Bats are famous for their ultrasound technique for space orientation while flying in the dark, called echolocation. This is efficient over short ranges and enables bats to locate prey and obstacles.
But scientists were puzzled about how bats manage to navigate long distances in the dark.
A recent study solved the puzzle: they use Earth's magnetic field, in a similar way to migrating birds.
Bats, like some birds, also use sunset as a directional marker.
Exposure to artificial magnetic fields confused such bats, causing them to fly in the wrong direction.
"It is interesting that our bats, which usually fly only a few miles, appear to use the same mechanism as migratory birds that fly thousands of miles," said biologist Richard Holland in Princeton University, New Jersey.
The team equipped big brown bats (Eptesicus fuscus, photo), a common species in North America, with lightweight radio transmitters and released them from
a location 12 miles (20 kilometers) north of their home roost.
Bats captured in a barn were exposed at dusk to an artificial magnetic field pointing east or west relative to the Earth's magnetic North, while other captured bats were left unexposed.
The bats were able to observe the setting sun.
The team tracked them using a small aircraft.
"The control bats, relying on their natural magnetic field, headed south, directly towards home," said Holland.
But those exposed to the artificial fields flew in the wrong direction.
Bats in the group exposed to the clockwise magnetic fields flew east, for example, while those in the opposite group flew west.
"But some of the errant bats eventually realized their mistake and made it home that night, suggesting either a recalibration of the compass or use of other navigational cues," Holland says.
Dr Holland said this, suggesting that the bats that may have recognized their compasses were faulty and switched to some other mechanism, as yet unknown.
"Given that it is possible to receive the signals of the transmitters used in our experiments from a low orbit satellite, it will hopefully soon be possible to track small animals, such as bats, birds and insects, globally with significant consequences for disease, agriculture and conservation.", said Holland.
A magnetic sense has been described till now also in birds, salamanders, lobsters and turtles.
"It's not just associated with long-distance movements or migration," said John Phillips, a biologist at Virginia Tech University in Blacksburg.
"We find even short-distance movements by simple animals are patterned by use of directional compass cues." Two types of magnetic orientation are known so far.
In one, a simple compass sense is based on particles of magnetite (a magnetic mineral).
Some birds can also "see" changes in light intensity in different locations within Earth's magnetic field.
This more sophisticated sensory apparatus provides an internal "map sense" of where the birds are on the globe.
"We don't know if bats have a light-dependent compass," Holland said.
"But they do have magnetite in their bodies."
"90% of the world's bats have barely been studied other than to give them their names - they rank as one of the least studied mammals," said Merlin Tuttle, president of Bat Conservation International.
"It does not surprise me that they use magnetic fields, although I am sure further research will disclose many other additional ways that they can navigate."