Are bats truly blind? While the animals aren’t exactly known for their vision, we tend to think of them as primarily creatures of the dark. However, new research indicates at least some of these flying mammals rely on light to guide the way.
Before darkness takes over, the greater mouse-eared bat, a species native to Europe, orients itself using a form of life that most humans can just barely detect.
According to a report published this week in the journal Nature, the bat actually calibrates an internal magnetic compass using the polarized light presented at twilight per Christian Science Monitor.
“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 study lead author Stefan Greif of Queen’s University Belfast in a news release. “But, until now, how they achieved such feats of navigation wasn’t clear.”
Day sunlight arrives on our planet unpolarized – meaning the light waves oscillate in many different orientations.
Depending on how much atmosphere the light waves have to travel through, orientations get blocked by air, water, dust, and gas molecules, leaving behind only polarized light waves.
When the sun is placed directly overhead, the most polarized light appears around the horizon, 90 degrees from its source, having passed through the most atmosphere.
As the sun sets, a band of polarized light rises overhead. As bats awake to this band, they use it to calibrate their internal magnetic compasses.
The researchers collected 70 female mouse-eared bats. The choice for females was based on them residing in one cave throughout the summer, while males move around.
Each of these homing bats were then put in a box to watch the sunset. Some of the boxes allowed for the bat to see the polarized band normally. This constituted the control group. The experimental group’s boxes had lenses that rotated the polarization 90 degrees, meaning the band appeared to extend from the sun to the east rather than the north to the south.
After dark, the bats were transported to one of two release sites. One was 12.5 miles away from the bats’ home caves while the other was 14.5 miles away.
When the bats were released, they were tasked with finding their way home. The control group flew in the appropriate direction towards home, while the experimental group 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,” says study coauthor Richard Holland, a zoologist at Queens University.
“Bats are really quite important animals for our ecosystem,” says Holland. “They eat millions of insects. People have actually estimated the cost to lose large populations of bats in terms of millions of dollars in insecticides.”
“Unless we fully understand the behavior of the animal, we can’t understand what they require from their environment, what we need to do to protect them within this environment.”