The American grey fox never digs its own den - it simply moves into another animal's home. If it is lucky, this grizzled fox - the only fox that regularly climbs trees - may find the abandoned earth of a red fox, preferably at the base of a tree. Otherwise, it will make do with a woodchuck's hole, a hollow log or a rocky crevice.
Other opportunist lodgers of the animal world include dwarf mongooses, found in Africa south of the Sahara, that make their homes in abandoned termite nests, and wild dogs on the grassy plains of South Africa, that often use abandoned aardvark burrows in which to rear their pups.
Some animals can lay claim to being the burrow diggers-in-chief of the animal world. On the North ASmerican prairies, horsemen must beware of holes dug by black-tailed prairie dogs - holes that are the entrances to an underground maze of burrows up to 10 ft (3 m) deep and extending over an area of about 160 acres (65 ha). These squirrel-sized diggers build whole 'towns', of narrow tunnels and cosy dens for a colony of thousands.
Burrowing owls are happy to adapt prairie dog burrows rather than dig their own, as are ground squirrels, salamanders, mice and snakes. A burrowing owl is not big enough to kill an adult prairie dog, but is not above making a meal of its host's pups. Otherwise the owl lives mainly on the prairie dogs' other lodgers - large insects and small snakes and lizards. But rattlesnake lodgers present far more of a threat. If one moves in, the prairie dogs seal up the section in which it has made its home.
Another unwelcome lodger is North America's rarest mammal, the black-footed ferret. This black-masked hunter nearly died out, but was bred in captivity and re-introduced to the wild in 1991. Not only do the ferrets live in prairie dog burrows, they also feed entirely on prairie dogs, which they pursue through the labyrinth.