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Photo of pine marten.

Looking into the Lives of Martens

Once nearly wiped out, American martens have regained their place in Minnesota's north woods. A new DNR study seeks to learn more about these secretive mustelids.

By Jason Abraham

Pam Coy and Barry Sampson ignore a biting mid-January wind on the edge of a wooded forest road at the southern end of Superior National Forest. They're absorbed in their effort to tranquilize a live-trapped American marten. The agile, brown weasel bustles about, intent on escaping the cage.

The Department of Natural Resources research biologists gently persuade the bouncing critter to move toward the rear of the live trap. They quickly lower a comblike divider to hold the marten firmly in place as they administer a syringe of tranquilizer and wait for it to take effect. With practiced hands, they remove the housecat-size animal, weigh and measure it, take a hair sample, and attach a radio collar. Then they place the tranquilized marten back in the trap to recover under a makeshift tent warmed by a small propane heater.

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Looking into the Lives of Martens

About 40 minutes later, the marten awakes and once again bounds around. Coy opens the trap and watches as the animal stretches its tubelike body and hurries for cover in a nearby thicket of underbrush.

In the early 20th century, martens were nearly extirpated from Minnesota due to extensive logging of their coniferous habitat and unregulated trapping for their valuable fur. Today the marten population in Minnesota is estimated at more than 10,000, and they occupy most of the area in Minnesota that they did when European settlers arrived.

To maintain these population levels into the future, DNR biologists have planned an ambitious study to learn more about marten (Martes americana), as well as its carnivorous cousin, fisher (M. pennanti).

In the years to come, Coy and Sampson will capture, release, and track dozens of radio-collared martens in the north woods. DNR furbearer biologist John Erb, who is leading the study, says the plan is to gather detailed data on this predator's eating habits, habitat needs, reproduction, and survival. The data will provide the first detailed look at marten ecology in Minnesota in more than 40 years.

"From a management perspective, we need to be particularly careful with the habitat and harvest of fisher and marten," Erb says. "These species have comparatively low reproductive rates and can't rapidly respond to significant changes in their environment. This study will help gather the information necessary to make sure we're managing these species for sustainable populations into the future."

Luxurious Fur

With its stiff, glossy guard hairs and dense, silky under fur, the marten has long been prized by trappers and furriers. When its winter coat grows in completely, the marten is usually medium-brown with darker legs and tail, a light-colored head, and a distinctive orange or white patch on the throat or chest.

Like the furs of its cousin the Russian sable (Martes martes), the furs of Martes americana often adorn the cuffs and hood liners of winter coats in Eastern Europe, Russia, and northern parts of China. The high commercial value of its fur is one reason the marten was nearly extirpated from Minnesota in the 1920s, says Bill Berg, retired DNR furbearer biologist.

"Both marten and fisher were highly valued by homesteaders in northern Minnesota in the early part of the last century," Berg says. "These farmers were barely eking out a living anyway, and a marten fur would bring in a lot of money. If they came across a marten or fisher track in those days, they'd just stay on it until they trapped that animal."

Although there is little information available regarding early marten harvest, there is evidence that harvests declined drastically by the mid-1890s. According to DNR records, a marten captured on the Northwest Angle in 1920 was the last one taken in Minnesota in the early 20th century.

Widespread clear-cutting in the early 1900s also contributed to the marten's demise. It changed the forest in northern Minnesota from mature white pines and red pines to relatively open areas dotted with new pine growth and aspen, Berg says. Because martens prefer mature conifer forests or mixed hardwood forests, extensive clear-cutting and large fires reduce their habitat, according to research in New England and eastern Canada. Such large disturbances remove fallen woody cover important to the marten's prey species such as white-footed mice, red squirrels, and red-backed voles. Factor in a lack of den sites and cover from predators, and open areas can be inhospitable places for martens.

In response to the rapidly decreasing marten population, the Minnesota Department of Conservation closed marten trapping in 1929. Marten sightings were rare in the ensuing decades. In the 1950s, Berg said the sight of a marten near Duluth was newsworthy enough to warrant mention in local newspapers. "As a wildlife biology student at the University of Minnesota in the late 1950s, I remember a professor telling us that marten were probably gone from the state and there probably would never be a harvest season on them again," Berg says.

Surprising Comeback

But the marten's demise wasn't to be. Reports of martens accidentally caught by trappers who were pursuing bobcats, raccoons, and other furbearers began increasing from nearly zero in the 1970s, to around 100 in 1978, and more than 200 in 1979. In the early 1980s, the incidental take of martens fluctuated between 100 and 250 per year.

"They were never listed as a species of special concern, and there were no reintroductions," Berg says. "Basically, there was a little bit of a maturation of the forest and a handful of other changes. It was enough to bring the species back."

