Living with a small dog often means walking past a pile of blankets and not realizing the chihuahua, or Pomeranian, is burrowed deep within. After calling for the little one, the blanket might move and reveal their position.

Dogs of all sizes (and many cats, too) enjoy going deep into a burrow of blankets. The darkness helps facilitate daytime napping, and the covers offer a sense of safety. Although domesticated animals burrow for comfort, scientists have found there is an evolutionary advantage to burrowing.

Burrowing is such an important behavior that scientists have argued that without burrows, some species wouldn’t exist.

What Is a Burrow?

A burrow is a space such as a tunnel or hole that is occupied by an animal. Insects such as ants, bees, and beetles create burrows, just as arachnids like spiders and scorpions do. Burrows are also known among birds, crustaceans, centipedes and millipedes, and certain vertebrates like alligators and lizards.

Mammals such as wolves and badgers also create dens. And as many hamster owners can attest — almost all rodents have burrowing behaviors. 

Animals create burrows for many reasons. Burrows provide protection from predators as well as harsh elements like extreme cold or heat. Burrows are also a place where an animal can store its food or keep its young away from danger.

Some animals live in burrows alone or with their young until they are old enough to survive on their own. Social animals, including many species of rodents, work together to create and occupy burrows. This teamwork mentality has many advantages, particularly when their homes are under construction. Building a burrow exposes an animal to predators and expends a lot of energy. Sharing the task means the animal uses less energy and spends less time in a vulnerable state.

Living in the burrow with roommates also comes with advantages. The group can pool resources to stand watch for predators and guard food reserves. Each member of the burrow then has more time for foraging and mating. 

Read More: 5 Animals Who Are Impressive Architects

How Animals Create Burrows

Some animals build burrows by collecting supplies and creating a den. Others dig into the ground (or, for sea creatures, the substrate) to build an underground lair.

When digging into the ground, an animal might dig straight down to create a shallow cavity. But depending on the sand or soil composition, an animal may instead dig a helix, which resembles a corkscrew shape.  

“You can’t do a straight-down burrow because the sand or sediment falls on you. The way to go straight down is to make a helix,” says J. Sean Doody, an associate professor in the department of Integrative Biology at the University of South Florida.

In hot desert areas, a helix shape can protect eggs until they hatch by preventing them from drying out. The shape may also serve as a deterrent for predators seeking a snack.

Read More: These Rare Adaptations Help Animals Survive in the Desert

Burrowing as a Survival Strategy

In recent years, catastrophic events have proven that burrows have protective properties. After massive fires in forests and grasslands, recovery teams have been surprised to find alligators still alive in their dens. The dens enabled the alligators to stay both wet and safe while the fire burned around them.

Burrows have kept animals safe during fires, hurricanes, and heat waves. Some scientists have argued that burrows have provided animals with an evolutionary advantage. 

There is evidence through trace fossils that burrows date back 540 million years. Although some of the species that created these ancient dens are now extinct, burrowing has long benefited others.

The American alligator, for example, still resembles how it looked 8 million years ago when it swam in the swamps of what is now Florida. Other than sharks and a handful of vertebrae, few animals can claim they’ve been on the fossil record that long with such few changes in appearance.

The American alligator belongs to the ancient Crocodylia group, and its relatives stretch back 84 million years and descend from ancestors dating back to the Triassic period, about 200 million years ago.

In the case of the alligator, scientists have argued that burrows are so important in protecting them that without their crucial dens, alligators wouldn’t exist. Fourteen of 23 crocodilian species dig burrows, including the Chinese alligator (alligator sinensis), which also creates an extensive tunnel system on riverbanks.

Evolutionary Advantage

Although evidence of ancient burrows can be scarce, many scientists argue that burrows provided an evolutionary advantage. 

“It’s difficult to prove, but there is a lot of indirect evidence for that,” Doody says.

Part of that indirect evidence comes from modern animals’ ability to survive extreme and catastrophic situations by remaining in their burrows. 

“When wildfires come through, the temperatures are in the thousands of degrees. There are animals that survive because they are in the burrows, and they literally can’t go anywhere else,” Doody says. 

As a result, Doody says there is evidence that burrowing animals have survived mass extinctions, and their burrows may have played a role in their success.

“Burrows are critical for avoiding environmental extremes and catastrophic events like fire, floods, and perhaps mass extinction,” he says.

Read More: 6 Unusual Traits of Animal Evolution

Article Sources

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Emilie Lucchesi has written for some of the country’s largest newspapers, including The New York Times, Chicago Tribune and Los Angeles Times. She holds a bachelor’s degree in journalism from the University of Missouri and an MA from DePaul University. She also holds a Ph.D. in communication from the University of Illinois-Chicago with an emphasis on media framing, message construction and stigma communication. Emilie has authored three nonfiction books. Her third, “A Light in the Dark: Surviving More Than Ted Bundy,” releases October 3, 2023 from Chicago Review Press and is co-authored with survivor Kathy Kleiner Rubin.