Smooth, manmade surfaces create a ‘blind spot’ for bats using echolocation

A bat collides with a flat, smooth surface in a tunnel set up by researchers.

The winged mammals have the ability to fly at top speeds in the dark, instinctively avoiding natural obstacles such as trees.

“Bats predominately rely on their echolocation system to forage, orientate, and navigate”, says a team led by Dr Stefan Greif of the Max Planck Institute for Ornithology near Munich in Germany.

Analysing the results, the scientists suggest that shiny upright surfaces disrupt echolocation signals, which in other circumstances bounce back from potential obstacles allowing bat brains to form a “picture” of their position and shape.

Bats navigate by making high-pitched squeaks – and then work out where to go by listening to the echoes of the squeaks bouncing back around them.

However, vertical mirroring surfaces such as window panes appear to trick them into thinking that the way ahead is clear.

Prof Gareth Jones of Bristol University, who is not connected with the study, is an expert on bat echolocation. Bats are sometimes found in nature with broken jaws and wings, or even dead, all situations that could be caused by running headlong into a flat, hard surface at high speeds. In fact, the bats that collided produced fewer calls and spent less time in front of the metal plate compared to the approaches in which they managed to avoid the collision.

Sometimes bats perceive a smooth, vertical surface as an open pathway, a unsafe error near buildings with glass facades, shown by injured or dead bats next to birds found underneath. They put a smooth, 1.2 x 2m metal plate against the wall, and with infrared-cameras and microphones, observed that 19 out of the 21 studied bats collided at least once with the plate within the first 15 minutes they spent in the room.

In the corner of the dark tunnel, they placed a metal plate either vertically or horizontally.

The authors report similar findings in field experiments outside of caves of three different bat species.

In their natural habitat, the vertical surfaces smooth are rare, while the extended horizontal, such as water, are many.

“One of our hopes of this study is people start paying attention to where they find these injured bats so we can get an idea of the extent of the problem”.

The researchers are calling for more evidence to be gathered on the scale of the threat to bats.

The greater mouse-eared bats and other bats often detect smooth glass surfaces only at the last moment. In our modern world, however, they encounter many sensory traps that lead to unsafe errors in interpreting their environment. Contrary to the popular expression, bats are not blind, he said, and one of the big questions in bat biology is how the animals respond if their “eyes tell them one thing and echolocation tells them something else”.

As important pollinators for many plants, and key predators of insects, their loss has serious consequences for the planet.

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