Echolocation: Bats are recognized as small, nocturnal mammals that
are able to truly fly, not merely glide as the flying squirrel
demonstrates, and form the second largest mammalian order,
outnumbered only by the rodent order, which has more species.
However, bats are also known specifically for their use of
echolocation to navigate and catch prey in the darkness: they are
constantly releasing ultrasonic waves during the whole time they are
awake, using it to enhance their sight, which exceeds the sight of
homo sapiens on its own. By emitting and receiving ultrasonic sound
waves, they retain the ability to see further distances than many other
organisms and can derive information which is much more refined.
Although not all bats use echolocation, such as some megabats
(large bats), many bats, many known as "microbats" do use sonar, however, the use versus
no use suggests different ancestors.

More info at:http://members.fortunecity.com/anemaw/bat.htm
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