Why our bats are really smooth operators

Bats transmit high-pitched sound pulses, using their echoes to picture the surroundings. 

These mammals aren’t blind, but their world seems dark and gloomy to us. Visual and sound images, however, are “virtual reality” constructs of our brains; they’re not real.

For all we know, bats may experience an echo-location equivalent of what we

call colour. A quarter of all mammals are bats.

To be that successful, a creature must have first-rate sensory faculties. Would we be better off today had our ancestors gone down the echolocation evolutionary road rather than the visual one?

Being able to “see” in total darkness is the great echolocation pay-off. A creature using it doesn’t need external light.

It can operate at night, in mist and in fog. But this self-sufficiency comes at a price; pulse generation squanders scarce energy resources and only objects within the immediate vicinity can be detected.

Vision, on the other hand, is not so restricted; we can even see stars millions of light-years away. Echolocation, though, might seem superior for local navigation but recent research in Germany suggests that it’s not foolproof.

Birds, like humans, use sight to find their way around. Vision never failed them long ago but now there’s a problem; they can’t see window panes.

Tempted to take short-cuts by flying through buildings, birds collide with glass. Even raptors, despite their extraordinary visual acuity, come to grief in our modern architectural death-traps. Echolocation pulses, however, are reflected from glass surfaces, so bats shouldn’t have a problem seeing them. But have they?

Dead and injured bats are sometimes found beneath windows, their injuries suggesting that they have struck the glass. Stephen Greif, of the Max Planck Institute for Ornithology, has investigated this. His team constructed a tunnel, with a smooth metal sheet standing vertically inside it.

Twenty-one mouse-eared bats were released into the tunnel and their behaviour recorded using infrared cameras and sound detectors.

Within 15 minutes of their release, 19 of the bats had collided with the metal plate at least once. This suggested that the animals had failed to detect it.

However, when the plate was placed flat on the ground, they could “see” it; 13 bats flew down and tried to drink from its surface. Some individuals did so repeatedly.

No bat was injured during the experiment; they weren’t able to fly fast enough in such a confined space to be at risk.

It’s clear from their attempts to drink from the plate on the ground, that bats can detect smooth surfaces, so why was there a problem when the plate was placed upright like a window?

The researchers believe that the bats became aware of the plate but misinterpreted what they were “seeing”. The only smooth surfaces they encounter normally are the horizontal ones of lakes and rivers.

When they come across a window, they decide that it’s a hole in the wall and try to fly through it.

The crucial difference between bird collision casualties and bat ones is that the birds fail to see the windows, whereas bats detect them but misinterpret the situation.

Flight and acoustic data collected during the experiment support this conclusion; when a bat got very close to the plate, and the echo from it became strong, the animal realised it was encountering an obstacle. Individuals which had hit the plate, spent less time in its vicinity subsequently.

“If there are no echoes coming back to the bat, except strong perpendicular echoes from underneath, it is a clear sign of water,” says Dr Greif.

“The exact same echoes coming from the side, however, signal an obstacle for the bat in the flyway that, so far, was perceived as open.”