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
The results showed that the bats carry around an internal,
cognitive map of their home range, based on visual landmarks, such as lights or
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.
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.
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
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
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.
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
To investigate further, the scientists took some of the
bats to a new area in the desert, 44 km. south of their normal
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.
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
This mechanism might involve sensing the magnetic fields or
directional odors carried on the sea breeze from the Mediterranean to the
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.