Bats Are The Only Known Mammals To Use Polarized Light To Navigate At Dusk

Bats are not entirely creatures of the dark as some of these animals rely on light to some extent to navigate, according to a new study, published Tuesday in the journal Nature Communications.

Scientists have discovered that greater mouse-eared bats use the pattern of light polarization -- light that vibrates in one direction -- in the sky to find their way around at dusk, an ability not yet found in other mammals. At sunset, bats use the way sunlight is scattered in the atmosphere to regulate their internal magnetic compass, which helps them fly in the right direction, scientists said, adding that how these flying mammals manage to detect polarized light remains a mystery.

“Every night through the spring, summer and autumn, bats leave their roosts in caves, trees and buildings to search for insect prey,” Stefan Greif of Queen's University Belfast in Northern Ireland, and the study’s lead author, said in a statement. “They might range hundreds of kilometers 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.”

Many creatures, such as bees, anchovies, birds, reptiles and amphibians, use polarization patterns as a compass to determine which way is north, south, east and west. While bees have specially adapted “photoreceptors” in their eyes to do this, birds, fish, amphibians and reptiles have “cone cell structures” in their eyes that help them detect polarization.

Although previous studies have suggested that bats might detect polarization patterns when they emerge from their caves at dusk, scientists are still unclear about the mechanism bats might be using to do so.

As part of the study, scientists collected 70 adult, female mouse-eared bats, some of whom -- the control group -- were showed the normal polarized pattern, while the rest were put in a box with lenses that rotated the polarization 90 degrees.

The scientists then released the bats in a site in Bulgaria nearly 15 miles from their home roost at 1:00 a.m. when no polarization was visible. After following the bats, the scientists found that the control group flew in the right direction back home, while the rest of the bats flew either 90 degrees to the left or right of the control group.

"That suggested that the rotation of the polarization pattern by 90 degrees had forced them to rotate their behavior," Richard Holland, a zoologist at Queens University, told the Christian Science Monitor.