Bats are nocturnal, but some need sunlight to set their internal compass.

"Recent evidence suggests that bats can detect the geomagnetic field," wrote Max Planck Institute ornithologists Richard Holland, Ivailo Borissov and Bjorn Siemers in an article published March 29 in the Proceedings of the National Academy of Sciences. "We demonstrate that homing greater mouse-eared bats calibrate a magnetic compass with sunset cues."

Previously, Holland showed that interfering with the magnetic field around bats impaired their long-distance navigation abilities. Those findings suggested that while bats used echolocation for short-distance steering, they rely on some geomagnetic sense to guide nocturnal flights that take them dozens of miles from home. The details, however, were hazy.

In the new study, Holland's team captured 32 greater mouse-eared bats. Half of them were placed inside a pair of giant, coiled magnets that created a geomagnetic field misaligned with Earth's, temporarily scrambling their own geomagnetic sense. All were released in an unfamiliar location 15 miles from their home cave.

Bats that were captured at night flew home unerringly, regardless of what the researchers had done. They'd already set their compasses by the sun. But if the bats were captured and magnetically disoriented at twilight, when they would normally be flying around calibrating their compasses, they could no longer find their way home. The bats appear to use the twilight as a point of reference while setting their compasses for the rest of the night.

How the compass works is still a mystery. Some birds use sunset for navigational calibration, but the similarities likely end there. While birds' eyes contain geomagnetically sensitive molecules that are activated by photons, Holland has previously shown that bats don't have this system. Instead, some of their cells appear to be laden with magnetite.

Bats that fly only in the dead of night, such as vampire bats, could provide an interesting comparison, wrote the researchers.

"The cues used by the bats to indicate their position can only be speculated on at this stage," they wrote, noting that ornithologists have argued over the bird compass for decades. "For animals that occupy ecological niches where the sunset is rarely observed, this is a surprising finding."

Images: 1) Greater mouse-eared bat/Gilles San Martin/Flickr. 2) The directions flown by control and experimental control bats when their magnetic fields were disrupted at sunset (above) and after dark/PNAS.

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Citation: "A nocturnal mammal, the greater mouse-eared bat, calibrates a magnetic compass by the sun." By Richard A. Holland, Ivailo Borissov, and Björn M. Siemers. Proceedings of the National Academy of Sciences, Vol. 107 No. 13, March 30, 2010.

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.