Scientists have discovered that greater mouse-eared bats use polarisation patterns in the sky to navigate -- the first mammal that's known to do this. The bats use the way the Sun's light is scattered in the atmosphere at sunset to calibrate their internal magnetic compass, which helps them to fly in the right direction, a study published in Nature Communications has shown.

Despite this breakthrough, researchers have no idea how they manage to detect polarised light.

"We know that other animals use polarisation patterns in the sky, and we have at least some idea how they do it: bees have specially-adapted photoreceptors（光感受器） in their eyes, and birds, fish, amphibians（两栖动物） and reptiles all have cone cell structures in their eyes which may help them to detect polarisation," says Dr Richard Holland of Queen's University Belfast, co-author of the study.

"But we don't know which structure these bats might be using."

Polarisation patterns depend on where the sun is in the sky. They're clearest in a strip across the sky 90° from the position of the sun at sunset or sunrise.

But animals can still see the patterns long after sunset. This means they can orient themselves even when they can't see the sun, including when it's cloudy. Scientists have even shown that dung beetles use the polarisation pattern of moonlight for orientation.

A hugely diverse range of creatures -- including bees, anchovies, birds, reptiles and amphibians -- use the patterns as a compass to work out which way is north, south, east and west.

"Every night through the spring, summer and autumn, bats leave their roosts in caves, trees and buildings to search for insect prey. They might range hundreds of kilometres in a night, but return to their roosts before sunrise to avoid predators. But, until now, how they achieved such feats of navigation wasn't clear," says Stefan Greif of Queen's University Belfast, lead author of the study.

Even so, previous studies suggested that bats might detect polarisation patterns when they emerge from their caves at dusk.

"Most people are familiar with bats using echolocation to get around. But that only works up to about 50 metres, so we knew they had to be using another of their senses for longer range navigation," says Greif.