Scientists have discovered that greater mouse-eared bats use polarisation patterns in the sky to navigate - the first mammal that is 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.
However, researchers could not determine how the creatures 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 in their eyes, and birds, fish, amphibians and reptiles all have cone cell structures in their eyes which may help them to detect polarisation," said 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," Holland said.
Polarisation patterns depend on where the Sun is in the sky. They're clearest in a strip across the sky 90 degrees 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.
Previous studies have 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," said Stefan Greif of Queen's University Belfast, lead author of the study.
In the study, Holland, Greif and colleagues from Tel Aviv University showed 70 adult, female mouse-eared bats one of two different types of polarisation patterns at sunset.
They then took them to one of two release sites in Bulgaria about 20 to 25 kilometres from their home roost.
They released the bats at 01:00 AM - when no polarisation is visible - and followed the direction they set off in using small radio transmitters attached to their backs.
They found the bats that had been shown a shifted pattern of polarised light headed off in a direction shifted at right angles from the controls released at the same time.
Bats probably use a suite of senses, including the position of the Sun or the stars, the Earth's magnetic field, smells, sight, and of course, echolocation to navigate, researchers said.
The study was published in the journal Nature Communications.