Once thought to be a species that could survive only in mature and old-growth forests, marten and fisher alike surprised biologists by thriving in younger and middle-aged forests dominated by dense underbrush with clearings and trees of various sizes. Younger forests may contain mice and voles, as well as an abundance of other potential prey species such as snowshoe hares and grouse. Provided there is adequate physical structure nearby for protection and den and rest sites, younger forests can sometimes be suitable for these forest-dwelling mustelids. An increasing deer population in northern Minnesota also helped, Berg says, because remains of deer killed by hunters and wolves may be an important source of food for martens and fishers in winter.

Harvest by trapping -- one measure of marten abundance -- has increased during the past two decades. When the DNR opened a 16-day trapping season with a limit of one marten in 1985, trappers harvested 468 animals. In the ensuing years, the season length, bag limit, and harvest increased. Today, martens and fishers alike have recolonized most of their historic range. Harvest is legal in the northern half of the state, with most martens harvested northeast of a line between Duluth and Roseau. In 2006 Minnesota trappers harvested a record 3,800 martens.

"Although the martens are doing well there is some indication [based on population models and winter track surveys] that the population may be declining," Erb says. "It's possible that we were harvesting too many. For that reason we took a fairly conservative approach with the trapping season in the past two years."

The decline also highlighted a need for more detailed information on marten ecology in Minnesota.

Closer Look

Martens are among the most efficient and inconspicuous predators in the forest. With long, slender bodies and semi-retractile claws on each of their five-toed feet, they relentlessly pursue squirrels, voles, and other prey through brush piles, treetops, and tunnels beneath the forest floor. Their secretive nature and low population density make estimating their population size difficult, Erb says.

Estimates of marten populations are based on a population model that depends on the annual harvest, which is obtained through mandatory furbearer registration, as well as estimates of survival and reproduction.

The long-term study will involve more direct observation of martens to get a clearer picture of how they survive and reproduce in Minnesota. The results, Erb says, will yield better population estimates, which will be used to better inform decisions on harvest and habitat management. By tracking radio-collared martens, researchers will be able to find den sites used by pregnant females. Then researchers will set up remotely triggered cameras to monitor how often females tend their young and when marten kits emerge from the dens. They will also count the number of kits in a litter by visiting den sites to look inside the dens with a video camera lens mounted on a pole. It is suspected that both fishers and martens use tree cavities, and sometimes hollow logs or brush piles on the ground, for birth and kit-rearing sites.

Erb says the direct observation of female martens and their litters in Minnesota will provide a more accurate picture of preferred den locations, pregnancy rates, and kit survival rates. Like most members of the weasel family (Mustelidae), martens reproduce through delayed implantation. Martens breed in summer within a month or two after giving birth, but the pre-embryonic blastocyst does not implant and begin to develop until late winter. Female martens typically give birth to two to three kits in spring.

Radio tracking will also help researchers investigate causes of death as each collar sends a special signal when it hasn't moved for a specified length of time. Researchers will recover carcasses to determine the specific cause of death based on field sign and a more detailed necropsy if necessary.

Detecting Diet

Erb and his colleagues also hope to pin down details of the marten's diet. To start, they will identify chemical "signatures" of red squirrels and other possible prey species. Relying on the old adage that "you are what you eat" chemically speaking, they will then chemically analyze hair samples from the martens they capture to look for similarities with the various potential prey species. If successful, Erb says this will allow researchers to better understand what prey martens are consuming and whether animals on certain diets may have higher reproductive or survival rates.

"There's not much of the forest that martens can't access," Erb says. "We believe their most common prey is likely small rodents like red-backed voles and white-footed mice. We believe they also take snowshoe hare and red squirrels, and suspect they take various birds and bird eggs.

"We also know that in some areas of the country, they feed on fruits and berries in the summer and fall and may be an important seed disperser, but we don't know how important this food source may be to them."

Researchers will also monitor prey populations each year to better understand how changes in prey abundance translate into changes in marten survival and reproduction.

Early Evidence

Back in the Superior National Forest, after releasing the marten, Coy and Sampson pack their gear and head off to check the next trap. In this first year of the study, the researchers were satisfied simply to get familiar with the study area, protocols, and equipment. Still, they captured 18 martens and tracked them throughout the summer of 2008. Five apparently slipped their radio collars, three were killed by raptors, and one marten traversed 15 miles before its radio signal was lost. Monitoring continues on the nine remaining martens.

At this preliminary stage of the study, conclusions are tricky, Erb says, but early evidence suggests that martens use dens on or below the ground (such as hollow logs, underground tunnels, brush piles, and rock piles) in winter, but they also use above-ground dens (such as tree cavities) during spring through late fall. If confirmed, these findings have important implications for land managers. It also appears that openings in the forest may be hazardous for a marten, as all three martens killed by raptors seem to have been killed near forest openings.

When the full-scale study begins this winter, researchers will attempt to collar 40 martens and 20 to 30 fishers. They will do so annually for the duration of the study, which should yield enough data to eventually draw firm conclusions.

Whatever the study reveals, this secretive creature of the north woods will remain a fascination for Erb and many others who live in northern Minnesota.

"I think there will always be a lot of interest in both fisher and marten," Erb says. "They live in the nooks and crannies of the woods, and seeing one in the wild is unique."

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