An Egyptian fruit bat landing on an apple in the lab. A recent study shows that the echolocation ability of these bats is much more sophisticated than previously thought.
Credit: PLoS Biology
CAMBRIDGE: Egyptian fruit bats adjust their field of view by controlling the angles of the sonar beams they use to navigate, allowing them to negotiate more complex environments, scientists have found.
This never-before-seen mechanism, described today in the open access journal PLoS Biology, demonstrates that the navigation systems of these bats are much more sophisticated than previously thought.
"We found that the more objects you have in the environment, the more angular extent the animal is trying to cover at once," explained co-author Nachum Ulanovsky, a neurobiologist at Weizman Institute of Science in Israel. "The effect is quite dramatic, and this is the first example of such behaviour."
Independent evolution of tongue clicks
Egyptian fruit bats (Rousettus aegyptiacus), which reside mainly in Africa and are named for their delectable diet of soft, pulpy fruits, use echolocation to navigate in dark caves, emitting beams of sound that reflect back to them and tell them about objects in their surroundings.
However, the 'Old World' Fruit Bats, including the Egyptian fruit bat, evolved echolocation independently from other bats and their mechanism is different; rather than using their vocal cords to produce sound, like laryngeal bats, they are lingual bats and use their tongue to produce clicks.
This lingual echolocation was widely considered to be a less-sophisticated form of navigation than laryngeal echolocation, but the new research shows otherwise.
Flying around to avoid collision
Ulanovsky and his colleagues trained Egyptian fruit bats to land on a sphere similar to fruit they would eat in the wild. The bats were in complete darkness so relied only on echolocation, and the sounds they made were monitored with an array of microphones.
To test how these sounds changed in different environments, the team simulated a forest setting with poles and nets that formed a narrow but variable corridor through which the bats had to navigate to land on the sphere.
According to Ulanovsky, this mimicked natural behaviour went well: "It was in the lab, but it was a large flight room and it [allowed] semi-natural behaviour - flying around, landing on objects and avoiding collisions."
Seeing with sound beams
In the artificial forest, the bats widened their sonic 'field-of-view' to obtain more information about the environment. They can do this because they produce clicks in pairs, emitting beams of sound on either side and integrating them to perceive objects in the space between these beams.
When the environment was more complex, they increased the angle between these beams of sound, thereby covering a greater space. What makes this mechanism more complicated is that the two clicks of a pair are consecutive.
"That means that they have to integrate information across time, which is a very interesting and puzzling process," explained Ulanovsky, "it'll be very interesting to try to understand how they can [do this] to produce this larger field of view."
Conventional view put to rest
The new findings demonstrate the fine control involved in the lingual echolocation of Egyptian fruit bats. Indeed, the control of the angle between sound beams to increase the field of view has never been seen in any other bat species or echolocation system.
"The conventional view has been that bat echolocation by tongue clicks is 'primitive' or 'unsophisticated' and this paper clearly puts that idea to rest," commented bat ecologist Brock Fenton from the University of Western Ontario in Canada.
He also thinks that similar systems may be found in other animals: "A next obvious step is to examine the click-based echolocation of birds."
Ulanovsky agrees that it may be seen in other species, and also thinks similar phenomena may be found in completely different sensory systems - even our own.
"Very little is known about how humans move their eyes when they're actually walking around and negotiating real environments. You could think something like that could happen in human vision," he said.
