"Deep inside Lewis and Clark Caverns in the Tobacco Root Mountains of southwestern Montana, a pale spider crawls across the Madison Limestone and vanishes behind a stalagmite. Scanning the ceiling for roosting bats, I realize the greatest concentration of wildlife here lies within the limestone itself. The corals, crinoids and brachiopods that once swayed in the warm sea waters that covered the area some 330 million years ago left a legacy of cave-forming stone. These creatures extracted dissolved calcite from the sea to form hard protective shells. The process continued for millions of years. As the animals died, their shells gradually formed a layer on the sea floor, pressed down by sand and mud and other sediments until the shells became rock. The limestone layer at Lewis and Clark Caverns is 1,400 feet thick.
During an off-season tour of the caverns one March day, the famed colony of Western Big-eared bats was nowhere to be found. Lee Flath, the park manager, explains that the caverns' year-round 50-degree temperature is too warm for hibernating. Instead the hundred or so bats fly a mile away to an abandoned gypsum mine where they can sleep the winter away at a comfy 35 degrees. We did find a lone little brown bat, hanging upside down from the ceiling of the exit tunnel.
For summer visitors to the caverns, bats are the reigning wildlife of the dark. About 95 female bats use the caverns as a nursery. Each will have one pup over the next couple of weeks. By mid-August these babies will be flying on their own and the females will join the colony. Males are excluded from the nursery, and young males will eventually leave to find their own hangouts nearby. Park tour guides point out this nursery colony of big-eared bats, but do not linger. Worldwide bat populations have been showing declines, in part due to human disturbance and in part to the use of pesticides, which kills or limits bat food sources. Talking while passing beneath the nursery colony also disturbs the bats, so now tour groups walk through in silence, allowing these bats to raise their young in peace and rest during the day. At night the bats emerge to sweep the skies in search of mosquitoes and other insects. Some bat species consume up to 3,000 insects in one night's foray.
For cave-dwelling insects, however, the presence of bats is a lifesaver. The spider we saw never needs the light of day to stalk its plentiful prey. Bat droppings, called guano, form a rich substrate for bacteria and fungi that in turn feed the appetites of mites and springtails. The latter fall prey to harvestmen (daddy longlegs) and ground spiders. Even centipedes and millipedes creep along the limestone, snacking on any insect they can catch. These eerie creatures of darkness have undergone genetic changes, such as the loss of eyes and protective pigmentation.
As we emerged into the brilliant sunshine after almost two hours in the company of stalactites, stalagmites and elusive cave creatures, we found ourselves blinking and a little dazed, as if leaving a matinee film into broad daylight. A descent through Lewis and Clark Caverns is more like stepping into a fantasy movie, nudging up against a world that test our fears of the dark. Yes, there is a moment in the tour when all the lights go off and the palpable darkness belongs entirely to the bats and pale creatures of eternal night."
(Broadcast: "Fieldnotes," 5/12/15. Listen weekly on the radio, Sundays at 12:55 p.m., Tuesdays at 4:54 p.m., or Fridays at 4:54 p.m., or via podcast.)