Polarized Light Cues Help Bats Find Their Way

It's not all about echo location. As dusk descends, bats quickly set an "internal compass" using polarized light that will help them find their way at night, according to a recent study.

The study, published in the journal Nature Communications, details how bats calibrate their internal magnetic compass once daily using polarized light.

Magnetic compasses are common among birds and even insects, who use an intrinsic knowledge of the Earth's poles to orient themselves as they travel. However, it's not as simple as waking up and instantly knowing which way is north. The Earth's magnetic field is in a constant flux, with the magnetic poles gradually migrating to ends opposite to where they are now. Human interferences, largely in the form of electromagnetic noise from urban settings, can also interfere with this natural compass.

To correct for this, birds and even monarch butterflies use environmental cues to orient themselves, much like a cabbie will use landmarks to recall which street to turn down next.

However, poor lighting at night makes orientation much harder for bats. According to this latest study, bats instead figure out where their compass is pointing by using cues from the sunset - namely polarized light.

Lead researcher Stefan Geriff and his team confirmed this after testing a group of about 70 greater mouse-eared bats.

Some bats were exposed to standard polarized light at dusk - a band of light that stretches across the sky from north to south. Others in the group were exposed to the same band, but only after it had been rotated 90 degrees.

These bats were then released more than 14 miles from their homes in the dead of night. Interestingly, while the unaltered group easily found its way back, the groups that had encountered the altered light set off in the wrong direction, often heading an exact 90 degrees from the correct direction.

Rachel Muheim, an expert who was not involved in the study, told National Geographic that this is the first evidence of a mammal using such a navigational mechanism, and "may indicate that other organisms do the same."

The study was published in Nature Communications on July 22.