Bats aren't exactly known for their keen eyesight. With their caves and sonar and whatnot, we tend to think of them primarily as creatures of the dark.
But according to new research, at least some of these flying mammals rely on light.
Before darkness descends, the greater mouse-eared bat, a species native to Europe, orients itself using a form of light 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 present at twilight.
"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."
How does polarized light help the bats?
Most sunlight during the day arrives on our planet unpolarized, that is, the light waves oscillate in many different orientations.
But depending on how much atmosphere the light waves travel through, some of these orientations get blocked by air, water, dust, and gas molecules, leaving behind only polarized light waves.
When the sun is directly overhead, the most polarized light appears around the horizon, 90 degrees from its source, having passed through the most atmosphere.
But just as the sun sets, a band of polarized light rises overhead, arching like a rainbow, with one end pointing north and the other pointing south.
As the bats awake to this band, they use it to calibrate their internal magnetic compass.
But how do we know?
The researchers collected 70 female mouse-eared bats from their home caves. Females were chosen because they spend the entire summer residing in one cave, while the males may move around.
Each of these homing bats were then put in a box to watch the sunset. Some of the boxes allowed the bat to see the polarized band normally. This was the control group. The experimental group's boxes had lenses that rotated the polarization 90 degrees, so that the band appeared to extend from the sun to the east rather than north to south.
After dark, the researchers transported the bats to one of two release sites. One was about 12.5 miles away from the bats’ homes while the other was about 14.5 miles away.
When the bats were released, they attempted to head home. While the control group flew in the appropriate direction towards home, 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 aren't the only ones to do this
These bats may be the first mammal known to use polarization patterns for orientation, but they're not the only animals.
Karl von Frisch, an Austrian ethologist working just after World War II, first noted that honey bees orient themselves according to polarized light in the daytime. Since then, researchers have observed similar perception among other insects, such as the dung beetle (which uses polarized light from the moon and even the stars). Some bird species also rely on polarized light.
Dr. Holland and his colleagues have not determined how the bats perceive polarized light. In fact, researchers don't fully understand the mechanism in any vertebrates. But they do know that insects have an eye structure that allows them to detect plain polarized light with ease.
Humans can just barely see polarized light. For instance, if you view a white space on an LCD screen, you might be able to perceive a fuzzy yellowish horizontal bowtie crossed by a vertical blueish bowtie. This faint optical phenomenon, known as Haidinger's brush, is an artifact of how our eyes evolved, says Holland.
"Fairly early on in the evolution of navigation mechanisms polarized light seems to have been playing an important role," suggests Holland.
Success springs from a dead end
The researchers originally thought that the bats used the location of the sun in the sky to calibrate their internal compasses, not the polarization patterns. They had found in a previous study that the bats used cues at sunset to orient themselves, but still had to determine what those cues were.
"We already knew from work on birds around the same time that birds actually showed this same kind of behavior and the birds were using the patterns of polarized light at sunset. But we were skeptical that the bats were doing the same," says Holland.
"The first thing that we tried to do was shift the position of the sun by putting the bats in front of a mirror so the sun's position was actually deflected," says Holland. "The bats weren't fooled by that at all."
Why should we care?
"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."