December 2010, Volume 8, Issue 12

Bats in the News

Listening for Water

If it’s smooth and horizontal, it must be water. That, at least, seems to be the conclusion bats reach – even when sight, smell and touch tell them otherwise, reports the ScienceDaily website. That suggests, according to scientists at the Max Planck Institute for Ornithology in Germany, that bats rely more on echolocation (their biological sonar) than on their other senses.

With echolocation, bats emit mostly ultrasonic sounds into their path and analyze the echoes that bounce back to identify obstacles and prey.

Bats need water to drink, of course, and they sip it on the wing. Many species also forage over rivers and lakes, where, ScienceDaily said, insects are easily spotted with echolocation. The water surface acts like a mirror, reflecting most of the echolocation calls away from the bat. An echo reflects back only if an insect (or other object) is on the surface.

Scientists Stefan Greif and Björn Siemers studied bats of 15 different species for their research, which was published in Nature Communications. In a large flight cage, they used metal, wood or plastic plates to simulate water surfaces and released bats in dim, red illumination to see if they would fall for the trick and try to drink from the smooth plate, ScienceDaily reports.

The results were striking: “The Schreiber's bat, for example, tried to drink up to 100 times in ten minutes from the smooth plate,” Greif said. Three other species also returned again and again to the three plates. Only the wooden surfaces proved slightly less attractive. ScienceDaily says the scientists tested one bat each from 11 additional species on the metal plate, and all of them were tricked.

The researchers were astonished, ScienceDaily reports, to find that bats seemed unable to learn that these acoustic mirrors are hard, dry surfaces rather than water. One bat even landed on the smooth plate, then simply took off, circled a few times and kept trying to get a drink.

The researchers told the science-news website that this association of smooth, horizontal surfaces with water seems to be hardwired in a bat’s brain. But how do bats handle the puzzle of contradictory information form other sensory systems? Echolocation is the only sense that would consistently treat a metal plate as water.

When the researchers eliminated vision by repeating the experiment in total darkness, ScienceDaily says, they found that the number of drinking attempts increased from 100 to 160 in ten minutes.

“It seems,” Grief said, “that the bats integrate and weigh up their sensory information, but echolocation dominates all the others.”

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