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Question Why do bats live in caves? Why don't they fly into objects at night?

Answer

Most bats are nocturnal.

Bats in a Texas evening sky. Insect-eating Brazilian Free-Tailed bats (Tadarida brasiliensis) provide a great pest-control service to agriculture and natural ecosystems. Paul Cryan, USGS photographer, 2009. USGS Science Explorer Media Gallery

Most bats are nocturnal. They fly and forage for their food (bugs) at night. This means that they need safe places to sleep during the day. Caves provide the kind of protected shelter in which bats can thrive. Hanging from the ceiling of a cave, bats are out of reach of most of their enemies. Some of the most successful species of bats live in large cave colonies. Some of these colonies have millions of members, even up to 20 million! Carlsbad Caverns in New Mexico once had 7-8 million, but now has about 1 million.

Bats hibernating in a cave. USGS Science Explorer Media Gallery

So, why don’t bats fly into things at night? They use echolocation to locate food and avoid obstacles. They have the ability to create and hear noises that humans cannot hear. The sound waves bounce off of objects and back to the bat, which can then judge the size and distance of the object. These ultrasonic noises vary in length and pulse frequency, and are unique to the individual. Each bat recognizes its own pulse reflections, or “voice,” and uses it to avoid objects and to identify food.

Most bat colonies leave their caves more or less together, in large groups after dark. Before leaving they fly around inside, preparing for departure. Flying around with thousands of other bats inside a cave creates a chaotic amount of noise! The bats simply ignore their personal navigation systems inside the caves. The echolocators are on, but the bats aren’t listening.

If you suddenly appeared in their cave among the flying bats, they would crash into you. The famous bat expert, Dr. Donald. R. Griffin, called this phenomenon the “Andrea Doria effect.” The Andrea Doria sank when it crashed into another ship out in the middle of the Atlantic ocean.

When bats are paying attention to their sonar signals, they can navigate without crashing into things. They can identify and capture food while it is moving. The echolocation system is designed to locate very small insects. Most of them are less than a centimeter in diameter. Compared to a bug, a human being is a very large, slow moving sound-reflecting surface. Outside of the cave, the chance of a bat hitting a person is very slim!

There are also bats that fly and hunt for food during the day. They sleep outdoors at night in trees, under bridges and other locations. Unlike nocturnal bats, they have well-developed eyes and poorly developed echolocation.

A solitary roosting bat in Mammoth Cave National Park. Dale Pate, photographer, 2018. National Park Service, NP Gallery Digital Asset Management System

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