The ability of bats to 'see' in the dark is mainly thanks to their ears. These animals have developed a sophisticated echolocation system, which enable them not only to locate their prey with a high level of precision, but also to avoid obstacles very adroitly. When airborne, there skilled fliers produce sounds with an ultrasonic frequency of more than 20,000 Hz, which are mostly inaudible to the human ear. Echoes of these generally vary loud cries are reflected from the bats' surroundings, giving the flying mammals a precise 'sound picture' of their environment. However, bats are not completely blind. They can distinguish between light and darkness and are therefore able to recognize contours. Bats also have a very good memory for places, which they constantly update during the course of their flights.