A burrow is a tunnel or hole that an animal digs for habitation (a place to live) or as a temporary refuge (a place of protection). Burrows can also be the byproduct of locomotion—moving from one place to another. Some burrows function as “larders,” where animals keep food.
Burrows provide shelter from predators and extreme temperatures. For these reasons, animals have used burrowing behavior for a very long time. In fact, a 110-million-year-old dinosaur burrow was recently discovered on the southeastern coast of what is now Australia. It is the oldest known dinosaur burrow, and is nearly identical to the first one ever found, in the U.S. state of Montana in 2006. This similarity suggests burrows were dug by similar dinosaur species on opposite ends of the Earth for millions of years.
Sometimes, entire families live in burrows. Beavers, for instance, construct complex lodges (sometimes called dams) that provide shelter for parents and offspring. Other times, burrows are dug primarily for pregnant mothers and infant offspring. Maternity dens used by bears are probably the most familiar example of this type of burrow.
Besides protection from predators and climate, some burrows function as food-storage facilities. Kangaroo rats are very small rodents—only weighing about 150 grams (5 ounces)—but they store grain in huge burrows many times their size. These “granaries” can store up to 120 liters (32 gallons) of food.
Burrowing is popular among many types of animals, including invertebrates, which are animals lacking a spinal column. Clams, crustaceans, insects, sea urchins, spiders, and worms all exhibit burrowing behavior. Various amphibians, including some species of frogs, are burrowers, as are a number of reptiles, including assorted snakes. Even some birds are burrowers. Kingfishers, Magellanic penguins, and puffins are among those known to make burrows instead of nests.
However, the most well-known burrowers are probably mammals, especially the mole, gopher, groundhog (also known as a woodchuck), and rabbit. Bears are most likely the largest burrowing animals. They use shelters such as caves, as well as dug-out earthen and snow burrows, as their dens. Most species spend the winter inside these dens in a long period of sleep similar to hibernation.
Animals construct burrows in many types of surfaces. Scabies mites dig into the skin of animals and humans. Termites chew through wood, including fallen or even living trees. Birds typically burrow in soft soil. Kangaroo mice use fine sand. Some clams and sea urchins can burrow into rock. Moles often burrow into lawns and raise molehills. The nine-banded armadillo builds its burrow in moist soil near creeks or streams. Pregnant female polar bears create maternity dens in earth or snow to give birth and nurture their cubs.
Burrows range in complexity from simple, short tubes to elaborate networks of connected chambers and tunnels.
Groundhogs are exceptional burrowers and their burrows are particularly large. It is estimated that an average groundhog moves 1 cubic meter (35 cubic feet) of soil when digging a burrow, which may have up to 14 meters (46 feet) of tunnels buried up to 1.5 meters (5 feet) underground. A well-developed rabbit warren—a group of burrows—may be thousands of meters long.
Beavers create an unusual type of burrow. Beaver lodges are constructed with tree branches and mud over banks or hills in creeks or ponds. Every year in late autumn, beavers cover their lodges with fresh mud. Only after the lodge is built do beavers dig burrows beneath them. All their burrows have underwater entrances, making it difficult for other animals to invade. When the frost comes, the mud on top of the lodge freezes, becoming almost as hard as stone and unable to be penetrated by predators, such as wolves and wolverines. Most beaver lodges have two rooms. The first is used for drying off after a beaver swims up to the burrow. The second, warmer and drier, is where the beaver family lives.
Some animals prefer not to dig their own burrows, but to use ones made by other animals instead. The meerkat is one example. A colony of meerkats, which averages 20 to 30 members and is called a “mob” or “gang,” often uses burrows dug by ground squirrels or mongooses. Meerkats do not compete with those species for resources, and sometimes even share the space with them. Meerkats have also been known to share burrows with snakes, although they most likely do not do so on purpose.
A meerkat burrow can have as many as 90 entrances and be 2 meters (6.5 feet) deep. The meerkats only leave them during the day, and when they do, one or more will stand guard while the rest of the gang is foraging or playing. As soon as a guard spots a predator, it will give a warning bark and the other gang members will run and hide in the many holes of the burrow.
Burrows and the Environment
Animal burrows can pose threats to the environment, as well as to human agricultural and residential development.
Gophers dig tunnels in the ground and place mounds of dirt and rocks at their entrances. These are often referred to as “gopher towns” or “gopher holes.” A gopher town can have a population in the thousands and take over large sections of mountain meadow or prairie, destroying plant life and leaving large stretches of bare dirt. This land is more vulnerable to erosion and flooding. Gopher towns can also disrupt garden plots, landscaping, and even some underground cables.
Groundhog burrows can damage farm machinery. Wheels or other implements, for instance, may abruptly fall into a large, burrowed area, delaying work and damaging the machinery’s structure. Groundhog burrows can even undermine the foundations of buildings.
Termites can do extensive damage to wooden structures and unprotected buildings. Once they have entered a building, they may destroy carpet, cloth, paper, and other materials containing cellulose, an organic compound found in plants that is the primary diet of termites. In the southwestern United States alone, termites cause approximately $1.5 billion in structural damage each year. They can also be major agricultural pests, especially in Africa and Asia, where crop damage can be extensive.
Prairie-dog towns, one of the most familiar types of burrow networks, are often found on land where livestock graze. Black-tailed prairie dogs, native to the Great Plains of North America, create enormous networks of burrows, some stretching more than 64,000 square kilometers (25,000 square miles) and containing about 400,000 prairie dogs. Prairie dogs prefer to dig their burrows in short grasses, making livestock pastures an ideal location for towns. This puts livestock, such as cattle and sheep, at risk for injury. Some predators attracted to prairie-dog towns, such as coyotes and bobcats, may also threaten livestock.
However, the prairie-dog diet (mostly grasses) encourages the growth of forbs, such as sunflowers and clover. The abundance of forbs attracts animals such as bison—and provides more nutrient-rich fodder for livestock.
In the 20th century, ranchers used poisons and hunting techniques to eradicate prairie-dog towns. The primary predator of the black-tailed prairie dog, the black-footed ferret, was almost driven to extinction.
Prairie dogs and cattle sometimes live and forage in the same areas, coexisting without major negative effects on either population. Some studies suggest an abundance of prairie dogs can correspond to minor declines in cattle weight.
The few prairie-dog towns that exist today are mostly in conservation areas and national parks.