As you drive the roads across eastern Colorado, humming along at highway speeds, the landscape flies past. It largely looks the same, with occasional changes in scenery – a hill, an arroyo, a row of cottonwood trees, a herd of cattle, a few horses, a farmhouse, a weathered barn, a windmill. Driving through the gate of the Chico Basin Ranch and down the road to the headquarters, you look out across the prairie and mostly see sparse grass, cholla cactus, prickly pear, yucca, sagebrush, and rabbitbrush. To some people, it isn’t particularly inviting, and perhaps, only of passing interest.
It’s close to noon, the sun is high in the sky. It’s hot, and the prairie looks largely devoid of wildlife. Coming in you did see a small group of pronghorn and a hawk, but not much else. That’s a bit puzzling as you read once that the American prairie has a high degree of biodiversity. So where are the animals?
When you visit some place like the Comanche National Grasslands in southeast Colorado, I suggest you stop and get out of the car. Take a few minutes to walk out across the open prairie. Or if you are staying on the Chico for a while, perhaps saddle up a horse and head out cross-country. Oh, and mornings or evenings are best, as more wildlife are active when the light is lower and it’s cooler. Look around. Look at the ground as you travel along. What do you see? You’ll soon start to notice holes; lots of holes, actually; holes of many different shapes and sizes. These are animal burrows, passages to a vast subterranean world that lies just under the surface of the prairie.
Animals need shelter, a place to escape the midday heat and burning sun in summer, the rain, snow, and cold winter wind. They need someplace cozy and dry to raise their young. Animals need protection from predators, a place to escape. Even for birds, there are few trees on the prairie in which to nest, so many nest on the ground or in the brush and cholla.
One reason you don’t see many animals during the day is that most of them are nocturnal, coming out at night and rarely seen in the daytime. The prairie is quite active at night. During the winter, the morning after a snowstorm, you’ll see hundreds of tracks and dozens of trails leading from burrows and running across the blank white canvas of snow. Several mammals hibernate during the long winter like the spotted ground squirrel and some species of mice.
Last year, I surveyed different burrows on the Chico Basin Ranch, looking primarily for active swift fox dens. With some study, I could eventually identify various animal burrows by the size of the entrance and other signs such as track patterns and scat. Roughly 50% of mammal species on the prairie are rodents. These include types of voles, mice, black-tailed prairie dogs, spotted ground squirrels, and pocket gophers–the bane of beautiful lawns and golf courses everywhere!
Voles (genus Microtus) build runways 1 to 2 inch-wide in passages under the dirt and through the grass mat under the snow. Voles are active all year round and often come above ground. There are also large numbers of both white-footed mice and deer mice of genus Peromyscus. They often build their nests underground in cavities beneath the roots of trees or shrubs, or beneath or inside logs.
The plains pocket gopher (Geomys bursarius) is one of 35 species of pocket gophers, so named in reference to their externally located, fur-lined cheek pouches. They are solitary rodents with huge front claws for digging who are active year round. They rarely appear above ground, spending most of their lives in burrows. Usually the entrances of their 2 to 3 ½ inch tunnels are closed. Pocket Gophers typically close the hole behind them as protection. In the winter, they pack dirt outside their tunnels under the snow, leaving long tube-shaped mounds called eskers.
Woodrats (Neotoma micropus), also known as packrats, build underground nest chambers with multiple entrances piled with dirt, rocks, cactus pieces, and cow dung. Ord’s kangaroo rats (Dipodomys ordii) can be seen primarily at night along country roads through sandy rangelands. Kangaroo rats will make two-footed jumps up to six feet long and one and a half foot high and can change direction in mid-air, using their tufted tail to steer. Although, like the pocket gopher, they plug their burrows as a defense against predators. Coyotes, badgers, swift foxes, snakes, and owls kill them in large numbers.
In the rodent family Sciuridae, the prairie dog (Cynomys ludovicianus) – not a dog, but a large ground squirrel – and the spotted ground squirrel (Xerospermophilus spilosoma) both live in colonies. Prairie dog colonies are a network of underground passageways with burrow entrances of about 4 to 6 inches formed in conical mounds or craters. Spotted ground squirrel burrows have two to four inch entrances. Many of the burrows are connected below ground with passages, long networks of tunnels and above the surface by networks of track-ways I call “scurries.”
