How Cave Biology Works

This blind species of scorpion was discovered by cave biologists in 2006 in a cave in Israel. See more pictures of arachnids.
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Spelunking. Go ahead, say it out loud -- it's a funny word. Spelunking.

Spelunking is the recreational exploration of caves. The scientific exploration of caves has a different name. It's called speleology. In the same vein, you can probably guess what biospeleology is. It's the biology of caves, or cave biology.

Biospeleologists study cave organisms, as well as the ecosystems of caves. The unique conditions of caves make cave life mysterious and challenging to research. Caves are in constant darkness with high humidity. Nutrients are hard to come by, and many caves contain mixtures of gases (sulfur, methane) that can be lethal to many life-forms. Yet many organisms have evolved and adapted to live in this dank, dark environment.

Although the environment seems uninhabitable, you'll find lots of animals, insects, fish and crustaceans deep in our Earth's caves. The most recognizable cave dweller is probably the bat. Along with these little flying beasts, caves play home to a host of other creatures, including fish, salamanders, insects, spiders, shrimp, crickets and a variety of fungi and bacteria. Some animals call a cave home only part-time like bears, raccoons and even humans.

The majority of cave-dwelling organisms actually descended from aboveground species, thousands or even millions of years ago. How did that happen? Some animals with traits suited to the cave environment split from the pack and migrated into caves. This separation from their aboveground counterparts is what allowed those species to flourish and adapt to living underground. We call this process regressive evolution. Some species actually lost features such as eyes or skin pigmentation. It sounds bizarre and fantastical, but if you think about it, you really don't need these things if you live in perpetual darkness.

Researchers believe that by studying the genetic adaptations of these organisms, we can learn more about how biology works. For example, cave animals are excellent models for studying the genetics of abnormal eye development. Cave organisms also have evolved metabolisms. Because nutrients are so hard to come by in a cave, cave organism's metabolisms adapt to work more efficiently on much less. Environmental researchers also look to cave fish as indicators of water quality. Much of our drinking water passes through a cave ecosystem at some point [source: MGS].

Let's tunnel down into the cave and see what lives there on the next page.

The Three Types of Cave Life

All cave animals and organisms fit into one of three categories of cave life. Placement in these categories depends on how much time the organism actually spends in the cave.

We call the first type trogloxenes. You can look at the word origin to figure out what kind of creatures fall into this category. Troglos is the Greek word for cave, and xenos is the Greek word for guest. So, you can think of trogloxenes as cave visitors. They come and go at will, but use the cave for specific parts of their life cycles -- hibernation, nesting or giving birth. A trogloxene will never spend a complete life cycle in a cave. The most familiar trogloxenes are bats, bears, skunks and raccoons. Even moths are trogloxenes. Trogloxenes have no special adaptations to the cave environment.

Next, we have troglophiles. From the Greek -- troglos for cave, and phileo for love. Love? Well, troglophiles are animals that can survive outside the cave, but may prefer to live inside it. They leave the cave only in search of food. Some examples of troglophiles include beetles, worms, frogs, salamanders, crickets and even some crustaceans like crayfish. A troglophile can live its entire life either inside or outside of the cave.

The creepiest -- and most fascinating -- types of cave life are the troglobites. Again -- troglos for cave and this time bios for life. Troglobites spend their entire life cycle within a cave. They're found only in caves and wouldn't be able to survive outside a cave. The troglobites are the animals that have adapted to cave life. They have poorly developed or absent eyes, little pigmentation and metabolisms that allow them to go a long time without food. They also have longer legs and antennae, allowing them to move and locate food more efficiently in the dark. Troglobites include cave fish, cave crayfish and shrimp, millipedes, as well as some salamanders and insects.

Keep reading to learn about the different habitats within a cave and how each area plays host to various animals.

Cave Zones


Trogloxenes, troglophiles and troglobites call different parts of the cave home. The environment at the mouth of the cave differs greatly from the environment deep inside the cave. A cave has several zones.

The entrance zone environment is closest to the environment above ground. It receives sunlight and has variable temperatures and green plants. Many animals like raccoons or bears utilize this space to eat their food, sleep or nest. In the entrance zone, you'll find organisms like moss, ferns, owls, snails and salamanders.

Venture a bit farther into the cave to enter the twilight zone. In the twilight zone, there's less light, so plants don't really grow there. The temperature remains a bit more constant but may still fluctuate in conjunction with weather aboveground. Organisms living in the twilight zone need moisture and coolness to survive. Here, you'll find the habitats of many trogloxenes, including moths, bats, spiders, millipedes and mushrooms. The animals found in the twilight zone usually leave and enter the cave at will.

Travel even deeper into the cave to experience the dark zone. In the dark zone, there is no light whatsoever. The temperature remains constant. Troglobites live in the dark zone. These organisms have undeveloped eyes, poor pigment and long antennae because they've adapted to live in this environment.

How do the organisms living in the dark zone survive? What do they eat? Read on to find out.

The Cave Ecosystem


As you probably learned in biology class, all life depends on sunlight -- a process known as photosynthesis. This is true even for the deepest, darkest part of a cave, otherwise known as the dark zone. We know that no green plants can grow in the dark zone. So how do those troglobites, the organisms that live solely in the dark zone, survive? The answer is simple -- through a food chain, just like everything else on our planet. Let's take a closer look at that chain.

