Almost every species of bat is nocturnal, meaning that these are animals that are active at night.  Moving around at night is bad enough (try walking outside without any lights nearby and see how difficult it is), but imagine how difficult it must be if you're flying through the air at several meters per second!  If a bat runs into a tree or a building, it's going to be hurt pretty badly.  There is also the fact that bats have to fly around any obstacles but they also have to find food while they're flying around.  This is a difficult problem to deal with, and bats have come up with two major solutions.


Solution 1: Excellent Vision


Some bats have solved this problem using their eyes.  These bats (the Megachiroptera) have large eyes that are capable of seeing well even in almost total darkness.  This pictures shows one of these bats.

Note that this bat has large eyes and relatively small ears compared to the size of its head.  This tells us that the bat uses vision for something very important, but sounds are not as important.  If you've ever read the book Stellaluna, it talks about how Stellaluna was able to see so well in the dark.  These bats primarily eat fruit and other plant material, so they use vision to avoid obstacles, while using their noses to smell the fruit and other plant material that they like to eat.


Solution 2: Echolocation


The other solution to this problem was the development of a system of biological sonar called echolocation as illustrated in the figure below:

The bat emits a loud sound (represented by the orange lines) that travels out into the environment until it hits something (such as the insect in this picture).  When the sound hits, some is reflected back as an echo (the gray lines in the figure).  The bat can hear the echo and use that to tell a lot about the obstacle, such as how far away it is and some fine details such as its size and shape.  This allows many of these bats to capture flying insects by hunting them down like a radar-guided missile.  These types of bats (the Microchiroptera)  tend to have large ears and small eyes.  While vision is not as important to these bats as it is to the Megachiroptera, these bats can see, so the saying "blind as a bat" is a myth!

Luckily for us, the sounds these bats make are generally not within the range of human hearing.  Humans can typically hear sounds up to about 20,000 Hertz, (Hz) while most bat echolocation sounds are 30,000 Hz or more, and some bats go as high as 200,000 Hz.  Bats are not the only animals to use such ultrasound, as it is found in many rodents in addition to some birds and frogs.  However, bats seem to be the masters of using their sonar system to navigate and capture prey.

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