Researchers use GPS to study fruit bat navigation

August 16, 2011 05:56

Results show that the bats carry around an internal, cognitive map of their home range, based on visual landmarks, such as lights or hills.

Researcher Asaf Tsoar

Researcher Asaf Tsoar 311. (photo credit:Hebrew University)

Egyptian fruit bats are picky eaters, flying dozens of kilometers a night to find their dinner on specific trees and then returning home long before sunrise. If they don’t find those trees, they die.

Now, for the first time, Israeli scientists have fitted the flying mammals with the world’s smallest GPS devices to track their homing flights after relocation far away. Published in the August 15 issue of Proceedings of the [US] National Academy of Sciences, it was the first comprehensive GPS-based field study of mammal navigation.

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The results showed that the bats carry around an internal, cognitive map of their home range, based on visual landmarks, such as lights or hills.

The researchers also suggest an additional, large-scale navigational mechanism. The study reveals for the first time how free-ranging mammals find their way around their natural environment.

Doctoral student Asaf Tsoar from the the Hebrew University of Jerusalem’s movement ecology lab and his supervisor Prof. Ran Nathan; Weizmann Institute of Science neurobiologist Dr. Nachum Ulanovsky who is studying the neural basis of navigation; and Giacomo Dell’Omo of Ornis Italica, Italy, and Alexei Vyssotski of ETH Zurich, Switzerland collaborated on the research.

Many scientists have already investigated the navigational skills of other creatures from birds and fish to insects, lobsters and turtles, but studies of mammalian navigation have been confined to the lab.

Unfortunately, lab studies cannot duplicate the large, complex landscapes an animal must navigate in the natural world.

The new GPS-based method gives researchers the best of both worlds and uses a new approach. The team developed miniaturized GPS devices, each weighing around 10 grams and containing tiny GPS receivers, a memory logger and battery.

They were used to track the movements of the Rousettus aegyptiacus fruit bats over several consecutive nights.

At first, the researchers collected data as the bats took flight each night from a cave near Beit Shemesh. The mammals flew in a straight line at speeds of more than 40 kms. an hour and at heights of hundreds of meters to trees that were about 12 to 25 kms. from their cave. They returned to the same trees, night after night, even bypassing apparently identical trees that were nearer to home. The data showed that bats’ navigational abilities rival those of homing pigeons.

The fact that the bats bypassed similar fruit trees to get to their favorite feeding site ruled out smell as their main navigational aid, while an analysis of the data suggested that the bats were not simply “beaconing” on any visual or other individual cue.

To investigate further, the scientists took some of the bats to a new area in the desert, 44 km. south of their normal range.

Some bats were released at dusk while others were fed in the new area and released just before dawn. Those released first had no trouble navigating to their favorite fruit trees, returning straight back to their caves afterward. Those who were fed first simply made a beeline back to the cave once they were released.

Based on a spatial model analysis and after discussions with pilots, it appeared, though, that the bats could have seen some familiar visual landmarks – hills or the lights of human settlements – from this release site near Beersheba.

To prevent the bats from using visual landmarks to guide them, the researchers removed the bats even further south, to a natural depression that limited their field of vision: the large crater near Mitzpe Ramon, located some 84 kms. south of their cave. Here, some of the bats were released from a hilltop at the edge of the crater and others were let go at the crater’s bottom.

Despite the distance, those flying from the hilltop oriented themselves right away and flew back to the cave. The bats inside the crater, however, appeared disoriented, wandering for quite a while before finding their way out of the crater and back to the cave.

This confirmed the idea that bats use visual information from a “bird’s eye view” to construct a cognitive map of a wide area. Navigational cues include these distant landmarks, and the scientists believe that the bats most likely compute their own location by employing a form of triangulation based on the different azimuths to known distant landmarks.

Because most of the bats released in the crater, when they finally left, exited to the north (the direction of home), Tsoar, Nathan and Ulanovsky believe that the bats may have an additional, back-up navigational mechanism to help when landmarks are unreliable.

This mechanism might involve sensing the magnetic fields or directional odors carried on the sea breeze from the Mediterranean to the Negev.

Although lab experiments based on distances of a meter or two had hinted at the existence of an internal map for navigation, this study is the first to show that such mammals as fruit bats use these maps to find their way around areas 100 kms. in size.

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