Animals that dig are described as fossorial, meaning they are adapted for a life spent primarily underground. This specialized behavior has evolved independently across a wide range of species, including mammals, insects, reptiles, and amphibians.
The Primary Reasons Animals Dig
The act of digging is primarily driven by three fundamental survival needs, determining the location and structure of the resulting tunnels.
Many animals dig for shelter, constructing complex burrow systems that provide a stable microclimate, offering a retreat from extreme temperatures or heavy rain. These networks also serve as safe havens from predators and are used as natal dens where young are raised.
A second major motivation is foraging, as many species rely on subterranean food sources like earthworms, grubs, roots, and bulbs. They dig small, temporary holes directly into the soil to access prey, often creating a scatter pattern across an area. The third, less common reason is for storage, where animals cache food, such as nuts or seeds, in shallow holes for later consumption during lean seasons.
Clues in the Dirt: Analyzing Tunnel and Hole Structure
The diameter of the hole is a direct indicator of the size of the animal, with small rodents like mice creating openings only about three centimeters wide, while a groundhog’s main entrance may measure 10 to 12 inches across. Examining the shape and placement of the excavated soil, known as the spoil, is also an element in identification.
A volcano-shaped mound of dirt, where the hole is often obscured or located directly in the center, typically suggests a species that pushes the soil up from below as it tunnels. Conversely, a fan-shaped or crescent-shaped mound, where the soil is spread out on one side and the entrance is often visible or plugged, indicates an animal that backs out of the burrow while pushing dirt away with its forelimbs.
The angle of the entrance further narrows the options; some animals create a shallow, sloped entry, while others, like certain spiders, dig a nearly vertical tunnel straight down. Burrows situated near solid structures like building foundations, rock piles, or fences are often the work of larger rodents seeking structural stability for their dens.
Many animals also create “quick-escape” holes, which are usually dug from the inside and lack the telltale dirt mound, serving as emergency exits from the tunnel system. The presence of tracks, droppings, or hair near the entrance can offer additional forensic evidence to confirm the identity of the occupant.
Profiling the Most Common Mammal Culprits
Moles are insectivores known for their extensive, shallow networks of raised ridges, or runways, that appear just beneath the lawn surface. Their characteristic molehills are small, symmetrical, volcano-shaped mounds of fine soil pushed up as they tunnel while searching for earthworms and grubs.
Pocket Gophers create a different pattern, distinguished by their crescent- or fan-shaped mounds of soil, which typically have a plugged, or sealed, entrance hole. These strictly herbivorous rodents dig deep, complex systems to access plant roots and store food, and the lack of a visible, open hole helps them conserve moisture underground.
Groundhogs, also known as woodchucks, are larger rodents that dig much wider, open entrance holes, often 10 to 12 inches in diameter. Their burrows are complex dens with multiple entrances, frequently found near the cover of woodlands, fences, or under sheds.
Nocturnal foragers like Skunks and Raccoons create numerous small, shallow, conical holes, which look like small scoops taken out of the soil. They are not creating permanent burrows but are digging for a quick meal, using their strong claws to root out insects, such as grubs, from the topsoil. This damage is often a cluster of divots rather than a single, deep entry point.
Beyond Mammals: Insects and Reptiles that Burrow
Solitary wasps and bees, such as Cicada Killers or ground bees, dig small, perfectly round, pencil-sized holes in bare soil or sandy areas. These insects create simple, temporary burrows to lay eggs and provision the nest with paralyzed prey.
Other invertebrates, including certain species of spiders, are primary excavators. Tarantulas and trapdoor spiders, for example, dig vertical tunnels often lined with silk and featuring a hinged door to conceal the entrance.
Most snakes do not dig their own burrows but instead occupy abandoned rodent holes. However, some species of turtles and lizards will excavate shallow, temporary depressions in soft earth for the sole purpose of laying their eggs before filling the nest back in.