The complex flight patterns of bats were successfully tracked, thanks to the world's smallest GPS devices, which revealed that Egyptian fruit bats are equipped with visual maps and internal compass of their home range, from a "bird's eye view."

Researchers from the Hebrew University of Jerusalem attached tiny GPS devices to the bats in a field study, in an attempt to figure out how the fruit bats identify specific fruit trees, flying dozens of kilometers each night.

The Egyptian Fruit Bat, or Egyptian Rousette, is a species of bat seen throughout Africa and the Middle East. It consumes large amounts of fruit each night, such as wild dates and soft fruit.

While the navigational skills of other creatures including birds, fish, insects and turtles have been investigated, studies of mammalian navigation have been limited to the laboratory.

This study marks the first-ever comprehensive GPS-based field study of mammal navigation. The GPS devices used to track the bats each weighed around 10 grams, containing tiny GPS receivers, in addition to a memory logger and battery.

During several consecutive nights of observation, the bats took flight from a cave near the Israeli city of Beit Shemesh, flying in a straight line at speeds of at least 40 km an hour, hundreds of meters above the tree tops. They traveled to the same trees around 12 to 25 km from their cave night after night, even ignoring apparently identical trees closer to home. This fact ruled out smell as their main navigation aid.

As a way for further investigation, the scientists brought some bats to a new area in the desert, 44 kilometers south of their normal range.

Some bats were released at dusk; 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. This made the researchers believe that bats were using familiar landmarks to find their way home, regardless of the distance.

The bats were tested with an even farther distance, moved to a natural depression which limited their field of vision. This time, some bats were released from a hilltop at the edge of the Large Crater 84 km south of their home cave while others from the bottom of the crater.
Those from the hilltop could immediately reorient themselves to fly back to the cave, while those from the crater's bottom seemed disoriented, unable to find clear landmarks to identify where they were. After wandering around for quite a while, the bats finally found their way out of the crater and to their home.

The results showed that the bats use visual information to construct a cognitive map of a wide area, carrying around an internal, cognitive map of their home range, based on landmarks such as lights or hills.

When landmarks are unreliable, bats may have an additional, back-up navigational mechanism, such as sensing magnetic fields or directional odors carried on the sea breeze from the Mediterranean to the Negev Desert, according to the researchers.

Researchers concluded that bats' large-scale navigational abilities could rival those of homing pigeons. The findings could bring new insight into both bat ecology and mental map system in the mammals, including human beings.