A burrow is a hole or tunnel excavated into the ground by an animal to create a space suitable for habitation, temporary refuge, or as a byproduct of locomotion. Burrows provide a form of shelter against predation and exposure to the elements, so the burrowing way of life is quite popular among the animals. Burrows can be constructed into a wide variety of substrates, and can range in complexity from a simple tube a few centimetres long to a complex network of interconnecting tunnels and chambers hundreds at thousands of metres in total length, such as a well-developed rabbit warren.
Examples of vertebrate burrowing animals include a number of mammals, amphibians, fish (dragonet and lungfish), reptiles, and birds (including small dinosaurs). Examples of burrowing invertebrates are insects, spiders, sea urchins, crustaceans, clams and worms.
A wide variety of vertebrates construct or use burrows in many different types of substrate. Mammals are perhaps most well known for burrowing, especially Insectivora like the voracious mole, and rodents like the prolific gopher and groundhog. The rabbit, a member of the family Lagomorpha, is a well-known burrower. There are estimations that a single groundhog burrow occupies a full cubic metre, displacing 320 kilograms of dirt. Even Carnivora like the meerkat, and marsupials, are burrowers. The largest burrowing animal is probably the polar bear when it makes its maternity den in snow or earth.
Burrows by birds are usually made in soft soils; some penguins and other pelagic seabirds are noted for such burrows. The Magellanic Penguin is an example of such a burrow constructor, making burrows along coastal Patagonian regions of Chile and Argentina. Other burrowing birds are puffins, kingfishers, and bee-eaters.
Kangaroo mice construct burrows in fine sand.
The burrows produced by invertebrate animals can either be actively or passively. Dwelling burrows which remain open during occupation by an organism are filled with passively, by gravity rather than by the organism. Actively filled burrows, on the other hand, are filled with material by the burrowing organism itself.
The establishment of an invertebrate burrow often involves the soaking of surrounding sediment in mucus in order to prevent collapse and to seal off water flow.
Burrows are also commonly preserved in the fossil record as a type of trace fossil.
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