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Bats use polarized light to calibrate internal compasses

Bats navigate the evening sky using patterns of polarized light, according to a new study. Researchers from Queen’s University in Belfast have found that in addition to their uncommon echolocation skills, bats use polarized sunlight at dusk to set their internal compass. They are the first mammals known to exhibit this ability.

Sunlight is naturally scattered in all directions but becomes directional when it is passes through gases in the earth’s atmosphere. The directional patterns it creates are determined by the position of the sun in the sky.

These patterns create stripes across the sky 90 degrees from the position of the sun when they are strongest at both sunrise and sunset. Many species of birds, fish and insects are capable of seeing the polarized light. So far bats are the first mammals aside from humans who are known to be capable of perceiving the phenomenon at all.

Dr Richard Holland, the paper’s senior author, was also a part of the team that
bats used a magnetic compass which they set using cues at sunset. “The question was, what cues? It was known that birds calibrate the magnetic field with the pattern of polarization at sunset, so we tried the same for bats,” said Holland.

To determine whether or not Bats had this ability, the zoologists placed bats in boxes that showed them polarized patterns before allowing them to fly their homes 20 kilometers away. Using small radio transmitters, the scientists found that the bats that had been shown a polarization pattern shifted by 90 degrees flew in right angles from the bats that were shown the natural polarization pattern.

The behavioral evidence supports bats using the polarized light at dusk to set their course, but scientists are unsure how they detect it. Unlike many insect species, vertebrates lack the specialized photoreceptors in their eyes, leading researchers to believe the structure of their cone cells may play a role.

The research provides a clearer picture of how bats use “map and compass” mechanisms to navigate longer distances outside the range of their echolocation.

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