Digging into Fall

TIP: Look for clues around holes in the ground, including size, tracks, and location, to determine what's lurking inside.

As fall settles in, this wildlife spy wonders about animals who dig in for the season — or all winter. People aren’t the only creatures who adjust their surroundings to their needs. Shelter is essential for survival, and some critters aren’t leaving it up to chance.


Animals dig out dens or burrows for several reasons, but they all boil down to one common factor: They’re staying in one place for a while and need shelter. An underground residence makes good sense; dirt is a good insulator, keeping temperatures cooler in summer and warmer in winter than open air.

What critters go to the trouble of constructing a hideout? Bears, both black and brown, are probably the first suspect you think of at this time of year. Hoary marmots, whose alarm whistles ring out in alpine meadows during summer, are also earth engineers.

In a month or so bears will bed down in a den for their long winter nap. Although they usually don’t stir for months on end, bears don’t actually hibernate. A bear’s body temperature lowers, but not drastically. If undisturbed, they most likely will stay asleep. But, they can wake up.

My first bear den sighting was in Montana, while taking a weekend bear-tracking class one summer. I’m not sure what I expected, a cartoonish tunnel with a large, spherical chamber at the end perhaps. What we found was much simpler. A dark hole, about two feet wide, gaped in a steep hillside. Since it was summer and the bear was out for the season; we decided to investigate.

We found some long hairs in the soil at the entrance. Based on the hairs and entrance size our instructor suspected it was a brown bear den. What surprised me most was that it was simply a tunnel that widened a little at the end. There was no spacious chamber like I’d imagined. To test it out, we tried crawling inside. I stand 5 foot 4; with my feet pressed against the back wall of the den, my head was level with the opening. It was a little larger inside than the opening, but not much.

Bears excavate a tunnel entrance that fits them just right, which helps keep out unwanted visitors (the bears in the Goldilocks story should have used this technique). The inside of the den allows enough space for the bear to settle in, but the snugger the fit, the cozier the winter snooze. A female with cubs of the year will dig a chamber big enough for her family so the den may be the same size as a larger male’s, but she won’t make a bigger entrance. A large male, on the other hand, will have an entryway that fits his bulk.

Black and brown bears have different styles for selecting den sites. Black bears are open-minded, and may try rock crevices, expand a hole under an uprooted tree, snuggle into a brush pile, remodel a coyote or fox den, dig a new winter retreat, wiggle into culverts, and occasionally wind up under someone’s porch. Brown bears are more self-reliant, usually digging out their own burrow. They are built for digging, with long straight claws to cut into the earth. The signature shoulder hump of brown bears consists of huge shoulder-blade muscles, all the better for moving earth.

Bears rarely use the same den twice, and even then not in consecutive years. Other bears, however, may use the space. The first winter a young bear is on its own, it might use a den where it previously hid out with its mother. A bear will visit sites in the summer, checking the premises. If that bear is a female with cubs, the cubs may return to claim the place as their own.

When a bear tucks itself into its den in fall and when it leaves in the spring, all depends on whether it’s male or female. A pregnant female is one of the first to snuggle in for the season and one of the last to emerge in the spring. The timing gives mom more rest and allows newborn cubs more time to grow. An adult male is one of the last to call it “lights out” in the fall and the first to wake in the spring. Being the biggest, they require more fuel than smaller bears so they need to be out foraging for longer periods.

Unlike bears, hoary marmots use their tunnel systems year-round. They are social animals, living in colonies, sometimes of more than two dozen animals. Each colony typically contains a dominant adult male, up to three adult females, and juveniles.

Marmots have much more complicated tunnel systems than a bears’ den, including rooms and tunnels with specialized construction. They may incorporate rocks or crevices in boulder piles in their subterranean dwellings. Marmots dig with sturdy claws, resorting to teeth to gnaw through roots or remove rocks.

A multitude of predators awaits the unwary marmot, including bears, golden eagles, wolves, coyotes, wolverines, foxes and lynx. The wily rodents excavate short bolt holes, usually less than six feet long, scattered across the colony grounds. If a colony has been in place for a long time, like a fine old country manor, each generation creates its own additions. The marmots, in such an established territory, could have more than one hundred emergency tunnels.

The next level of construction for marmots are the burrows where they spend their time in summer. They sleep at night in the safety of a burrow, larger and more extensive than the bolt holes. These burrows may have multiple exits — one main door marked by a mound of dirt and other, hidden, secondary doors. Tunnels also provide shelter from mosquitoes on days when there isn’t a breeze to blow away the pesky insects.

Marmots, being rodents, have short pregnancies. They mate in April or early May, with females giving birth a scant month later to litters of up to six. Babies are blind and hairless at birth, so the tunnels provide a nursery for the first month of their lives. When they’ve grown enough, their mother weans them and they take their first reconnaissance outside.

With short Alaska summers, the part of the manor marmots spend the most time in is the “hibernaculum,” the chamber where they hibernate in winter. Since this can mean up to eight months underground, marmots line the chambers with plants to make a cushy mattress. They blockade the door against cold air and intruders with a wall of dirt, plants, and droppings (hey, you can’t be picky about building materials when you’re a rodent). Marmots are unusually long-lived for rodents, sometimes reaching a ripe old age of 10 years or more. They take longer to reach maturity, and the young animals hibernate in the family home their first winter.

As you check your insulation, seal up the windows, and do other chores to ready your house for the cold months, in the surrounding mountains the bears and marmots run through their own checklists before settling in for a long winter’s nap, perhaps dreaming of spring.


• Beth Peluso is a freelance writer and illustrator and avid birder. She enjoys spying on wildlife across Alaska


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