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.