Bats Set Their Internal Compass at Dusk—A First Among Mammals

Bats are creatures of the night. Even under the cover of total darkness, they can maneuver around trees, chase down moths, and find their way home—but they would still be lost without light, according to a new study.

Greater mouse-eared bats set their internal magnetic compass using the pattern of light polarization—light that vibrates in one direction—at dusk, according to the study, published Tuesday in the journal Nature Communications.

A greater mouse-eared bat (Myotis myotis) drinks from the water surface. Photograph by Dietmar Nill/FN/Minden Picture

Without the proper light cues during that critical period, the bats become disoriented and have a hard time finding their way home in the darkness. (See “5 Amazing Animal Navigators.”)

This marks the first time a mammal has been documented making use of polarized light.

“We [humans] can perceive polarization if you know what to look for, but there is no functional meaning that we know of,” said study leader Stefan Greif, a biologist at Germany’s Max Planck Institute. This study, therefore, may “shine a little light on how it works in humans as well.”

How Bats Navigate

One drawback to bats’ internal compasses is that the Earth’s magnetic field varies across time and space. To compensate for this, some animals—like birds—calibrate their magnetic compass daily using more reliable geographic cues.

Prior studies have indicated that bats figure out what direction their magnetic compass is pointing using cues around sunset. However, until now, it wasn’t clear how they were doing that.

But the study authors had a hypothesis: The flying mammals get directional information from polarized light. (See “Bats Drawn to Plant via ‘Echo Beacon.'”)

At dusk, there is a strong band of polarized light that runs like a rainbow from north to south, a phenomenon that provides a consistent geographic reference and is a known orientation cue for birds.

This light pattern occurs because polarization is maximized when the sun’s rays are scattered at a 90-degree angle from their original path. (Related: “Dung Beetles Navigate Via the Milky Way, First Known in Animal Kingdom.”)

Polarization Lights the Way

Greif and his colleagues tested the role of polarized light by experimentally manipulating 70 greater mouse-eared bats in Bulgaria.

The team placed each bat in a box that simulated polarized light at sunset. Some bats saw the natural pattern; others saw a band of polarization that was rotated 90 degrees.

Next, they displaced the radio-tagged bats more than 14 miles (20 kilometers) from their roosting cave and tracked their movements in the night. (See video: “Bat Hunts in ‘Stealth Mode.'”) 

The result: Bats shown the altered polarization pattern did not seem to know what direction was home. In fact, many went in directions that were rotated 90 degrees from the correct orientation, just as you’d expect if they were navigating using a polarization-calibrated magnetic compass.

The fact that bats “use the exact same compass calibration method [as birds] is remarkable” noted Rachel Muheim, an expert on bird navigation at Sweden’s Lund University, in an email to National Geographic.

It “may indicate that other organisms do the same,” said Muheim, who wasn’t involved in the research.

Bats may be known for their stealth in the dark, but light guides their path after all.

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Meet the Author
Katie Langin holds a Ph.D. in ecology and was a 2014 AAAS Mass Media Fellow at National Geographic.