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In the crowded canopy, fruit bats get flexible with sonar

By Martin Leggett - 13 Sep 2011 21:1:1 GMT
In the crowded canopy, fruit bats get flexible with sonar

Image Caption: An Egyptian fruit bat landing on an apple in the lab. The study shows that the echolocation ability of these bats is much more sophisticated than previously thought.

Most newcomers to a party, when greeted with a jostling crowded room, will narrow their field of vision down to what interests them the most - whether its the drinks at the other side of the room, or a particularly attractive party-goer shaking their thing in the corner. But fruit bats it seems, have developed a different strategy for dealing with a chaotic jumble of obstacles. When they fly through a cluttered environment, their sonar gets both louder and broader in scope - helping them to track their target and obstacles, simultaneously.

This bag of ultrasonic tricks was discovered, being employed by fruit bats, during experiments led by scientists from the University of Maryland, and Israel's Weizmann Institute in Israel. Their joint paper is being published today in PLoS Biology, the open-access journal. And the researchers feel this may be one of the first studies to show just how sophisticated a sensory system animals can achieve.

Batty about mangoes

The focus of their work was the humble Egyptian fruit bat, Rousettus aegyptiacus, which feasts, unsurprisingly, on fruits hanging the boughs of trees. But a network of branches, twigs and trunks is a dangerously cluttered feeding ground to have to navigate through - especially at speed, and in the dark. So scientists have been puzzled as to exactly how these bats could manage it. The answer to that question may now have been answered, with results from the team's experimental trials on five trained fruit bats.

The researchers set up a pitch-black room, complete with mango-shaped plastic balls, and a 'sound-a-like' forest, in the shape of nets supported by poles. The fruit bats had already been trained to seek out the pretend mangoes. The path to these 'mangoes' was varied - sometimes clear, sometimes narrow, with different approach paths possible. Because the room had 20 microphones placed all around, a sensitive map of the sounds produced by the bats was recorded by the researchers.

Louder and wider works best

What was found was that bats adapted their sonar-clicking to the complexity of their surrounds. A demanding environment saw them making louder clicks, with the their beam ranging much wider - covering three times the area of that covered in simpler, clearer layouts. And as the bats homed in on their 'mango' targets, they increased their clicks' volume, and widened the beam angles even more. It seems that both these techniques help to keep the bat abreast of the details of the more tricky-to-navigate spaces.

Nachum Ulanovsky of the Weizmann Institute claimed this flexibility is hitting new frontiers of animal sensory perception. "This is the first report, in any sensory system, of an active increase in field-of-view in response to changes in environmental complexity." In some ways that's not so surprising - echo-location is a very different sensory ball-game to more passive forms of sensing, such as sight, sound and smell.

It involves both the sending out of signals, as well as the receiving of them, and so the potential for an interactive method of mapping the world. But this research may be the first to confirm that fruit bats are taking advantage of exactly that feedback potential. "The work presented here reveals a new parameter under adaptive control in bat echo-location", concluded Ulanovsky.

Top Image Credit: © 2011 Janelle Weaver - Source: PLoS Biology