Some terrestrial and aquatic animals are capable of excavating holes in the ground (burrowing) for protection from adverse environmental conditions, as well as for storing food. Burrows vary from temporary structures of simple design (for example, the nesting burrows of some birds) to more permanent underground networks that may be inhabited for several generations (for example, rabbit warrens, badger sets, fox earths, and prairie dog burrows). They vary in structure from blind burrows with a single opening to extensive systems with several openings. Some animals (for example, some species of moles) live permanently underground, and their burrows have no obvious large openings to the surface. Burrows may be shared by a number of species, and abandoned burrows may be used by other species. Animals with limbs usually excavate their burrows by using their legs, but many burrowing animals are limbless and the mechanism of progression is not always obvious.
Worms, slugs, many insects, and many vertebrates live in burrows. Earthworms are important soil organisms because their burrows improve drainage and aeration, their feeding habits enhance leaf decomposition, and their droppings increase soil fertility. Earthworms burrow by contracting circular muscles in their body wall to push forward, and contracting the longitudinal muscles to widen the burrow. Termites (Isoptera) and ants (Hymenoptera) are social insects, most of which live underground. Termites are major consumers of vegetation in warm climates, and many construct extensive underground galleries extending from the mound located on the surface. Most ants are predators or scavengers, but leaf-cutter ants feed on a fungus that grows on the harvested pieces of leaf in carefully tended underground galleries.
Many marine animals, including flatfish, crabs, and shrimps, take temporary refuge or live more permanently in sand or mud by burying themselves just below the surface. Aquatic sand and mud pose several problems for burrowing animals. First, the particles are usually tightly packed together, restricting movement and requiring these organisms to expend 10–1000 times as much energy to move a given distance compared to other forms of locomotion. Second, burrows readily collapse unless reinforced or consolidated. The wall of burrows may simply be consolidated with mucus, but some animals make a more permanent, substantial tube of particles stuck together with mucus. Finally, all burrowing animals must be able to create a current of water through the burrow so that they can breathe. Many animals also feed partly or wholly on particles carried in such currents.