One way for food to get into a cave is through natural weather events like a flood. Excess rainwater washes leaves, twigs and plants into a cave, providing munchies for insects and other animals. Another way that nutrients enter a cave is simply when animals -- trogloxenes and troglophiles -- bring it in.

Yet another food source for cave organisms is one you probably haven't thought about -- guano. Guano is full of organic matter, and troglobites love to feed on this stuff. What is guano? It's the scientific term for bat poop. Bats nest deep inside the cave, and their droppings will pile up several feet high and several feet wide. Very few animals can feed directly on these droppings, but bacteria and fungi found in the cave can decompose guano into basic food and nutrients.

As you can see, all the different organisms in a cave depend on each other for survival. We call this a food chain and here's how it works, starting at the bottom:

Organic material, such as guano, other animal droppings and washed-in plants provide a haven for fungus and microscopic bacteria, which feed on the organic material, breaking it down into simple nutrients. Then, millipedes and tiny crustaceans feed on the fungus, bacteria and the nutrients left behind. Bigger insects, such as cave beetles, feed on these millipedes, crustaceans and even the eggs of cave crickets. Centipedes, cave spiders, salamanders and cavefish feed on insects smaller than them. Some cave centipedes grow so large, they've been spotted feasting on bats [source: Krajick].

Because there are more animals and organisms at the bottom of the cave food chain than at the top, it might be better to call it the cave food pyramid. You'll find many decomposers at the base of the pyramid, with the more evolved carnivores at the top.

On the next page, we'll discuss the science of studying cave life and the challenges of working in the dark zone.

Studying Cave Biology

Perhaps we can trace the origins of cave biology to the late 1700s, when locals spotted foot-long (30.5-centimeter) salamanders in some Slovenian caves [source: Krajick]. These salamanders -- called Proteus salamanders -- were and are among the largest troglobites known. Even though we only became aware of cave biology in the past few centuries, some of the types of organisms dwelling inside caves have been around for hundreds of millions of years.

Cave biologists have to get deep inside a cave to study what lives there. Depending on the cave, this process may entail literally squeezing inside. Scientists wedge themselves through narrow openings 60 feet (18 meters) long before emerging into a cold, damp area. But once they get inside, there are countless discoveries just waiting for them.

By turning over rocks, shining flashlights into damp, dark recesses and peering into pools of water to find creepy-crawlies, cave biologists are many times the first humans to lay eyes on these creatures. It's imperative to treat these habitats and organisms with respect. The Federal Cave Resources Protection Act of 1988 enacted laws to protect and preserve caves. This act made it a criminal offense to disturb, destroy or deface any type of life within a cave, including the cave structure itself.

As we learned earlier, all species in a cave depend on each other to survive. Even just entering a cave can alter its ecosystem -- we all carry lint, hairs and even dandruff on our bodies. Shedding these inside a cave can introduce new fungi and bacteria to the environment. Visiting a cave is fine in small doses, but if humans over-visit and pollute a cave, the entire bionetwork can collapse and die. Many serious cavers and biologists won't even reveal their best research spots for fear that others will enter and disturb the ecosystem.

Speaking of altering the ecosystem, researchers are finding that climate change also affects cave biology. The dark zones of a cave have constant cool temperatures. Cave organisms have evolved over millions of years to adapt to this stability. If the planet's temperatures continue to ascend, scientists fear that many troglobites won't be able to adapt quickly enough to keep up [source: Krajick].

So how do cave biologists study within a cave? Many enter and leave jars full of bait to attract creatures. A favorite bait is stinky cheese, likely because of its strong odor. Remember, cave-dwelling animals have very advanced senses to make up for their lack of sight. Some cave biologists spend hours inside a cave, watching and recording all the action. And some will simply lie in wait and pluck up an organism with tweezers, put it in a jar, and take it out of the cave for further research.

For more information on caves and cave creatures, see the links on the next page.

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More Great Links


  • AOL Television. "Dirty Jobs - Cave Biologist." Jan. 22, 2008. (April 9, 2009)
  • California Underground. "Cave Life Gallery." Oakland Museum of California. 1999. (April 9, 2009)
  • Cave Biology Research Group. "Cave Facts." New York University Department of Biology. 2009. (April 9, 2009)
  • "The Federal Cave Resources Protection Act of 1988." 2009. (April 9, 2009)
  • Krajick, Kevin. "Discoveries in the Dark." National Geographic. September 2007. (April 9, 2009)
  • Conkwright, Bob. "Caves in Maryland." Maryland Geological Survey. Feb. 6, 2007. (April 9, 2009)
  • Missouri State Parks & Historic Sites. "Food Chains and Pyramids." June 6, 2008. (April 9, 2009)
  • National Park Service. "Great Basin - Cave Life." U.S. Department of the Interior. February 2004. (April 9, 2009)
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  • Roach, John. "Scientist Journeys Into Caves for Clues to Extreme Life." National Geographic News. April 30, 2001. (April 9, 2009)
  • Taylor, Steve J. "Biospeleology." Illinois Natural History Survey. Dec. 12, 2006. (April 9, 2009)
  • United States Fish & Wildlife Service. "White-nose Syndrome in bats: Something is killing our bats." April 3, 2009. (April 9, 2009)