blouin pulled the moose onto a flatter spot. “We’ll go tongue to tail,” Debow said, “a full field necropsy.” They would find and observe every organ, note its condition, and take and pack samples to send to the University of New Hampshire veterinary diagnostic lab for microscopic examination. They would try not to slip and fall into the moose.
They laid the moose right side up, because the organs tend to present themselves better that way, and then set about removing the two right legs. Blouin, holding a leg near the hoof, would lever it ever farther away from the torso as Debow used a scalpel-sharp, white-handled, eight-inch fillet knife to carve through the basketball-sized mound of muscle holding the leg to the joint. It took a few minutes to cut through all the muscle, ligaments, and finally the joint itself. Blouin would then lift the released limb, itself almost as long as he was tall, and lay it gently on the ground.
Debow next removed the hide from the animal’s right side. After slicing through the skin along the abdominal midline, he started the knife up at the top of the opening where the front leg had been and shimmied it down along the crease between skin and chest wall, slicing the subcutaneous connective tissue so that he and Blouin, pulling steadily on the animal’s thick, tick-encrusted hide, slowly peeled it away. In less than five minutes, they had exposed the calf’s rib cage and sack of belly. Blouin picked up a heavy pair of Fiskars tree-branch loppers and snapped through the top of each rib near the spine, the loppers making a crunchy cracking sound each time, then again at their junctions with the sternum. Finally they lifted the right half of the rib cage out as one piece and set it on the snow, concave side up. With its smooth, deep-red intercostal muscles and elegant curves, it looked like a huge, glossy serving dish.
The organs of the chest and abdomen now presented like an anatomy diagram. Despite the gore, the smell of digestion, and the animal’s emaciated state, the calf’s innards possessed an acute and unexpected beauty. His depletion—his body’s desperation to extract from itself every joule of energy—had turned the calf’s epithelia, the thin, stretchy linings that surround many organs, and which normally have a milky translucence marbled with pallid blotches of fat, into a gorgeously clear membrane. It was like a shrink-wrapped looking glass. When Debow pulled back the calf’s head and opened the underside of its throat, the animal’s thick windpipe, so cleanly displayed and perfectly formed and isolated, had the quality of a museum piece—with futuristic overtones, in its distinct, hoselike, mathematically regular segmentation, of the bones and strange tubes of the monster in Alien. No atlas or animation ever so beautifully displayed a trachea.
When possible, they dismantled him in systems. The entire trachea and the lungs soon lay in one piece on the snow. Blouin, snipping a sample of lung tissue for the lab, showed me the thin strands of lungworm: less a cause of illness than a sign of the moose’s failing defenses. It went in a sample jar. Debow showed me a fat slice of the kidney, inch-thick like a steak but far smoother, its two lobes a glistening cherry red, brilliant and beautiful in its symmetry. The four chambers of the stomach—the moose is a ruminant—were sliced open and lain flat. The lining of each stomach is smoother and more granular in structure than the one previous. The rumen was like a 1970s shag carpet, color included: a deep pea-soup green from the moose’s chawed, swampy vegetarian fare. The reticulum was more a modernist honeycomb; the omasum, minimalist, with stiff, short spikes like a startled pufferfish. Finally, the abomasum, far fleshier and less green, was folded into long, curved, anchored leaves that splayed, as Debow’s anatomy teacher taught him, “like the pages of a Bible opened on a stand.”
At the end they showed me the brain, first in its cup, with furry ears still behind it, then scooped out and held in Debow’s black-gloved hands. It looked distinctly human. My cortex couldn’t help but wonder what had been in his.
When done, Debow and Blouin realized they had forgotten to bring a garbage bag in which to carry out the hide and some other parts. They decided to leave all that here. The site was a bit of a slaughterhouse: a leg as long as Debow; the huge head, sans cranium, as big as many a human torso. The entrails, the rib-cage serving dish, and the main carcass with its underside and two attached legs still befurred. Most of it would be gone in three days, Debow said, if not before, as soon as the coyotes found it.
In May, when the snow was gone and the dead counted, the Vermont moose team found that of the 29 calves in their study that winter (one having shucked its collar), 15 died of tick infestation. Fifty-two percent. New Hampshire and Maine had similar results. Northern New England’s moose had officially had their fourth tick epizootic event in five years, their sixth in 11.
Over the summer, the Vermont moose team, pondering this mess and reading the literature and talking to colleagues nearby and elsewhere, reached the uncomfortable conclusion that the only way to slow this wintry slaughter was ... to let more people shoot moose in the fall.
They were painfully aware that this would strike many people as nuts. Increase hunting to rescue a failing wildlife population? It appeared to be not just counterintuitive but contrary to the department’s straight decade of shrinking the hunts. From 2008 through 2018, as Vermont’s moose population dropped from almost 5,000 to fewer than 2,000, the department had reduced permits from 1,251 (in 2008) to just 13 for the 2018 season, which itself was down from 80 the year before. (Even for 2018, they had to settle on 13 because they couldn’t get 14 approved.) Seven moose were taken. Walter Medwid, a founding member of the Vermont Wildlife Coalition, told Vermont’s Seven Days in early 2018 that he was baffled that the Fish and Wildlife Department “continues to feel the need to put hunting pressures on a species that is truly imperiled.” The moose population had plummeted straight past the target population in 2015—and they were going to hunt more? How the hell was that supposed to work?
The answer lies in the aforementioned relationship between moose density and tick numbers per moose. The magic density number seems to be about 0.75 to one moose per square mile, according to Pekins; let moose get denser than that, and you exponentially increase the moose’s average tick load and the calves’ mortality rate. Thus, as it were, the more moose, the fewer moose.
Fish and Wildlife can’t do much about snow cover. But through hunting permits, they can do something about moose density when needed. On February 27, the department recommended to its board that no moose-hunting permits be issued this year to allow for uninterrupted research.
They could, of course, just let repeated tick irruptions reduce moose numbers. That would take longer and be less controllable. And it would leave unaddressed the huge hole that tick epizootics are eating in the moose population’s age structure, with ever fewer moose of prime reproduction age—a trend that could make the moose population older, less fertile, and less resilient at a time when it is facing new challenges.
To some people, of course, letting the ticks do the job might seem a more natural solution. It’s not, of course. One of the tragedies of this dilemma—the essence of it—is that whether we shoot the moose or let the ticks suck the young dry, it is we humans, whether through gunshot or climate change, that are killing moose. Jake Debow thinks the hunt would simply be more humane. These tick-ridden calves, he notes, don’t just lie down peacefully one day and die. They suffer for months. He has found sites where coyotes came upon a tick-weakened calf and took it apart. “There’s always blood,” he says, “all over the place.
“And if you’ve seen one of these calves on the verge of death ...” We were back at the truck now. He thought a minute and then pulled out his phone. “Probably shouldn’t do this. But here’s one I came across last spring.” The video shows a pathetically emaciated calf, probably only 20 feet away from Debow as he filmed it with his phone—far closer than a healthy calf would allow—trying to move away among some young trees. Except the calf can barely move. The slope is gentle and the snow only a couple of inches deep. Yet the calf, his legs a-wobble and his head moving uncertainly, looks ready to fall at any second. Repeatedly he tries to lift a foreleg and stride forward, but cannot. He seems to sense that if he falls, he will not rise. After several seconds, he finally succeeds. One step. Debow followed him until dark, he said, before he had to head home. The next morning he found the calf dead, just a few yards from where he had last seen him.