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.

Follow Katie Langin on Twitter.

Katie Langin holds a Ph.D. in ecology and was a 2014 AAAS Mass Media Fellow at National Geographic.
  • Tom Harnish

    Wow! That means they have an internal calendar too, because the Sun sets in a different place every day! Evolution sure has produced some amazing adaptations.

  • Ricardo Manuel Mugerema Junior

    I think so the bat doesn’t walked on presence of light… But when I see the information, Iam suprise with the study.

  • wwiddy

    Are we looking at a separate evolutionary process that resulted in similar methods of navigating, or are we looking at a shared genetic thread between avians and mammals that re-emerged in the bats?

  • Tony Cooley

    All of these studies show how an animal orients to north, which is fine if the animal only has to go north or south or can use its memory of the route traveled to return home. However, only knowing the direction of north does not provide enough information to return to a distant location when the animal is moved there without experiencing the route (as when carried in a box). It is the same problem sailors had before the nautical chronometer was invented. To get an absolute position on the earth (so you can return to home by a direct route between the two points), you need longitude (the east-west component) as well as latitude. Things like the dip of the magnetic field or orientation to stars may help establish north-south position relative to home, but I have not yet seen an explanation of how an animal determines the east-west component. The earth rotates so every position on the earth over time will have the same north-south orientation relative to stars.

    Some east-west location information could be obtained from variations in magnetic declination (deviation of magnetic north from true north), but the animal would have to experience this by having been all those places to know how this changes because it does so irregularly. It is also a very small effect.

    The only way I could credit an animal with knowing its east-west location relative to home when taken there in a box is for it to have a very accurate internal chronometer that allows it to compare local time at its present position to its internal clock. It would also have to be able to determine local time very accurately and at any time of day (rather than wait until noon or some other special orientation of the sun or stars to local position).

    Simple calculations can outline the precision required. A nautical mile is about 6076 feet, which corresponds to one minute of arc of a great circle around the globe, say at the equator. There are 360° X 60′ = 21600 minutes of arc in a circle, so each hour of rotation the earth moves 21600/24 = 900 minutes of arc or 5,468,400 feet an hour at the equator. It moves 1519 feet/second of time. As you move toward the poles, the width of an arc of rotation decreases, eventually reaching zero width at the poles where all longitude lines come together. At any given latitude, the ratio of arc length to equator arc length is the ratio of radii from the rotation axis. Draw a triangle inside a circle and you can see the radius at any latitude is the cosine of the latitude because the latitudes are measured as the angle from zero at the equator. My latitude in Kentucky is about 36°. Cos(30°) = 0.81, so the ground is moving about 1230 feet/second of time. Thus, if you move a bird or other animal 5 miles east-west from home, that is (5280 X 5)/1230 = 21.5 seconds of time shift between that animal’s internal clock and its local time. Only an error in this determination of a few seconds of time would be allowable for the animal to determine a travel heading that would take it straight home. This requires a very high precision both for an internal clock and also for the animal to determine from star position or other clues the exact local time. If this actually happens, I would really be impressed and want to know how an animal manages this precision for both body time and determination of local time. Of course, if moved 100 miles from home, the time difference is a little over 7 minutes. An error of half a minute or so would still be usefully close to the bearing derived from a more accurate answer.

    An alternate way the animal could navigate back home when taken to a destination without seeing its route would be do this the way sailors did before nautical chronometers. It could travel north or south until it matches the right latitude and then turn right or left to follow that latitude home. It would be a longer route, but reliable. However, the claims I have seen for snakes were they oriented directly on the home, which requires an east-west fix as well as north-south.

    If animals actually do have an internal clock and ability to perceive local time to compare them, an imprecise east-west difference would be very helpful as the animal could “guess” the direction based on its approximate east-west distance and its more accurate north-south difference. It could then obliquely approach the right latitude using its east-west guess, then shift to follow the latitude line east or west as it gets closer. This compensates for the increasing demands for precision in time difference discrimination as it nears home. Unless researchers were watching for this behavior, they might simply assume the animal’s deviation from a straight line home was simply an animal following its own agenda that doesn’t require it to go straight home rather than being limited in its ability to define the precise direction. Of course, it is true animals have no obligation to go directly home. That complicates research, so the experiment design would have to work around or allow for this animal motivation and priorities factor.

    As noted in the calculations above, the farther an animal is shifted east-west from home, the less precisely it needs to identify the time difference for the result to be useful. This orientation capability based on time demands higher precision as it nears its home to maintain the same angular movement orientation precision. However, as an animal returns to its home range, it will have many additional clues, including its familiarity with its territory, that could then take over from its ability to identify time-differences.

  • Joe Haughawout

    So, the manipulated bats NEVER found their way home?!?

  • Liselle S

    Interesting! So aside from echolocation bats also need this polarized light to orient and navigate at greater distances. So how do these bats detect the polarized light? Is it through visual perceptions?

  • Bob frank

    This subject is really cool

  • Budder wolf

    That is very interesting facts about the bat…if u find more interesting thing about them, plz tell me more.

    Peace out!

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