Bats Have a Vertical “Blind Spot” When Using Echolocation

Bats colliding with the vertical plate produced fewer calls and had higher flight speeds relative to the bats that avoided collision. | Stefan Greif

Bats are well known for their sophisticated use of sound waves and echoes to navigate through areas riddled with obstacles, but now a new study reveals that their use of echolocation can be hindered in the face of smooth vertical surfaces. The results , published in the September 8 issue of Science, may help explain why injured or dead bats are often found near buildings.

To navigate through the dark, bats rely heavily on echolocation, where they emit high-frequency sounds and use the returning echoes to detect and classify objects in their environment. However, when Stefan Greif of Eötvös Loránd University was studying how bats use echolocation to recognize water surfaces, he noticed something odd.

"I found that bats sometimes collided with our smooth plate when I temporarily positioned it [vertically along] the wall [while] rearranging the setup," he explained. "That made me wonder why and how those bats would perceive this unusual situation. After my attention was focused on this issue, I started to notice bats sometimes bumping into a metal information plate at the entrance of a cave where we caught bats."

Furthermore, several observations of bats colliding with smooth vertical surfaces (such as glass windows) suggest that bats have problems recognizing them. Very few smooth vertical surfaces occur naturally in the wild, with objects such as trees and rocks exhibiting rough and uneven surfaces; however, bats do encounter smooth horizontal surfaces in the form of water.

To explore this issue in greater detail, Greif and his colleague Sándor Zsebők monitored greater mouse-eared bats ( Myotis myotis) as the animals flew through a continuous, rectangular flight tunnel in the dark. In the corner of the dark tunnel, the researchers placed a metal plate either vertically or horizontally.

Field experiments show bats colliding with vertical smooth surfaces placed in natural environments. | Stefan Greif et al., Science (2017)

Of 21 individual bats, 19 collided with the vertical plate at least once (on average, colliding in 23% of passes) but never with the horizontal plate. The researchers found that when the bats collided with the vertical plate, they were producing fewer calls, spending less time in front of the plate, approaching the plate at a more acute angle, and had higher flight speeds relative to the bats that avoided collision. The authors report similar findings in field experiments outside of caves of three different bat species.

"It is surprising and intriguing how under certain circumstances even the most sophisticated sensory systems can be tricked into misinterpretation of its environment," said Greif. Now that scientists are aware of this sensory loophole for bats during navigation, he noted, they need to gather reliable data about the regular occurrence and extent of these collisions in natural settings.

"In 'high-risk' areas, like the vicinity of important bat roosts, we should try to avoid setting up unnecessary smooth, vertical surfaces. In areas where surfaces like extended glass fronts [exist], we should experiment with mitigation tools like acoustic bat deterrents. These emit ultrasonic sounds, inaudible to humans, that are supposed to repel bats. Even if that fails and curious bats would check on those sounds, they would be very attentive and hopefully realize the obstacles in their way," said Greif.

[Credit for associated photo: Stefan Greif]