Greater mouse-eared bats use polarized light to help them navigate at sunset and sunrise, according to a new study. They are the first mammals known to use this method to find direction.

When the sun starts to set over the horizon, the animals use light scattering patterns to reset their internal compasses. Although researchers now realize polarized light plays an important role in calibrating navigational systems in bats, they do not yet understand how the animals detect the rays.

"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," Richard Holland from Queen's University Belfast, co-author of the study, said.

Polarized light is used in 3D movies to project a pair of images with every frame of film. Viewers wear glasses with polarized filters over each eye, turned at right angles to each other. Light from one frame of each pair passes through a single eye, and is nearly completely blocked by the other filter. In nature, the sky acts like the filters, with the brightest light found 90 degrees away from the setting sun.

Researchers from Tel Aviv University exposed 70 adult female mouse-eared bats to polarized light. They later released the animals at 1 p.m., when polarization is negligible. Flight patterns were recorded, and followed the paths expected from the polarized light.

Bats are able to orientate themselves, even at night or on cloudy days. Researchers are uncertain how they continue to navigate without being able to see the sun. Bees have special photo-detectors in their eyes, while reptiles, amphibians, birds and fish each possess cone structures in their retinas that may detect polarized light. Dung beetles have been shown to use polarized light from the moon to find their way around.

The flying mammals leave their caves and trees each evening to search for insects. During a single journey, the creatures can fly hundreds of miles. Biologists did not know how the flying mammals found their way back home after these long hunting expeditions.

"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," Stefan Greif from Queen's University Belfast, lead author of the study, stated in a press release.

Discovery of the role of polarized light in the navigation systems of bats was profiled in the journal Nature Communications.

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