Fruit bats navigate long distance exactly like you do when your smartphone is on the blink: using conspicuous landmarks such as Tel Aviv’s Azrieli Towers and the Reading Power Station, scientists at Tel Aviv University report this week in Science.
Even better, fruit bats can build a cognitive map of their environment, as evinced among other things by the fact that experienced bats take shortcuts – which means that once they have the map in mind, they don’t necessarily need the landmarks anymore. They can efficiently flap in straight lines to their favorite fruit trees and back to their caves, a separate team at Hebrew University write in Science’s cover story this week. Ergo, the bats know where they are in the environment.
Yes, there are two separate reports by two separate Israeli fruit bat teams in the journal this week, as there should be. Both address issues of batty navigation, which as both Prof. Yossi Yovel of Tel Aviv University and Prof. Ran Nathan of the Hebrew University explain, was hitherto one of the great mysteries of the world.
How exactly do fruit bats navigate double-digit kilometers and find their way back? Where does their famed facility for sonar come in? Do urban bats in Tel Aviv differ in their navigation skills from rural ones at the Hula Valley?
The Hebrew University report in Science is by led by Prof. Ran Nathan and Prof. Sivan Toledo (of Tel Aviv University), David Shohami, Ingo Schiffner, Emmanuel Lourie, Yotam Orchan and Yoav Bartan. The Tel Aviv University study is by Yovel with students Amitai Katz, Lee Harten, Aya Goldstein and Michal Handel.
GPS for baby bats
The Tel Aviv scientists had the advantage in that they maintain a colony of fruit bats, which, for the first time in the annals of science, were monitored starting from the time they began to fly. The Tel Aviv paper is based on tracking 22 bat pups with the help of the smallest GPS devices in the world, developed by the fruit bat researchers, as they scoured the city for food. The bats that is.
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It bears adding that infant fruit bats cling to their mothers like baby monkeys do, just upside down, until they can fly on their own.
And the scientists discovered that the pups came to know Tel Aviv by looking for large, unique buildings such as the Azrieli Towers, which are indeed conspicuous – a cluster of three blue-and-white skyscrapers, one square, one circular and one triangular. The bats navigated in much the same way as Tel Aviv’s humans do: by sight.
Moreover, the animals clearly created mental maps of landmarks, including another Tel Aviv icon, the Dizengoff Center mall in the center of town. And the mental maps they created let them conserve energy and time by developing shortcuts to their target trees and home.
“Since we knew the flight history of each bat since infancy, we could always tell when a specific bat took a certain shortcut for the first time,” Yovel says. “We discovered that when taking new, unknown routes, the bats flew above the buildings. Sending up drones to the altitude and location where a bat had been observed, we found that the city’s towers were clearly visible from this high angle.”
What about bats’ famous facility for sonar? “Bats use their sonar to navigate over short distances – near a tree, for example,” Yovel says. “The sonar doesn’t work for greater distances. For this, fruit bats use vision.”
This mental mapping ability is no artifact of the bats’ dwelling in civilized conditions in the university’s colony when they’re not flapping about Tel Aviv. The team in Jerusalem studied wild Egyptian fruit bats in the Hula Valley in northern Israel, and in their paper they report on the aviating mammals’ sophisticated navigational abilities using a “reverse GPS” system developed by Ran Nathan of Hebrew University teamed up with Sivan Toledo of Tel Aviv University, which they called ATLAS.
Ordinary GPS, Nathan points out to Haaretz, began as a military technology and has been happily adopted by civilian society. But it remains extremely demanding of energy, which is why keeping allowing our phones to keep track of our location burns up our phone battery faster. The phone is continually communicating with satellites and calculating our location.
David Shohami, Nathan’s PhD student from the Hebrew University team was responsible for trapping wild Egyptian fruit bats in Hula, tagging them (gluing tags onto their backs or by collar), releasing them to the wild and collecting, collating and analyzing the data. The bats did not suffer from this handling, Nathan vows. Shohami also mapped all the tens of thousands of fruit trees in the Hula valley, so he could tell the target of each bat flight.
Each ATLAS tag emits a unique ID signal – which instead of being picked up by orbiting satellites, is picked up by the 14 base stations the team placed around the valley. The burden of satellite communication and position calculation of GPS is relieved, as the control system takes charge.
Hence, ATLAS reverses the roles compared to GPS. Nathan adds that these tags are not only much smaller, but also significantly cheaper than GPS tags.
Altogether the team collected data on 172 foraging Egyptian fruit bats, achieving more than 18 million localizations collected over 3,449 bat-nights across four years. And thus they could figure out where the bats began, where they were headed and their routes.
The critters seldom, if ever, search randomly, and their flight patterns cannot be explained by simpler navigation mechanisms, write Nathan and the team. The evidence suggests they were building maps in their little heads: the wild bats seldom searched for food randomly, and alternative navigation mechanisms such as the use of multiple landmarks or because to smell or other cues, had no support. Instead, the bats repeatedly foraged for their fruit in goal-directed, long, straight flights that included frequent shortcuts, once they have the map in their heads. We do the same thing.
And that, the Hebrew University team concludes, constitutes the best evidence for cognitive mapping ability in any animal since scientists first hypothesized the existence of a human-style cognitive map in 1948, Nathan says.
Nathan adds that while they worked with wild bats, not a colony of captives, they were tracking bats from the age of about three months – and the juveniles also flew in straight lines, in both regular tracks and shortcuts, indistinguishable from adults.
Over the period of their research, they also discovered that bats fly straight regardless of the time the shortcut was recorded, days, weeks and even up to five months as of being tagged. In other words, this line of evidence suggest that bats could retain their spatial memory of particular goals in their landscape for at least five months.
There is a theory that the use of navigational apps such as Waze depresses our natural navigational skills. After some resistance, this author eventually succumbed to the siren song of the smartphone’s directions on the road, but like the fruit bats of Tel Aviv, still checks out her progress down the Ayalon Highway using the Azrieli Towers. Nathan, in turn, invites this author to the beautiful Hula Valley, where nature and wildlife flourish, and fruit bats skillfully roam between caves and fruit trees, as their ancestors did long time ago.