July–August 2023

The Secret Lives of Cave Critters

At Forestville/Mystery Cave State Park, researchers have discovered tiny, transparent invertebrates—some of which might be new to science. What can these animals teach us about their fragile underground home?

Emily Sohn

When Dawn Ryan moved from California to Minnesota to work at Forestville/Mystery Cave State Park in 2019, she encountered a prevailing—if outdated—belief that the state was too far north for its caves to have much life in them. With the area’s long history of glaciers and cold, the thinking was that life forms hadn’t had enough time to adapt to subterranean habitat. 

Ryan, who has a passion for caves, had long been traveling around the country and volunteering on cave projects. In 2000, while still living in California, she started helping survey and restore Mystery Cave, where she formed close friendships with cave managers and, despite expectations, frequently encountered animals inside the park’s extensive network of underground passageways. So, after she left her job at the Sequoia and Kings Canyon National Parks to sign on as manager at Mystery Cave, she commissioned a study to see what was living down there. 

In the park’s caves, according to preliminary results released earlier this year, live a rich diversity of small invertebrates, many with transparent bodies and no eyes. Some species, Ryan suspects, are new to science. The findings emphasize the need for more careful protections of the region’s underground communities. 

“Some of these animals probably live in Mystery Cave and nowhere else in the world,” she says. “That’s the cool thing about it.” 

A Cave Mystery
Discovered in 1937 near the southeastern corner of Minnesota, Mystery Cave is the state’s longest cave system, encompassing 13 miles of maze-like passageways. Located in what’s known as the Driftless Area of the Midwest, which remained free of glaciers during the last ice age, the landscape features river valleys, high cliffs, and lots of limestone. Caves formed there over millennia as rising and falling floodwaters carved out the rock. 

Although Minnesota contains some 400 caves, the Mystery Cave complex is the only one protected as a state park. On guided tours, visitors can walk, stoop, and crawl through lofty rooms and narrow tunnels that remain a chilly 47 degrees year-round. Surrounded by a musty odor—a smell produced by bacteria that live in the mud—people are likely to see stalagmites, stalactites, and otherworldly rock formations. What they don’t usually notice are the cave’s wild animals, which are predominantly tiny. 

But wildlife is what concerned Ryan most when she came to work at the park. In 2016, a fungal disease called white-nose syndrome was first confirmed in the state. By 2018, the disease had wiped out 94 percent of Mystery Cave’s bat population, and that wasn’t just a bat problem. Bat poop, called guano, and the bacteria that grow on it, provide important nutrients for other animals that live in caves, where nutrients tend to be scarce because plants can’t grow in the absence of sunlight. Without bats and their waste, Ryan worried that cave invertebrates would starve to death before anybody had taken the time to catalog them. Only a few studies had been done to identify the cave’s wild animals in the 1980s and ’90s, and those focused mostly on fish, bats, and swimming invertebrates. “When you lose one food source in an area that doesn’t have a lot of food in the first place,” says Ryan, “what’s going to happen to these other animals?”

To conduct the first comprehensive survey of all wildlife at Mystery Cave, Ryan turned to the Texas-based consulting firm Zara Environmental, which sent two cave experts to Minnesota in September 2021. With help from Ryan and other Minnesota DNR staff, the team began setting traps that were, in some cases, reminiscent of elementary school science projects: little plastic cups buried in the dirt, dotted with dabs of glue, and baited with bits of stinky limburger cheese or anchovy paste. In flowing springs at cave openings, the team used mop heads and drift nets to snag floating organisms. And they scoured rocks, scat, standing water, and cave walls for any signs of life they could find.

Sampling wasn’t always easy. To explore Coon Cave, also known as Goliath’s Cave, the cavers entered through a sinkhole, then crawled on hands and knees through up to two feet of extremely cold water, passing leeches and earthworms to get into a dry room that was large enough to stand in. From there, they encountered a waterfall-like rock formation called Jabba the Hutt, a series of pools, and a dry passage called Main Street that looks like a subway tunnel filled with rubble, among other features. In one area, they encountered 15 raccoon carcasses. In another, they found a dead woodchuck.

After eight days of collecting, the samples were shipped to a lab in Texas, where the Zara researchers sorted them and identified as many species as they could. 

New Finds
At Mystery Cave, the team noted a variety of common and expected animals: flies, beetles, mice, springtails, millipedes, snails, mites, and microscopic crustaceans called copepods. The survey also identified several creatures never before recorded in the area, including some that may not have been recorded anywhere on earth. 

The list of intriguing finds included a large population of Bathynellacea—extremely small, totally blind crustaceans that live in muddy spaces between water bodies and have stringy, wormlike bodies with a multitude of short, feathery legs. Bathynellacea are missing from the fossil record in places that were free of ice during the last glacial period, Ryan says. Experts suspect the invertebrates were washed into the region through floods, changing river flows, and other water-based mechanisms after the glaciers last retreated from the area more than 11,000 years ago. A subset of them, driven underground by predation or other forces, adapted to cave environments.

In one small, isolated pool, the researchers found eyeless amphipods. These shrimp-shaped invertebrates with clear outer bodies, long antennae, and many legs have never been seen at the park before, and a 1993 report specifically flagged their absence. The team suspects the find represents either a new species or a sign that the creatures have extended their range. The survey also turned up what seem to be the park’s first recorded examples of certain types of mites, millipedes, and spiders. More work is necessary, says Ryan, to confirm species identifications and other details. 

The Big Picture
The range of interesting creatures discovered in Mystery Cave illustrates the unique ways that cave animals adapt to cold, wet, dark, and nutrient-starved environments that are often isolated from one another, says Rickard Toomey, a cave resource management specialist and research coordinator at Mammoth Cave National Park in Kentucky. Because caves tend to maintain consistent temperatures equivalent to mean annual outdoor weather, cave species might be able to survive only within a narrow 10-degree range from 45 to 55 degrees. That sensitivity makes them potential canaries in coal mines—the first to disappear as climate change makes caves warmer, Toomey says. And wiping out one species could cause entire ecosystems to collapse, with consequences that could affect people. Certain microbial species that live in caves even have cancer-fighting potential, some research suggests.

“Caves are incredibly exciting and important places, and they preserve some unique biology,” Toomey says. “It’s important to understand these adaptations and to be careful not to lose these creatures.”

There is likely more yet to be discovered at Mystery Cave and other underground environments, says Julian Lewis, a biologist who owns and operates Lewis and Associates, a biological consulting company in southern Indiana. Caves remain relatively unexplored, and studying them is like peeling off layers of an onion. “The outer layer of what was in Mystery Cave probably got scraped off 30 or 40 years ago” with earlier studies, he says. “This new study takes another layer off.”

Caves face many threats, Lewis adds. He has seen cases where people have dumped toxic waste into holes in the ground because nobody was looking. Agricultural runoff, fertilizers, and pesticides often end up in caves, too. Better protections may depend on learning what’s living down there. “How do you put an animal on an endangered species list,” Lewis says, “if you don’t know what it is?”

Ryan plans to monitor populations annually to see how the animals respond to variations in food supply with floods, mammal activity, and other unpredictable events.  Knowing what’s living there can guide management decisions about where to put trails and where to keep people out, Ryan says. For visitors, she adds, learning about the life hiding in plain sight can enrich a cave experience and make the world a more interesting place. That’s what drives her. As a kid, she wanted to be an astronaut, but later found caves to be an equally enticing frontier. “It’s just the idea of going places and seeing things that most people don’t get a chance to see,” Ryan says. “Humans have that drive, I think.”