Moose Mystery

Wildlife researchers have learned that moose die from many causes. the puzzling demise of seemingly healthy moose has led them to mount a new study of moose mortality.

By Greg Breining

In northeastern Minnesota during the past year, at least five emaciated but otherwise healthy-looking cow moose simply keeled over. So inexplicable and spontaneous were their deaths that researchers call them "tipovers."

Which begs the question: If a moose falls in the forest and no one is there to hear it, does it make a sound?

Each tipover did, in fact, make a sound—a fast-paced beep, beep, beep as a sensor on its radio collar failed to detect motion for several hours. The signal allowed researchers to scramble through the woods to examine each apparently uninjured carcass. It is just such "mortality signals" that Mike Nelson listens for today as he flies over the forest between Ely and Two Harbors in northeastern Minnesota.

It is early April with a sprinkling of snow beneath us in a patchwork of conifers, second-growth hardwoods, and stubble-covered clear-cuts crisscrossed by an occasional highway, logging road, and railroad track.

No mortality signals, so Nelson listens to his headphones for the much slower-paced beep of a healthy, mobile moose. Sitting in the passenger seat of the DeHavilland Beaver with a map, clipboard, and GPS unit on his lap, Nelson moves his finger left, right, up, down—subtly, like a high-roller at a Sotheby’s auction. Wayne Erickson, the U.S. Forest Service pilot who has flown with Nelson for 23 years, responds to these signals, sending the plane into a dive, pulling into a tight turn at 500 feet until Nelson spots the moose on a logging road and punches a waypoint into his GPS. Through the morning we do this—silver Lake Superior see-saws in our windows, the bouncy plane dipsy-doodles like a woodcock, and I, in the back seat, try to heave a breakfast I didn’t eat into a plastic bag as Nelson tallies moose.

These moose are some of the 60 (27 bulls and 33 cows) wildlife biologists darted or netted from a helicopter and radio collared the previous two winters. Now, twice a week, Nelson or one of the others collects waypoints of the beeping moose. The results are downloaded after each flight to produce a map of the animals’ locations and peregrinations. If the receiver picks up a mortality signal, a search party is quickly assembled to find the moose, conduct a necropsy, and try to determine the cause of death.

Mysteriously Dying. Nelson, a wildlife biologist for the U.S. Geological Survey, and his colleagues are trying to answer several questions about northeastern Minnesota’s moose population: How many moose die? At what rate? And from what causes, other than hunting? What proportion of cows are pregnant? How accurate are wildlife managers in counting the state’s moose by airplane?

The five-year study, now in its second year, is supported by the Minnesota Department of Natural Resources, the 1854 Authority, the Fond du Lac Band of Lake Superior Chippewa, and the USGS. The 1854 Authority, a natural resources management agency representing the Bois Forte and Grand Portage bands, has contributed $40,000 to start the study, and Fond du Lac kicks in $12,000 a year. The DNR has spent more than $118,000.

Wildlife managers have concerns over both of Minnesota’s moose populations.

In the northwest, where about 250 moose live in scrubby bogs and aspen parklands, only about 55 percent of the cows are pregnant in any given year, an abnormally low rate. The reason for such poor reproduction? Still unknown, despite a long study, said Mark Lenarz, DNR Forest Wildlife Populations and Research Group leader. Clearly, however, the northwestern population is much smaller than it was a decade ago.

In the northeast, site of the current study, research indicated more than 90 percent of the adult cows were pregnant last year—a healthy, normal rate. By rights, the moose population "should have been going through the roof," Lenarz said. Instead, the DNR estimated 4,200 moose lived in northeastern Minnesota last winter, down from 5,200 the year before. Did the drop suggest a real problem? Or simply an undercount because cows and calves were tough to spot from the air with last winter’s patchy snow?

An unexpected concern was the tipovers. All had full stomachs, yet all were malnourished. None was particularly old. None had obvious injuries, illness, or lethal parasites. They simply tipped over.

