Cave-Dwelling Harvestmen in Cold Stream Caves of Montana

When I first started exploring caves, I was surprised by the lack of life I encountered in these cold environments (30-45 degrees Fahrenheit). Contrary to the ideas that classic cave horror movies like “The Descent” propagate, there is not enough food in these systems to support large cave-adapted predators–especially ones interested in eating people.

But on closer inspection, even frigid alpine caves harbor smaller organisms. I have observed small winged insects, spiders, and beetles a relatively long distance from the surface in these cold, wet caves. Most pronounced  are harvestmen, a type of arachnid which gathers in some caves in northwestern Montana in large clusters during the winter.

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Harvestmen clustered by the thousands on the ceiling of a cave near the border with Canada.

Organisms that have adapted to live in caves full-time exhibit a few well known traits: shrinkage or loss of eyes, loss of pigmentation, and lengthened forelimbs (with which to feel prey).  This process can occur over millions of years in species like spiders, salamanders, and fish. Cave ecosystems are, effectively, small islands where individuals are separated from their above-ground relatives for many generations. These species, once cave-adapted, can be separated from similar subterranean habitat by dozens of miles (or more). Therefore, any isolated cave system could host new species.

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A blind, depigmented harvestman (Sclerobunus) in an alpine cave in cental Montana. (Photo courtesy Carl Froslie)

Since 2006, research efforts in our state have focused on bat roosts due to the enormous population losses of bats in the US due to White-Nose Syndrome. However, there are plenty more species to learn about in cave systems of the American west. The photo above, snapped by a curious caver in 2016, is an undescribed species of harvestman, according to Dr. Shahan Derkarabetian (San Diego State University). There is still much to learn about this cave-adapted animal.

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