“Blind as a bat?” Pshaw. Bats are known for their ability to navigate their environment through echolocation, but new research shows that the greater mouse-eared (Myotis myotis) bat also uses polarized light in the sky to calibrate their internal compass. The research was led by Richard Holland of Queen's University Belfast, and the results were published in Nature Communications.
"We know that other animals use polarization 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 polarization," Holland said in a press release. "But we don't know which structure these bats might be using.”
As sunlight comes through Earth’s atmosphere, it is given directionality and creates polarization patterns. These patterns are most pronounced when the observer is 90 degrees away from the Sun at sunrise and sunset. Research has shown that animals are able to detect patterns long after dusk and even in bad weather when the Sun is not clearly visible. Certain animals are able to use these patterns and use them to reorient themselves.
"Every night through the spring, summer and autumn, bats leave their roosts in caves, trees and buildings to search for insect prey,” lead author Stefan Greif explained. “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.”
Holland’s team split 70 M. myotis females into two groups that were each exposed to different polarization patterns, one normal polarization and one shifted 90 degrees to the right. All of the bats were fitted with transmitters to track their locations and the two groups were then released in the middle of the night, when they would not be able to detect polarization patterns. The group that had been exposed to the shifted pattern oriented themselves about 90 degrees away from the control group.
Unfortunately, the scientists still don’t understand the mechanism that is allowing the bats to sense and interpret this polarized light pattern. It is possible that the orientation and structure of their retinal cells are causing this, but that will be explored in future research.
"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," added Greif.