By Jonathan Webb
Bats use the pattern of polarised light in the evening sky to get their bearings, according to a new study.
As well as having unusual echolocation skills and their own magnetic compass, bats are now the first mammals known to make use of polarised light.
Other animals with this ability include birds, anchovies and dung beetles.
To make the discovery, published in Nature Communications, zoologists placed bats in boxes with polarising windows before watching them fly home.
Light waves normally wiggle all around their direction of travel, but when they pass through special filters – or are scattered by gases in the atmosphere – they can become polarised, so that the oscillations all line up.
“We initially didn’t think that the bats would use polarised light,” said the paper’s senior author, Dr Richard Holland from Queen’s University in Belfast.
Dr Holland was one of the scientists who discovered in 2006 that bats navigate by somehow sensing the earth’s magnetic field – but that in-built compass needs to be calibrated. Other experiments showed that the calibration was happening at sunset, when the bats’ day begins.
“We thought that surely, the sun’s disc itself would be a more likely cue,” Dr Holland told the BBC. But his team recently tested how bats responded when the sun’s image was shifted by mirrors, and found no difference.
So they switched their attention to the pattern of polarised light that appears at sunset, which is already known to be important for various other animals, particularly birds.
It’s invisible to humans, unless we wear polarising glasses. “If you were standing looking at the sun, you’d see a dark band going directly over your head, from left to right,” explained Dr Holland.