Image credit: Andrew Recher

Summer 2011

In the Dark

By John Grossmann

How do animals adapt to cave life?

Avid cavers aside, most humans tend to think of caves as dark, damp, and dangerous. But to a host of animals, caves are actually quite hospitable. Though some cave denizens arrive accidentally, perhaps swept underground by water flowing into a sinkhole or a through a crack in a creek bed, most seek out caves as protective havens from the elements and predators; some have evolved over millennia to exploit a previously unoccupied niche. That’s not to say the living is easy. It’s not. Round-the-clock darkness and a scarcity of food make living in caves challenging.

So how do cave-dwellers survive below ground in Carlsbad Caverns, Jewel Cave, Mammoth Cave, Wind Cave, and some 15 other national park units? Daily and seasonally, they employ a host of behavioral strategies. And on the Darwinian clock, they adapt to their subterranean environment, while random mutations alter their bodies and physical abilities and even extend their lifespan, making caves the equivalent of underground Galapagos Islands—living laboratories of evolution.

Cave biologists spotlight the most obvious adaptations by distinguishing among three types of cave animals: Troglobites (or troglobionts) like tiny cavefish dwell in caves and nowhere else; troglophiles may live in caves most of the time or in similar environments outside; trogloxenes spend time inside and outside caves. “The most basic survival strategy,” says William Elliott, a cave biologist and resource scientist with the Missouri Department of Conservation, “is not to stay in the cave your whole life. Trogloxenes, or cave foreigners, like bats and cave crickets, come and go frequently.” They leave primarily to feed—bats, famously, on insects like mosquitoes.

Like fussy renters, bats carefully choose their cave (their neighborhood) and the most hospitable part of the cave (their apartment). Some even have winter and summer homes. Gray bats, explains Elliott, migrate to summer caves in the Southeast, where they take to high-ceilinged rooms by the tens of thousands to incubate their young. “Their own body heat accumulates there,” says Elliott. “We’ve measured the ambient temperature at over 90 degrees.” At the other end of the thermometer, Indiana bats seek out the coldest regions of caves, which, contrary to what you’d hear on a typical guided cave tour, are not uniformly 55 to 60 degrees Fahrenheit year-round. Those conditions allow them to lower their metabolic rate and conserve energy, a key survival skill.

Indeed, many cave creatures, especially those short on food sources, live in slow motion. “Some of the cavefishes I’ve watched take a stroke with their pectoral fins and then glide for about a minute before they bother to take another stroke,” says Elliott. Above ground, most crayfish live for only three years, but some Alabama cave crayfish grow so slowly they aren’t able to reproduce until they’re 40; when a female lays eggs, she lays fewer, larger eggs that bear more protein for the next generation. These cave-adapted crayfish can live for 80 years.

“Caves are microcosms for studying evolution,” says Kurt Helf, an ecologist and cave biologist with the Cumberland Piedmont Network, one of 32 eco-regional networks in the National Park Service’s Inventory and Monitoring Program. As suggested by the Methuselah crayfish, the adaptations go well beyond the loss of unnecessary eyes and pigment in animals like salamanders and fish. In Texas, some of the more adapted cave-dwelling millipedes—those that leave their normal habitat in the soil—grow as much as three times as large as millipedes in the same genus. “The key is humidity,” says Elliott. “These animals were accustomed to living between soil grains, where it’s very moist. Once they got into an open cave that was still close to 100 percent humidity, they were not in danger of drying out. [In that environment] it’s advantageous to have longer legs and a larger body size, so some species evolved into a larger, free-ranging troglobiont.”

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Some cave creatures gained a better sense of smell. Others developed longer, more sensitive antennae to navigate better in the dark. Cave crickets, which must venture into the dangerous outside world to forage for food, limit their exposure to predators like mice and spiders by leaving the cave only every couple of weeks. “They have a distensible crop, a storage organ that enables them to eat more than 100 percent of their body weight in one or more sittings,” says Helf. “They essentially tank up.”

Someday, the secrets of cave life may even play a role in human survival. Pharmaceutical companies are searching for distinctive adaptive metabolic properties of cave bacteria and fungi, says Rick Toomey, director of the Mammoth Cave International Center for Science and Learning. “Because caves are such unusual places, especially from a nutrient standpoint, there’s a premium on making sure someone else doesn’t come and use your fuel source.” Cave microbiologists, he explains, are looking for bacteria that might successfully fight other bacteria, probing in the dark for new medicines that would make cave-lovers of lots of humans.

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This article appeared in the Summer 2011 issue

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