Bats use sound and echolocation to navigate, but they have another trick up their sleeve, researchers say; they use polarized sunlight at sunset to set their internal compass for nighttime use.
The atmosphere scatters sunlight and it becomes polarized, or directional, in patterns determined by the sun's position at it moves across the sky.
The patterns are strongest at sunset and sunrise, in a strip in the sky positioned 90 degrees from the sun.
A number of animals from birds to fish to reptiles can use these patterns for orientation cues, so researchers at British and Israeli universities set out to find if bats had the same capability.
"We had already demonstrated that bats used a magnetic compass that was calibrated by cues observed at sunset," says Richard Holland of Queen's University Belfast. "The question was, what cues? It was known that birds calibrate the magnetic field with the pattern of polarization at sunset, so we tried the same for bats."
It is known how some other creatures do it, he says; specially adapted photoreceptors are present in the eyes of bees, while fish, birds, reptiles and amphibians have all evolved cone cell structures allowing their eyes to detect polarization.
"But we don't know which structure these bats might be using," says Holland.
Working with colleagues at Queen's and at the University of Tel Aviv, Holland gathered 70 greater mouse-eared bats and exposed them to different types of sunset polarization patterns, manipulated by filters on experimental boxes in which the bats were kept.
Bats exposed to the manipulated patterns, when released, flew off at a heading exactly 90 degrees from bats that experienced natural polarization patterns, the researchers found.
"By shifting the pattern 90 degrees, we shifted the bats' orientation after sunset (when only the magnetic compass was available) either 90° east or west of the control bats," Holland says. "It shows that the bats calibrate the magnetic field with the pattern of polarization at sunset."
Although the experiment confirmed that bats' use of polarized light, it was unable to determine how they were able to detect the patterns, the researchers acknowledged.
However it's done, it just one more way the bats use to get around in addition to sound, sight, and the Earth's magnetic field, they say.
Echolocation only reaches out about 160 feet from a flying bat, they say, so some other sense or senses must come into play for navigation over longer distances.