Both black-tailed prairie dogs and spotted ground squirrels are diurnal, meaning they are active and visible outside their burrows during the day, While prairie dogs are active all year, spotted ground squirrels hibernate for long periods during the winter from about October through April. When checking burrows, I look for tracks and scat as clues of activity. In winter, signs of recent activity ruled out hibernators like ground squirrels and several species of mice.
These are the prairie’s primary tunnel builders, but other animals utilize prairie dog or other rodent tunnels as shelter. These include bull snakes, whiptails, prairie rattlesnakes, and burrowing owls.
Several predators live underground building their own dens, but often they dig out and reuse prairie dog tunnels. In particular, American badgers (Taxidea taxis) will forage for rodents by quickly digging out and widening their tunnels in 8-12 inch wide horizontal ovals. They will sometimes sleep in these new burrows before moving on. They often push the dirt behind them, giving the appearance the hole was filled in, but a badger may still be in the burrow asleep. After mating, they will select a den site where they will stay for weeks while they raise their young in the spring and early summer.
The nocturnal swift foxes (Vulpus velox) will dig their own burrows for a den, also called an earth, but often just widen out prairie dog burrows or reuse badger burrows. I have seen numerous cases of this. I have watched and videoed adult foxes digging out a tunnel. A continual stream of dirt shoots up and out from the tunnel in a rooster tail, resembling a small volcanic eruption if the light from the setting sun catches it just right. The swift fox will maintain as many as seven or more dens at one time. They’ll move every couple of weeks or so to a different den to make it harder to be found by coyotes or bobcats. Their den entrances are about 7 to 9 inches, and are usually round or keyhole shaped. Many dens will have several entrances connected by passages from five to thirty feet long with rooms for sleeping. They also have single entrance burrows within their territories in which to hide.
Similarly, coyotes (Canis latrans) and bobcats (Lynx rufus) will also dig dens for raising their pups or cubs, respectively. Den entrances are round or vertical ovals about 9 to 12 inches in diameter. Coyotes prefer steeper sloped banks. Bobcats build shallow dens for sleeping or deeper dens for raising cubs. Their dens are often dug underneath logs, rock overhangs,or in caves.
On Colorado’s short-grass prairie, lives the Desert cottontail rabbit (Sylvilagus audubonii), also known as Audubon’s cottontail. Cottontails give birth to their kits in warrens; burrows vacated by other mammals. I currently have a family of cottontails living in an old red fox den under my barn. I have seen several of them on the Chico, but the black-tailed jackrabbit (Lepus californicus), also known as the American desert hare, is more commonly seen on the ranch. I have seen three or four cases where black-tailed jackrabbits have taken over swift fox dens after they were vacated. Both rabbits will dig shallow depressions or pans in which to rest and keep warm or cool in the earth depending on the season. I frequently find them lying in these pans.
Burrowing owls (Athene cunicularia) return from southern climes to the Colorado prairie in spring to breed and raise their young. These ground-dwelling birds rarely dig their own burrows; rather they occupy old prairie dog tunnels. They’ll often line their nests with cow dung or soft grass. I read in Audubon that cowboys sometimes called these little owls “howdy birds” due to their tendency to nod their heads and bob up and down in greeting. They eat mostly small mammals and insects that live in or near the prairie dog towns.
Black widows (Latrodectus mactans) and other spiders live inside abandoned tunnels of prairie dogs or foxes, weaving their funnel webs across the opening. Yet others like the wolf spider are free roaming or build their own burrows about the size of a penny where they hide and ambush unsuspecting prey before darting back inside.
And after living deep underground all winter, once the frozen soil layer thaws, earthworms begin to migrate vertically towards the surface. The robins know when it’s time for the earthworms to emerge at the surface; a sign that spring has come.
So, next time you are traveling across the Great Plains, visit a national grasslands or a ranch like the Chico. Take a moment to stroll across the prairie. Look around for these hidden doorways, passages to a mysterious subterranean world, scout the ground for other signs of life, and maybe you will see some of the creatures that live both on and under the prairie.