Everyone is quiet. Our hike has gone from a casual stroll through the northern Minnesota woods to a silent, slow creep. That's how I know we are close to the bear's den. The four researchers ahead of me, one of them holding a VHF antenna high in the air, are searching for the signal of a GPS collar. They have tracked the bear wearing it since 2016, but now she has created a new den, one that the research team has never been to. I lean against a tall paper birch tree and cringe as my metal water bottle hits a zipper on my jacket, sending a sharp clank out into the silence. The researchers communicate in hushed whispers and hand gestures before making the final movement: a single index finger pointing down toward a hole in the snow. We have found our bear den.

Conducting research on wild animals has never been easy, but when faced with tracking one of Minnesota's largest mammals, the black bear (Ursus americanus), scientists must take extra precautions. Although it is winter and the bears are in the midst of hibernation, they have developed an adaptation that allows them to wake suddenly from their slumber. Thus the whispering. I almost let out a laugh as four snowmobiles whiz nearby, completely oblivious to our cautious crawl. Luckily for us, the bears don't seem to mind.

Some black bears build open nests on top of the forest floor, some crawl into cavities under brush piles or the roots of fallen trees, and others excavate their own dens. This particular den is dug into the earth, hidden under a mound of snow and surrounded by young saplings. It's just deep enough to fit three yearlings and their mother.

I hold my breath as Andrew Tri, a research wildlife biologist for the Minnesota Department of Natural Resources, kneels down, carefully clears snow from the den entrance, and covers it with a large black cloth to keep researchers' silhouettes from rousing the hibernating bears. Then, to my complete surprise, he sticks his head under the cloth and inside the den.

"There was no tense moment to speak of; I just calmly looked in, stayed quiet, and looked for a place to administer the Telazol," Tri later tells me, speaking of the drug they use to anesthetize the bear. "Sometimes a bear is more aroused, but almost always, they look toward you, try to determine what's going on, and then put their head down again."

I follow suit and crawl underneath the black cloth to peer into the den. When my eyes finally adjust to the darkness, I'm surprised by the serenity of the scene in front of me. Twisted roots dangle from the den's dirt ceiling above a giant sleeping black bear. She is curled up like my family's dog used to do, and she is gently snoring. Tucked behind her, hidden from view, are three hibernating yearlings. We lie there for a moment, the two of us, before I pull my head back up above the snow.

The Minnesota DNR and the University of Minnesota Department of Surgery have been collectively sticking their heads into active bear dens since 2001 to gather information on these uniquely adapted mammals. Their team is made up of professionals from a variety of backgrounds, including Dave Garshelis, an international award-winning conservation biologist; Paul Iaizzo, a systems physiologist and professor of surgery; Tim Laske, a biomedical engineer; and Tinen Iles, an assistant professor of surgery. Over the course of several generations of bears, they have been using surgically implanted cardiac biologgers to record data on exactly what happens when a bear hibernates. The bears are given GPS collars to allow the researchers to track individual bears and gather information about their range and habitat.

The scientists believe that a better understanding of the basic physiological mechanisms of hibernation may lead to breakthroughs in human medicine. Their research could support scientific advances that help prevent heart failure, shorten post-operative recovery time, improve muscle retention, or aid in wound reconstruction.

"What we have in fact learned has been largely unanticipated and unexpected," says Laske. "Bears may be the most interesting model of hibernation as it relates to human applications."

Black bears are one of Minnesota's finest hibernators. When temperatures begin to drop, they spend their time fattening up and preparing to live in a state of dormancy for as long as five or six months. During this time they will not eat or drink aside from the occasional bite of snow, and they typically don't urinate or defecate. They will enter a state of inactivity with suppressed metabolism and slowed heart rate and breathing. Yet as it turns out, the hearts of black bears do more than was previously understood.

"These bears don't simply have a slow heart rate in the winter. Rather, they have a few fast beats during inspiration [inhaling] and then rest the heart between breaths—a respiratory sinus arrhythmia," Laske explains. "The fast beats maintain blood flow to the brain and organs, and presumably provide a level of exercise for the cardiac muscle."

Other large mammals including humans have respiratory sinus arrhythmias, Iaizzo says, but none are anything like those found in the black bear.

"As a human, your resting heart rate is typically 60 beats per minute," says Iaizzo. "If you inhale, it might go up to 61 or 62, but with bears, it's quite different. Their resting heart rates during hibernation are about 5 to 10 beats per minute. Then, as they inhale, the heart rate will go up to 60 or 70 beats per minute. It will stay elevated for about five beats, and then as they start exhaling, their heart will pause, and that pause can be between 5 and 30 seconds long. So it's really pretty remarkable."

This pause in between heartbeats proves that what we learn about hibernation as children—that bears are getting a nice long sleep through the winter—is far from the truth. When the bears emerge in the spring, Iaizzo says, they are likely not well rested.

"If your heart stops between every breath ... you'll never have that normal sleep pattern," he explains. "Basically the bear will kind of be passing out. In the spring, they're getting out of this respiratory sinus arrhythmia and going into a more normal sleep behavior to recover from the lack of sleep during hibernation."

