Burrow

A burrow is a hole or tunnel excavated into the ground. Animals create these spaces for habitation or as a temporary refuge. They may also be a byproduct of locomotion, which is the act of moving from one place to another.

Burrows serve as vital shelters against predation and exposure to the elements. They can be found in nearly every biome on Earth. These habitats exist within many different biological interaction types, including symbiosis and competition. Burrows range from simple tubes a few centimeters long to massive networks of interconnecting tunnels. A complex network, such as a rabbit warren, can reach thousands of meters in total length.

Animals can be categorized by how they interact with these structures. Primary excavators are the animals that originally dig and construct the burrow. These creatures are generally very strong, such as the prairie dog, the aardvark, or the wombat.

Secondary modifiers do not build the original burrow. Instead, they live inside a burrow made by others and improve or change it for their own needs. The pygmy gerbil is an example of a secondary modifier. The third category is simple occupants, also known as commensals. These animals neither build nor modify the burrow but simply live inside it. Some bird species make use of burrows originally built by tortoises.

Vertebrates, which are animals with backbones, use a wide variety of substrates for their homes. Mammals are perhaps the most well-known burrowers. This group includes Insectivora, such as the mole, and rodents like the gopher, great gerbil, and groundhog. Some mammals, like the platypus, pangolin, armadillo, and weasel, are also known to burrow. Even Carnivora, such as the meerkat, and marsupials, like the wombat, utilize these spaces. The largest burrowing animal is likely the polar bear. It constructs a maternity den in snow or earth to protect its young.

Other vertebrate groups also rely on burrows for survival. Reptiles, such as lizards, construct burrows and may show territorial behavior over them. The Adelaide pygmy blue-tongue skink may even use a burrow as protection while fighting. Birds often construct burrows in soft soils. Examples include puffins, kingfishers, and bee-eaters. The Magellanic penguin constructs burrows in the coastal Patagonian regions of Chile and Argentina.

Even fish, such as the dragonet and lungfish, are known to burrow. Invertebrates, which lack backbones, also play a major role in creating these structures.

Invertebrate burrows are created through different methods. Some are filled passively by gravity, meaning the organism does not participate in the filling process. Others are actively filled, meaning the organism uses material to fill the space itself. Many invertebrates, such as insects, spiders, crustaceans, and worms, use specific techniques to maintain their tunnels. For example, the establishment of an invertebrate burrow often involves soaking surrounding sediment in mucus. This prevents the walls from collapsing and seals off the flow of water. Some invertebrates, like sea urchins and clams, can even burrow into hard rock.

Burrows provide essential benefits for survival and resource management. They offer protection from harsh environmental conditions and predators. An animal might face a burrow toward the sunlight or away from cold winds to help with insulation and heat retention. Some species, like the earwig, use burrows for physical protection during the winter. Burrows also serve as storage sites for food. This allows animals to keep a steady supply of food away from competitors and during seasons when food is scarce. Furthermore, burrows provide a stable temperature for animals in areas frequently destroyed by fire. They also offer a safe environment for vulnerable newborn animals.

The impact of burrowing can be seen on a massive scale. Some rodents, such as great gerbils, live in family groups in extensive burrows. These networks are so large they can be seen on satellite images. Even when unoccupied, these burrows can remain visible in the landscape for many years. A groundhog can construct a burrow that occupies a full cubic metre, displacing a large amount of dirt. Beyond living structures, burrows are often preserved in the fossil record. These are known as burrow fossils, which are a specific type of trace fossil. This helps scientists understand the behavior of animals from the distant past.