1. Three Groups of Animals That Live in Caves
Scientists classify animals associated with caves into three categories. The first group includes creatures that only wander into caves occasionally. The second group consists of animals that can survive both inside and outside the cave environment. The final group is made up of animals that are usually found only within caves. These true cave dwellers have developed special traits to handle the darkness [1].
2. Why Blind Cavefish Lose Their Eyes as They Grow
Among the animals found only in caves, some have lost their eyes entirely. A type of fish called the blind cavefish lives in caves in Mexico. Scientists believe these fish evolved from surface-dwelling ancestors that entered caves multiple times independently. When the blind cavefish hatch from eggs, they actually have the beginnings of eyes. But as they grow, the cells in the lens of the eye die on their own. Soon after, the cells in the back of the eye, called the retina, are also lost. This process causes the fish to lose their sight completely [2].
3. How Eyes and Brains Use a Lot of Energy
Why would an animal lose its eyes? Eyes and the parts of the brain that process vision require a large amount of energy to work. One study found that for young cavefish growing up, about 15% of the energy they use while resting goes toward their eyes and visual brain parts. That is about 15% of the energy used by young surface-dwelling relatives while resting. In caves, where food is scarce, not having eyes might help the fish save enough energy to survive [3].
4. How Blind Cavefish Use Other Senses
Because they cannot see, blind cavefish rely on other senses. They have a body organ called the lateral line, which detects slight movements in the water. Scientists also think their sense of taste becomes sharper. These enhanced senses allow the fish to find food in the dark water even without eyes [4].
5. The Olm: A Cave Animal That Barely Moves for Years
Some animals lose their eyes and become remarkably inactive. In European caves, there is a creature called the olm. Olms have no eyes and pale, white bodies. In one study, an olm moved only a few centimeters over a period of 2,659 days, which is about 7 years. Another experiment showed that an olm can live for about 10 years without food. Research based on captive populations in Moulis, France, estimates that the average lifespan of an olm is about 68.5 years. The predicted maximum lifespan for these animals is over 100 years [5].
6. Why Cave Animals Around the World Look Alike
Animals in caves around the world often develop similar physical changes. They may become pale, grow slightly larger, or have longer legs. A study comparing different groups of millipedes in caves in southern China found these three changes in every comparison made. For species that still had eyes, there was a trend of fewer eye spots. However, because some compared species had no eyes to begin with, this change was not seen as clearly as the other three. It is like people in different countries dressing similarly for cold weather without knowing each other. They evolved similar shapes independently because they faced the same challenge of living in caves [6].
7. A Safe Activity: Exploring Touch While Seated
You can try to understand how other senses help in the dark. Before you begin, make sure there are no dangerous objects like knives or hot drinks on the table. Then, sit down, close your eyes and try to find things using only your hands. This simple activity shows how much you rely on touch and hearing when you cannot see.
Safety Note: Choose a place with no dangerous items like knives or hot drinks. Keep your eyes closed and try to find objects around you. This helps you experience how your mind focuses on touch and hearing when sight is not available.