Imagine a warm summer night, just after twilight. You startle at a flitting shadow or faint whoosh—a bat. Now imagine that sight and sound multiplied by a thousand or more. Thats a typical summer night near the Alaska shaft at Soudan Underground Mine State Park in Tower.
From late August into September, thousands of bats make their way deep into the mine to hibernate. Those that arrive early in the season spiral up the mine shaft to the surface every night to hunt for insects. And every night their numbers increase until the weather turns cold enough for hibernation to begin.
Because natural caves have so many human visitors or are closed for safety reasons, hibernation spots for bats are dwindling, according to Bat Conservation International. This makes underground mines such as Soudan, with their consistent temperatures and high humidity, some of the nations premier bat hibernacula.
To make access to the mine easier for the bats but more difficult for predators and people, the park recently added a cage 16 feet by 16 feet by 7 feet tall over the entrance to the Alaska shaft. Before the cage was installed, bats had to crawl through old railroad ties, making them vulnerable to predators such as hawks and cats. Now they can fly in and out through the cage.
"We hope the cage will encourage the bats to use the Alaska shaft, keeping more of them away from the Number 8 shaft, where we transport people and equipment," says park manager Paul Wannarka. He says people visit only about one-half mile of the 54 miles of horizontal tunnels in the Soudan, leaving a lot of room for the bats.
Between 5,000 and 10,000 bats winter in the Soudan, says Dave Olfelt, state parks resource management specialist. Most are little brown myotis, a common species in the forests of northern Minnesota—or your attic. There are also many northern myotis, a species of special concern. In fact, the mine is the most important hibernaculum of the species in Minnesota.
Getting an accurate count is nearly impossible. "We counted about 4,000 one winter while they were hibernating," says Olfelt. "But we can only get into half of the mines areas." To count bats as they leave or enter the shaft in the dark, the staff is looking into using infrared cameras.
That technology would give an estimate of numbers at a specific time. "Bats also use the mine as a rest stop during migration and congregate in even larger numbers in one of the caverns in the spring," says Olfelt. In May 2001, Olfelt estimates, there were about 10,000 bats in the pumping station, a human-made cavern that was used for large equipment during mining days.
Gerda Nordquist, DNR bat researcher, has been counting the Soudan bats since 1983. She and bat specialists from BCI were called in before excavation began on a new cavity for the University of Minnesota MINOS experiment, which makes use of the half-mile of rock overlying the mine to investigate properties of subatomic particles called neutrinos.
"We knew the excavation could affect the bats," Olfelt says. "Bats have so little fat reserves; if theyre disturbed during hibernation, they die." So Nordquist and the specialists suggested excavation begin before the bats came back for the winter. That way the bats would be more likely to select hibernating sites deeper in the mine, farther from the disturbance.
Some bats, mostly males and barren females, hang around the mine all year. Other bats move out for the summer. Some migrate long distances to find suitable habitat. One banded bat from Soudan was recovered in Park Rapids, Minn. Away from the mine, males live alone, while females and their young form colonies of as many as several hundred.
Although bats can live more than 30 years, BCI studies show bat populations throughout the country have been declining because of pesticides (bats ingest contaminated insects) and destruction of habitat, and because many people dislike bats and kill them. But bats eat their weight in insects (usually mosquitoes) every night and many people are starting to see their benefit.
DNR public affairs officer