This unusual cardiac activity is also connected to the bears' ability to wake suddenly during hibernation. This adaptation is thought to be a fight-or-flight mechanism that allows bears to protect themselves from predators, especially wolves, during the winter.

"The bears can rapidly increase their heart rate. If they hibernated at a lower body temperature, they wouldn't be able to do that," says Garshelis, the DNR biologist who's been leading Minnesota black bear management research since 1983. "That can make our work harder, because sometimes they will bolt from their den at our approach—especially an open nest den. We must approach very quietly, and even then, some bears are alert enough that they detect us."

The team's medical researchers are intrigued by this ability to wake suddenly from a state of inactivity and immediately resume normal function. They have recently begun working with Dr. Andrew Shaffer, a cardiothoracic surgeon at the University of Minnesota, to connect the data to potential human benefits.

"The fascinating thing about the research ... is that these bears serve as a good analogy to humans after surgery in the ICU," explains Shaffer. "The bears are able to regulate their metabolism and function at full capacity if a threat comes near their den. As a comparison, humans in the ICU take weeks to be able to breathe, eat, or walk again. If we could bridge the gap between the difference, we could make some amazing advances in medicine."

The team has discovered other physiological mechanisms with potentially groundbreaking implications. Black bears have blood that takes three times longer to clot in winter than in summer, are able to give birth during hibernation, and can conserve muscle and bone mass despite six months of disuse. The scientists even discovered that the bears are able to rapidly heal wounds during hibernation. Some bears have been found with significant injuries in December that have all but disappeared by March.

"The bears look great, like nothing ever happened," says Iaizzo. "They don't put scar tissue down, they actually remodel it, which is pretty amazing. You and I—or, say, a hibernating ground squirrel—would just put down scar tissue, and then it would take months and months to do a reverse remodel of an injury that big."

While these findings are promising, this type of research is not without its challenges.

One of the main complications of the study has been the rejection of the cardiac biologgers from the bears' bodies. The devices are sterile, use the same materials used in human medicine, and are rarely rejected by humans or in preclinical research studies, says Laske. Biologger devices have been used to collect data from many large animals without complication, including wolves, moose, lynx, and even elephant seals. However, the free-ranging black bear in Minnesota remains resistant, sometimes rejecting and expelling the monitors. The biologgers have a battery life of about three years, meaning a rejected implant can result in the loss of important data.

"It is an inherent response to a foreign material under the skin, much like a splinter," says Laske. "We don't think the bears remove the devices [themselves] since many of them come out more than a year after implantation."

Biologgers that remain intact give researchers additional insights into bear life outside the den. One thing they've noticed is that human development affects free-ranging black bears. Structures such as roads and railways are responsible for significant increases in the bears' heart rates. Laske notes that extremes in heart rate are commonly seen during hunting season.

Minnesota is home to a population of about 12,000 to 15,000 black bears, with most of the animals found in the northern third of the state, meaning the bear den we are visiting in Grand Rapids is right in the middle of Minnesota bear country.

You certainly don't have to go to bear country to find a bear, though. In 2018, in order to track the spread of the population southward, the DNR launched a website (mndnr.gov/hunting/bear/bear-sightings.html) where people can report bear sightings outside the northern range. The number of reports was impressive.

Says Garshelis, "We've suddenly come to realize how many sightings outside the normal range there really are: over 1,000 in 2018!" The reports seem to indicate that bear range is slowly expanding southward and westward.

Andrew Tri is barely visible now—just two boots sticking out of the snow as he tries to maneuver the anesthetized yearlings out from behind their mother. He shimmies back and forth on his stomach until he is out of the den and holding a year-old male like an oversized baby. He and the other biologists then remove the other yearlings one by one, laying them atop tarps on packed snow. Finally, it's time for the mother, more of a challenge at 157 pounds.

"I need a rope!" calls Tri from inside the den.

While the yearlings are being measured and weighed for growth, several biologists carry the mother to a tarp. Her implanted biologger is near the end of its battery life. Iaizzo dons a pair of white gloves and gingerly shaves a patch of fur on the bear's sternum. With a steady hand and a surgical scalpel, he makes an incision in the skin on the bear's chest and removes the device. He then places a new, fully charged biologger in the opening and stitches the wound closed.

As the day's research comes to an end, the biologists pick up the bears, one by one, and pass them to Tri to be returned safely to their den. Garshelis hands me the female yearling—the largest young bear, weighing around 60 pounds—and as I look down at her sleepy face, her limp weight is heavy in my arms. Slowly stirring from her anesthesia, she stares at me, pawing gently at the air between us.

How odd this experience is—to be close enough to a wild predator that I can feel her heartbeat through my glove. I can't help but notice that the rhythm feels familiar, almost the same as my own pulse. I hand her back to Tri, and he lowers her into the den. Soon the anesthesia will wear off completely and her heart will fall back into its regular respiratory sinus arrhythmia, returning to hibernation as if nothing ever happened.