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Bats Use Sun to Calibrate Geomagnetic Compass

from - Newsletter of the Bedfordshire Bat Group April 2013
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.”

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

“Blind as a bat” goes the saying – but that’s a myth. Small bats have perfectly good eyesight for their size, but they can also call upon “extra senses” which humans can only achieve with technology. These include the famous ability to navigate using echoes of sounds and the ability to track Earth’s magnetic field based on light from sunsets.

Bats use echoes to hunt and navigate around familiar areas. However, studies from the late 1960s have shown that beyond a certain distance (about 10 miles), they need something else to navigate.

If a bat is to return home from an unfamiliar place, it needs something like a compass and a map to tell it where it is and where to fly. In other animals, such as birds, we have known how they do this for a long time.

But research on the flying mammals has for some reason lagged behind. In part, it may be because of the difficulty of studying small and highly mobile nocturnal animals. With the help of some colleagues, some eight years ago, I set out to discover the map and compass bats used to navigate. Our recent results have now been published in the journal Nature Communications.

The first place to look was to find if bats have magnetic sense, which is what birds use when faced with similar navigation challenges. To do that, we put bats that we had captured at a local barn in a box with an electromagnetic coil. When inside, the box changed the direction of the magnetic field of the surrounding area by 90 degrees. Once the sun had set, the pigeons were let out, after fitting a small radio transmitter to their backs and displacing them 20km north of the home roost.

The transmitter allowed us to track the paths the pigeons took. What we found was that bats that had been in the altered magnetic field flew off in a path that was deflected by 90 degrees compared to the bats that hadn’t been in an altered magnetic field. But this only happened when those bats in the altered magnetic field were released after the sun had set. If they were put in an altered magnetic field and release before sunset, they flew home without any deflection.

It seemed then that bats, like birds, calibrate their magnetic compasses based on cues observed at sunset. But what could these cues be?

The most obvious seems to be the sun’s position. Unlike the Earth’s magnetic field, which can be variable depending on location, the sun predictably sets in certain locations throughout the year. Thus, if the bats could learn that position, this would be a reliable cue on which to calibrate the magnetic compass.

To test this, instead of putting them in altered magnetic fields, the bats were shown a mirror at sunset. It was angled such that the sun appeared deflected by 90 degrees. Then, when released, we observed the bats' flight path. But the mirror seemed to have had no effect on their orientation. Clearly, something else must be going on.

Perhaps bats, like birds again, used the pattern of polarised light at sunset and sunrise to calibrate a magnetic compass. Polarisation measures the angle at which light waves move in relation to the direction in which they are travelling. Human eyes don’t have the ability to detect the differences in polarisation.

The polarisation cue appears as a dark band running across the sky from north to south as the sun sets in the west. To test our hypothesis, we put our bats into boxes with polarisation filters that changed this pattern so that it was rotated by 90 degrees. Then the birds were released from the boxes after sunset and 20km away from from home.

This time we found that their paths were indeed deflected by 90 degrees, compared to bats that had been in boxes that mimicked the natural polarisation pattern. This is exactly how they had reacted when their magnetic orientation was altered after sunset.

So it seems that bats use the Earth’s magnetic field as a compass, and that this is calibrated by the pattern of polarised light at sunset. This makes bats the first mammal we know to show the use of such cues for navigation.

Although this is an exciting result, it raises the question of how the light-sensitive cells in bat’s eyes have adapted to detect it. Also, it only provides the answer to how bats' compasses help them set off in the right direction. We still need to find out how bats work out their position to navigate long distances in the dark.



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BatsRule!: Bats Use Sun to Calibrate Geomagnetic Compass
Bats Use Sun to Calibrate Geomagnetic Compass
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