Bluff County

Large number of hibernating bats in area cave a pleasant puzzle.

February 2008

Avid caver and cave conservationist John Ackerman-who spends most weekends underground in Fillmore County-has added a discovery of a different type to his resume. It’s something that drives him a little batty, but in a good way.
Last August, the Bluff Country Reader shared the story of a major cave system discovery in Fillmore village area by seasoned cave explorers Ackerman, John Preston, and Phil Gemuenden. The cave is appropriately named Bat River Cave because of the incredible number of bats found inside. Until recently Ackerman had no idea how many there really were.
Clay Kraus of Chatfield, Charles Graling of Cherry Grove, Ackerman and David Gerboth of Dakota County, have continued exploration trips deep into the cave. Currently, they've mapped 2.5 miles.

This winter, the first three conducted a bat inventory. Ackerman said he was astonished when the total number of hibernating bats was tallied over 4,100.

“This means Bat River Cave is home to the largest colony of hibernating bats in any natural cave in Minnesota and possibly in the tri-state area.” Said Ackerman.

Now he's been spending time doing research on them, in between his thrilling caving trips.


Ackerman states that four species of bats hibernate in winter throughout Minnesota, roosting in various places. They are the Eastern Pipistrelle, Northern Myotis, Big Brown and Little Brown. He said almost all of the bats in Bat River Cave were Little Browns.

“That is not surprising,” said Ackerman. But he did wonder why the bats chose this 2.5 mile long cave as opposed to other large cave systems. His Spring Valley Caverns, with its 5.5 miles of passages, only has a handful of hibernating bats. Ackerman is the founder and owner of the Minnesota Cave Preserve, which includes 605 scenic acres in Fillmore County, along with 1,274 subterranean acres to 42 caves. Most were discovered by Ackerman with the help of fellow cavers.

All told, the preserve provides access to 36 miles of cave passages to fellow cavers, nature groups and scientists. Five of the cave systems are considered world class caves and have received national attention.

Bat River Cave joins those ranks with the discovery of the hibernating bats.

“My well-thought-out theory is that a large swamp zone borders Bat River Cave to the north and that may be the source of their food. Swamps are almost non-existent in Fillmore County. Most of the water that falls disappears into underground cavities.”

Bats eat insects, which also happen to include agricultural pests. He cited further statistics he'd found. During summer a colony of 150 bats can consume 38,000 cucumber beetles; 16,000 June bugs; 19,000 stink bugs and 50,000 leafhoppers. They feed at night on flying insects. Interestingly, nursing females ingest their body weight in insects each night.

He noted that hibernating bats do not eat, surviving instead of fat reserves. They may loose over 25 percent of their body weight over the winter.


Ackerman cautioned, “It is important to minimize or halt visitation in areas of caves where bats hibernate and here is why: when bats hibernate their heart rate, breathing rate and body temperature are reduced to conserve energy.”

“A hibernating bat can live on a small amount of stored fat for about 6 months. It’s estimated that a hibernating bat uses up to 10 to 30 days of fat reserves when it is disturbed. That could be a death sentence for the bats,” stated Ackerman.

The bats can live up to 20 or 30 years, usually having only one young a year, which is called a “pup.” Bats in this area give birth in late May and early June. Every year they will return to the same cave.


There has always been a relationship between bats and caves. Ackerman reminds people that bats are not blind. When in darkness and to locate prey, however, they emit a high frequency sound and figure out their location by “echo location,” where they process the echo of their emitted sound.

Echo location helps bats use caves for their habitat. Ackerman stated, “There are over 40,000 known U.S. caves, but only 5 percent are ideally suited for bats. This makes Bat River Cave a critical refuge for our bat population. That is another good reason why I have spent so much of my own money to perpetually protect large cave systems in Fillmore County and make them available to scientific research.”

In the United States nearly 30 percent of the bat species are either listed as endangered by the federal government, or are candidates for such a listing.

Ackerman also credits area residents with their help.
“The major discovery within Bat River Cave would never have occurred had it not been for the owners of the natural entrance to the cave, John and JoAnn Glady of Wycoff. They have invited cavers and scientists into this cave since day one and are enthusiastic supporters of exploration and scientific studies.”

When Bat River Cave stretched well beyond Glady’s property, Ackerman approached several other neighbors in the hopes that they would sell him enough land to create a new safe entrance to the cave. One such landowner, Wycoff native William Baker, listened to what Ackerman was proposing and then referred Ackerman to his daughter, Carolyn Baker Meyer.

“Carolyn was receptive to the idea at once” said Ackerman. “When she learned that there was a large cave system under the property she stated that she had always thought that such a cave did exist because of the numerous surrounding sinkholes and karst features.”

“Like the Glady family, Carolyn is a strong proponent for scientific study and protection of the cave, and has even accompanied cavers into the cave with some of her family for a tour.”

He said she also asked a question that he, himself, and other cavers have asked: “Where are all those bats entering the cave?”

At this time, Ackerman and fellow cavers in their mapping explorations have not yet located the passage or passages through which the bats are entering and exiting the cave system. That will be a goal for them.

To find the true extent of a cave, you've got to be willing to take some chances. Caver John Ackerman-who was named one of several “Adventurers of the Year” in the December 2005/January 2006 issues of the National Geographic Adventure magazine-is just the person to lead that expedition.

Bat River Cave, northwest of the village of Fillmore, turned out to be a large cave system once experienced cavers Ackerman, John Preston and Phil Gemuenden dove through a pitch black, water-filled passage at the end of the known cave in 2006. They surfaced to hear thundering sounds of pouring water. Removing their diving gear, walked around a bend in the passage and discovered a tremendous waterfall crashing down into a large, cavernous room.

After scaling the waterfall they found that the source of the water was coming from an upper level stream, running along the floor of a 20-foot tall passage. By evening they had followed the passage for over one mile before calling it a day.

Ackerman stated he understood the importance of the amazing discovery. Later, he was successful in purchasing land above the cave, so a safe entrance could be installed.

His ambitious discovery of caves and creation of new entrances has at times caused concern to the Department of Natural Resources officials and purists cavers. Ackerman has assured he is operating within the system. The Fillmore County zoning office agreed, saying no permit is needed to create a small hole on one’s own land to go into a cave.

When asked about his techniques, Ackerman always stresses cave preservation for generations to come. He also shares the explorations, for example, providing his Spring Valley Caverns facility for wild caving tours and participants from Quarry Hill Nature Center in Rochester. (At no charge.)

At Bat River Cave, another trip was conducted through the water filled passage in May 2007 with cave diver Tami Thomsen. Its purpose was to pinpoint a suitable man-made entrance. Using a highly specialized “cave radio” the task was accomplished. That summer a 30-inch diameter access shaft was created to the cave below, allowing for safe access to further exploration and scientific studies.