Many Ways To Die. A moose’s stature belies the hardships of its life, and its cartoonish appearance, the essential tragedy that a moose can die in many ways.

Most simply, by starvation. Moose on Isle Royale in Lake Superior, numbering as many as 3,000 during the 1920s, overate their range and dwindled to about 500 during a series of hard winters in the mid-1930s.

Most obviously, by predation. Wolves are a chief predator, though they prefer deer and caribou, which are smaller and easier to kill. Moose, long-legged and tireless, glide swiftly through deep snow and over downed timber that a pursuing wolf must surmount or dodge.

When a moose stands its ground, backed up against a conifer to protect its rump, its mane erect, ears flattened, front and rear hooves striking at its attackers, it is nearly invincible. L. David Mech, a wolf expert with the USGS and University of Minnesota, studied wolves as a graduate student on Isle Royale more than 40 years ago. He witnessed 120 instances in which wolves detected and confronted a moose. Thirty-six moose stood their ground, and all survived. Another Isle Royale researcher, Rolf O. Peterson, watched 200 confrontations between wolves and moose. In only 10 did wolves kill their prey. Wolves occasionally exhibit broken ribs and fractured skulls from such encounters.

Once Peterson discovered a moose that had been wounded and remained standing for seven days as wolves waited nearby for it to die. On another occasion Peterson and his pilot found a pack of wolves near a bull moose that, they discovered when they put down on the Lake Superior ice, was blind. They checked throughout the day, but wolves and moose never made contact. The next day, however, they again spotted the blind moose, standing on the ice, surrounded by wolf tracks. They saw no blood; no obvious wounds. The sound of the plane triggered stomping and kicking—the means by which the moose had beaten back its attackers. On day three, they again spotted the moose, still standing, farther out on the ice, surrounded by new wolf tracks. A snowstorm blew in and it was three days before Peterson could fly again. This time, the moose—and the ice on which he had been standing—were gone, apparently carried out into the lake by the wind. Superior, not the wolves, finally claimed the old fighter.

Wolves are not the only predators of moose. In southeastern Alaska, swimming moose are occasionally eaten by killer whales. Elsewhere, grizzlies and even black bears are deadly, especially to calves. Research in east-central Saskatchewan revealed black bears killed nearly half the year’s crop of calves.

Much of the sparring and fighting between bulls during the autumn rut is ritualized, to test dominance without the danger of a full-tilt battle. But occasionally bulls get into it. Mismatched antler tines put out eyes. The swipe of an antler causes festering puncture wounds, and even slashes from which intestines ooze. Battered moose die of exhaustion, dehydration, and blood loss.

Moose are killed by cars, at the rate of about 70 a year in Minnesota. In winter they are attracted to road salt and the easy traveling of a plowed roadway.

For the same reason—to travel a clear path—moose follow railroad tracks. They may flee a speeding train by running along the tracks until they are overtaken. During the rut, testosterone-addled bulls may challenge an oncoming engine. "The locomotive would blow at them and at times they would stand their ground and charge the locomotive," said Dale Reno, road master for the Duluth, Missabi and Iron Range Railway in northeastern Minnesota. "They would lose."

Moose die in accidents of their own making. They break through ice and drown. They tumble over cliffs, sink into mud, and become entangled in fences. They become trapped in deep, steep-sided, insurmountable "foraging trenches" created as moose pace back and forth in deep snow. While browsing, they catch their legs in the forks of trees and then starve. Moose have even been found rendered immobile by "snowballs"—huge accumulations of snow on the wound site of a fractured leg.

Mighty moose are laid low by various ignominious illnesses and parasites. (If you are eating, put this aside.) They are susceptible to brucellosis, pus pockets, pinkeye, abscesses in the lower jaw, degenerative joint disease, osteoporosis, toxoplasmosis, parasitic flatworms, liver flukes, various species of tapeworms, tissue worms, lung nematodes, gastrointestinal nematodes, arterial worms, footworms, abdominal worms, and rumen worms. Throat and nasal bots (beelike flies) occasionally land on their muzzle to produce larvae that then crawl into nostrils and throat. Moose are plagued by horseflies, blackflies, mosquitoes, biting midges, moose flies, deerflies, snipe flies, and stable flies.

