Identifying Wildlife Holes

There's a fascinating world of fur and claws underfoot. Here's how to tell who's down there.

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My wife and I labored up the brutally steep trail to Hidden Lake as a young couple hightailed it downhill, breathing every bit as hard as we were.

“If you’re going to the top,” the young fellow gasped and pointed toward the mountain, “you’d better be careful. They’re everywhere!”

Everywhere? They? I scanned the ridgeline for thunderstorms, ever a

threat in Washington’s North Cascades National Park. Or perhaps “they” meant pollen-crazed killer bees in this time of sweet-smelling wildflowers.

Not even close.

“Wolverines!” the hiker announced with a shudder. “We must have seen a dozen up there. And their holes are all over the place!”

This boy had read one too many Jack London stories. How else to explain his misidentification of tunnel-loving, pacifistic yellow-bellied marmots for a passel of bloodthirsty cat-bears? And while I had to snicker at his poor grasp of mammalogy, I certainly understood his confusion over zoological aperture ecology, aka what-critter-you-reckon-made-that-hole?

After all, holes in the ground are among the most common animal signs a backpacker is likely to see, yet most hikers pass them by with little consideration. “People are overwhelmed by all the big stuff in a landscape,” says Pinau Merlin, an Arizona-based naturalist and author of A Field Guide to Desert Holes. “When they do notice an animal hole, they have no way of knowing what made it or why. Holes are a big mystery.”

They don’t have to be. Like animal tracking, identifying dens and burrows is a great way for backpackers to get to know the wild neighborhood. Besides, it’s good to know if that burrow next to the tent is an innocent mole hole or the abyss of a venomous spider.

Down The Hatch

Animals burrow beneath the earth for one reason: security. “Whether they seek a retreat from heat, cold, or predators,” explains Susan Morse, founder of the Vermont-based tracking organization Keeping Track, “animals want to worm their way into a tight place out of the elements-one that’s small enough to prevent a bigger animal from getting in.”

Burrows are often much smaller than you’d expect. A marmot den, for instance, will be just big enough for that marmot, so a larger coyote can’t run in after him. A 1,000-pound grizzly bear might excavate tons of dirt when it digs its winter den, but the entrance to the den will be just large enough to accommodate the bear. That’s why the classic cartoon image of a big bear snoring away the winter in a huge cave “is all Walt Disney,” Morse says.

Many holes in the ground aren’t necessarily long-term residences; they are dug to provide temporary safety from pursuers and extreme weather. A red fox, says Morse, might dig 10 or 15 quick-escape security dens in an area of less than 100 acres, but it actually sleeps out in the open.

Natal dens, on the other hand, are dug solely for reproduction. Coyotes, wolves, and foxes construct complex systems with multiple chambers and entrances for use only during the young-rearing season. (Lone among the canids, kit foxes of arid western regions use underground dens year-round.) Frequently, says Merlin, these animals will excavate a number of intricate natal dens and move the pups to a different den to escape fleas and ticks, or if the den is discovered by humans or other predators. And it’s not just four-footed animals that go underground with the kids. Burrowing owls line old mammal burrows with cow chips, grass, and feathers for a nest chamber, and belted kingfishers excavate nesting tunnels as long as 15 feet in the banks of rivers and creeks.

But those who spend serious time in their underground homes dig the most impressive catacombs. Prairie dogs and badgers build subterranean superhighways up to 10 feet wide at depths of up to 30 feet. Groundhogs and marmots fashion elaborate burrow systems accessed by numerous “plunge holes”—hidden shafts that they can dive into at a moment’s notice. If you’re hiking across alpine fields in the West or meadows and pastures in the East, watch for these often camouflaged openings, lest you step in one and wrench an ankle or knee.

In the desert Southwest, says Merlin, pack rats and pocket gophers dig separate chambers for defecation, sleeping, and food storage. Staying underground does more than keep them cool; it keeps them alive. “For a little animal like that,” Merlin explains, “the more you’re aboveground, the easier it is for somebody to get you.”

Insect burrows often serve double duty, offering security plus the perfect trap for unsuspecting prey. The tiger beetle larva tunnels into the ground, then plugs the entrance with the seamless camouflage of its flat head. When a smaller insect blunders by, the tiger beetle larva snatches it with powerful jaws and drags it into the lair. The trap-door spider takes the ruse to an architectural height by capping a burrow with a door constructed of soil, saliva, and silk, and camouflaged with moss, leaves, and small sticks. Silken hinges on the rear edge allow the spider to crack open the door at night and pounce on nearby prey.

Hole Knowledge

If you wonder what lurks within some curious crevice, use the size, shape, and structure to determine the possible residents (see “Whose Hole?” on page 40). A den site littered with fur, bones, and partially eaten prey is likely where a fox, coyote, or wolf is raising young. Move along so you don’t disturb the babies. Otherwise, find a comfortable spot, preferably downwind of the hole, and wait for a little head to pop out, confirming your guess. Sit quietly, and keep these clues in mind:

  1. Fresh dirt piled by a hole indicates an active marmot or woodchuck house. Quick-escape holes are dug from the inside and don’t have the telltale mound.
  2. A snake slithering out of a hole is just a visitor-or more likely a snacker-not a resident. Snakes don’t dig holes, but will check rodent burrows for food.
  3. Skunks often defecate near their holes. You’d better step lightly, or move away, if you spy 3/8- to 3/4-inch piles (similar in shape to dog scat) that contain lots of insect parts.
  4. Why build when there’s a den in move-in condition? That’s what skunks, mink, and raccoons do. These animals often take over abandoned dens, while opossums and foxes have been known to move in with the original owner of a multichambered den.
  5. Bear in mind that you might not even see some holes. Many pocket mice of the Southwest plug their entrance holes with soil during the day to stave off moisture loss. Tarantulas read the same book: During the day, they weave a film of silk that covers their burrows.

“On the ground,” declares Merlin, “everybody has a trick up their sleeve.”

Or, of course, down their hole.


The following books are available from

A Field Guide to Desert Holes, by Pinau Merlin (Arizona-Sonoma Desert Museum Press, 520-883-3028; $7.50).
Animal Architects: How Animals Weave, Tunnel, and Build Their Remarkable Homes, by Wanda Shipman (Stackpole Books, 800-732-3669; $16.95).
Stokes Nature Guides: A Guide to Animal Tracking and Behavior, by Donald and Lillian Stokes (Little, Brown, 800-759-0190; $15).
Tracking and the Art of Seeing: How to Read Animal Tracks and Sign, by Paul Rzendes (HarperCollins, 800-331-3761; $25).

Ground-dwelling wasps recently sent North Carolina writer Eddie Nickens to the emergency room, sharpening his interest in zoological aperture ecology.

Hole-Watching Tips


  1. Use a mirror to reflect sunlight into the hole instead of putting your face near the hole to see inside it. The beam is far brighter than that of a flashlight.
  2. Look for shed hair or hair caught on sharp objects like rocks and tree roots around the hole opening to identify the occupant.
  3. Carry a small measuring tape. Field estimates of hole dimensions often are way off.


  1. Get too close to a suspected natal den. You might force the mother to move her young. Instead, observe with binoculars or a spotting scope.
  2. Put your face close to a suspected animal den. You might inhale the eggs of the host’s parasites.
  3. Probe holes with sticks, stems, fingers, or hiking poles. If the occupant is an insect or young animal, you might injure it.