Bats rely on the position of the Sun at sunset to navigate, even though they are still in their caves when it becomes dark, a new study shows.
Researchers at the Max Planck Institute, Germany and Bulgaria's National Museum of Natural History studied greater mouse-eared bats (Myotis myotis) to see if they could find their way home from somewhere they had never been before.
The researchers captured the bats and released them 25 kilometres from their roost cave.
When they followed the bats with the help of small radio transmitters, the researchers found that within one to three kilometres of flying most of the bats were heading in the direction of their cave.
"I was quite sceptical that this first part of the experiment would work," says study co-author Dr Björn Siemers. "Therefore I was very impressed that the fastest bats arrived back in their cave only two hours after release."
Using the Sun
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, like birds, calibrate their magnetic compasses 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 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 as 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.
"The manipulation of the magnetic field was only effective in combination with the sunset," says co-author Dr Richard Holland.
They believe the bats seem to know that the Sun always sets in the west whereas Earth's magnetic field is less reliable.
The researchers say the result is remarkable given that this species usually emerges from their caves after sunset.
"After the bats became active, we were able to see where the Sun had disappeared even an hour after sunset", says Siemers.
In another study appearing in the same edition, researchers believe they have identified how animals use echolocation to navigate through a cluttered environment.
A team led by Professor Hiroshi Riquimaroux of Doshisha University, Japan, tested the ability of big brown bats (Eptesicus fuscus).
By attaching microphones to the heads of the bats, they recorded the sounds emitted by the bats as they navigated through an obstacle course of plastic chains suspended from the ceiling.
After training the bats to follow a specific flight path, the researchers rearranged the chains to observe how the bats negotiated the unknown environment.
The researchers found the bats responded by emitting additional sounds at slightly higher and lower frequencies, which aided them in identifying ambiguities and helped identify a different flight path.