Why do bats fly at night?
(Lansing State Journal, October 29, 1997)

Bats have wide hairless wings that would dry out in the sun if they flew during the daytime.  Some bats eat fruit, many bats eat insects and vampire bats in the tropics feed on the blood of animals.  Bats are good at flying at night because they use sound rather than sight to navigate.  This method of navigation is know as echolocation and it is similar in principle to the sonar that submarines use to navigate the ocean.  Bats send pulses of sound through their mouths or noses, and  these pulses echo back outlining the objects in the bats’ flight path.  The ears of a bat are large and oddly constructed but they help it to determine where the echoes are coming from.

To search for insects a typical bat will sweep an area with sound, sending out 10 beats per second.  When the echoes come back from the prey, such as a fly, the sound pulses increase to more than 25 per second.  This gives the bat a better idea of where the fly is moving.  As the bat hones in on its prey, it sends out more and more sound blips; as many as 200 per second.

This method of navigation is very efficient for the bats.  They can eat 25% of their body weight in insects in 30 minutes.  Some insects, like gnats are so small that they weigh almost nothing.  So some bats may catch more than 1200 insects in an hour, or one insect every 3 seconds.  Some bats are so good at echolocation that they can detect and avoid wires as thin as human hairs.

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