Bats rely on the position of the sun at sunset to navigate, even though they are nocturnal creatures and are still in their caves when the sun goes down, a study published Monday showed. Researchers at the Max Planck Institute studied greater mouse-eared bats to see if they could find their way home even from somewhere they had never been before or if Earth's magnetic field, which earlier studies have shown are used by bats to navigate, had been tinkered with. First, the researchers captured bats and released them 25 kilometers (15 miles) from their roost cave. When they followed the bats with the help of small radio transmitters, the researchers found that within one to three kilometers (0.6-1.8 miles) most of the bats were heading in the direction of their cave. The fastest were back home within two hours. Once the researchers had worked out that bats could find their way home from a strange place, they wanted to know how they did it. They tested the hypothesis that bats do as birds do and calibrate their magnetic compasses -- which use the Earth's magnetic field to navigate -- to the sun. The researchers altered the magnetic field for half the bats at night, shifting it from north to east using a Helmholtz coil after the sun had completely set. The bats with the altered magnetic field flew in the same direction as the control bats -- in other words, they headed home. When the researchers altered the magnetic field when the sun was setting, the bats flew off course, heading east instead of south, towards home. The researchers concluded that the bats used the position of the sun at sunset as the most reliable indication of direction. Bats seem to know that the sun always sets in the west whereas Earth's magnetic field is less reliable, because its iron deposits can change its qualities, the researchers concluded. 'This result is remarkable given that this species usually emerges from their caves after sunset,' the study said. By watching bats' flight pattern, the researchers were able to determine where the sun had set, even an hour after it had completely disappeared, the researchers said. 