But of all the pests to pester a moose, only three seem to cause their death.

A tapeworm that inhabits the guts of wolves passes through feces to land or water, where it is ingested by the moose, its intermediate host. It travels by bloodstream to the lungs, where it forms hydatid cysts, often reaching golf-ball size. Several dozen cysts may clog a moose’s lungs enough to reduce stamina and make the moose vulnerable to predators.

The winter tick, which preys only on moose, does its damage during winter, when moose struggle with food shortages and severe weather. Up to a dozen ticks, each engorged female growing to the size of a peanut, may cling to a square inch of moose hide. Ticks may total up to 100,000 on a single moose. Infested moose rub against trees continually, eventually losing much of their hair before the ticks drop off in spring. So-called ghost moose may lose more than three-quarters of their pelage. Researchers don’t know whether ticks often kill moose, but some suspect the act of grooming, by consuming energy and distracting moose from predators, might be more damaging than hair loss.

Finally, moose are susceptible to "moose sickness," caused by a meningeal nematode (brain worm) passed in a complicated cycle from white-tailed deer to moose via snails and slugs. The worm, which bores into the spinal cord and membranes surrounding the brain of its mammalian host, is harmless to deer but debilitating to moose. The afflicted animal stumbles, charges for little reason, and walks in circles. Moose sickness is often blamed for far-flung wanderings of some moose, such as two that roamed from northern Minnesota deep into Iowa and were killed, one shot by a poacher and the other struck by a semi.

According to a summary of moose mortality in northeastern Ontario over several years, nearly half that died were killed by traffic, 10 percent by trains, 10 percent by poachers, 17 percent by subsistence hunters, and fewer than 3 percent by predators.

No Moose. Sometimes you might think a moose is dead, but it’s not. In mid-April Nelson, flying his survey, detected two mortality signals. Lenarz rounded up a search party and hiked in the next day. Following the first signal, he found the beeping collar with a hint of blood, but no bones, no hair, no moose. Perhaps the moose had been killed by wolves. Wolf pups can carry radio collars for hundreds of yards as trophies, like spaniels with their owner’s socks. Or perhaps the collar had been able to slip over the moose’s head because of the loss of hair from winter ticks.

The second signal led searchers into a cedar swamp. They followed the signal on a straight bearing for two hours. Then the source of the beep seemed to move off to one side. The direction changed 90 degrees again, and Lenarz realized the moose wasn’t dead at all—its collar was stuck in mortality mode.

By last spring one collared bull had been shot by a hunter. A bull and two cows had been killed or scavenged by wolves. Two collars were found without their moose. Signals from two bulls simply disappeared, as though the bulls had wandered out of range.

"We’re just going to stay tuned," Lenarz said. "It’s conceivable that they made a long movement and we haven’t picked them up yet."

And, of course, five cows tipped over. Their deaths are most alarming and puzzling.

As study moose die, researchers hope to come up with the money to capture and collar replacements. Meanwhile, they will track their moose, dead or alive. After annual aerial surveys, they will compare the number of radio-collared moose they know to be in the woods with the number spotted from the air to see if aerial surveys are significantly underestimating Minnesota’s moose population. Blood and fecal samples taken at the time of capture will be used to determine the pregnancy rates of study animals and may lead to a field test using moose pellets to determine pregnancy rates.

In the end, the researchers hope to answer these overriding questions: Why do moose in northeastern Minnesota appear to be in decline? What explains their mysterious deaths? How might the herd be better managed?

These are difficult questions. For not only is Nature a complicated laboratory. But also, there are many ways to kill a moose.

Greg Breining is a freelance writer. His most recent book is Wild Shore: Exploring Lake Superior by Kayak. Visit his web site at gregbreining.com.