An excavated hole or tunnel in the ground, commonly called a burrow, represents a widespread survival strategy across the animal kingdom. This behavior, known as fossoriality, is found in species ranging from tiny insects to large reptiles. The structure serves as a controlled environment, offering a degree of stability that the surface environment lacks. Understanding which animals dig and why they do so provides insights into the ecological pressures driving this behavior.
The Reasons Behind Excavation
Animals construct burrows to meet several fundamental biological needs. Protection from predators is a primary motivator, as an underground space provides an immediate refuge from threats above. The burrow entrance is often the only point of vulnerability, allowing the animal to escape observation and direct attack.
The subterranean environment is also utilized for thermoregulation, which is relevant in areas with extreme temperature fluctuations. Excavating below the soil line allows animals to escape scorching surface heat during the day or insulate themselves from cold temperatures during the winter. Many species use burrows for reproductive purposes, creating secure, temperature-stable nurseries for their young. Finally, some animals use the structures for food management, either as larders for caching stored food or as ambushing points for hunting prey.
Common Mammalian Burrowers
Mammals often create the most noticeable disturbances in lawns and gardens. Moles are insectivores that create shallow subsurface tunnels, or runs, while searching for earthworms and grubs. The most recognizable sign of mole activity is the molehill, a conical or volcano-shaped mound of soil pushed upward from the tunnel network, which usually lacks a visible entrance hole.
Groundhogs are large rodents that dig substantial burrows for habitation and hibernation. Their entrances are much wider, typically measuring between six and ten inches in diameter, and are often located near the base of a shed or a dense shrub. Groundhog activity is characterized by a prominent fan-shaped apron of dirt that accumulates directly outside the main, open entrance.
Smaller burrowing rodents, such as gophers and voles, create different signatures. Pocket gophers construct extensive, deep tunnel systems and leave crescent-shaped or fan-shaped mounds of soil, with the actual hole often plugged with dirt. Voles create numerous small, open burrow entrances, generally one to one-and-a-half inches wide, that connect to a network of visible surface runways worn into the grass.
Smaller Diggers: Insects, Arachnids, and Reptiles
Burrowing behavior extends far beyond mammals, with many invertebrates and reptiles creating specialized excavations. Cicada killer wasps construct individual nests by digging a U-shaped tunnel in dry, sandy soil. The entrance is often a perfectly round hole, approximately a half-inch to one inch in diameter, sometimes surrounded by a small collar of loose soil. The wasp paralyzes a cicada and deposits it inside the burrow’s chamber to serve as food for her developing larva.
Arachnids, notably the trapdoor spider, build a silk-lined burrow up to 12 inches deep. The spider conceals the entrance with a hinged lid made of soil, silk, and vegetation, camouflaged to match the surrounding substrate. This specialized door allows the spider to ambush passing prey, quickly lunging out before retreating into its sealed home.
Reptiles, such as turtles, also excavate temporary holes. Female turtles utilize their hind legs to dig flask-shaped nests in soft soil, where they deposit their eggs. These temporary scrapes are usually shallow and are characterized by scattered, loose soil.
Identifying the Builder from the Hole
Distinguishing the animal responsible relies on analyzing the hole’s characteristics. The most immediate clue is the hole’s diameter. A large hole, six inches or more across, with a significant dirt pile indicates a groundhog or a similar large mammal. Conversely, a small, perfectly circular hole less than two inches wide suggests an insect, such as a cicada killer wasp, or a small rodent like a vole.
The presence or absence of a dirt mound is a strong indicator. Moles create distinct, volcano-shaped mounds with no open hole. If a mound is present but asymmetrical or fan-shaped with a plugged entrance, it is more likely the work of a gopher. A hole that is open and clean, especially if found in conjunction with worn surface trails, points toward a vole.