Fruit Bats Use “Cognitive Map” To Navigate

Today, many cars and cell phones are equipped with GPS navigation devices that make it relatively easy for us to find our way to specific localities. Other mammals successfully navigate long distances without such technologies – and their survival depends on it. Just how they do it is still not clear. However, Asaf Tsoar from the Movement Ecology Lab and his supervisor Prof. Ran Nathan from the Hebrew University of Jerusalem report in the Proceedings of the National Academy of Sciences (PNAS) how Egyptian fruit bats (Rousettus aegyptiacus) locate individual fruit trees dozens of kilometers from their caves each night (Figure 1).
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Figure 1. An Egyptian fruit bat (Rousettus aegyptiacus). (From Flickr/Arran Edmonstone)
Tsoar and his colleagues (2011) attached tiny GPS loggers and radio telemetry receivers to 70 adult bats. These bats were then asked to preform a variety of homing and navigational tasks.
At first, the researchers collected data as the bats took flight each night from their roost cave. These bats flew in a straight line at speeds of 40 km an hour and more and at elevations of hundreds of meters to specific fruit trees that were about 12 to 25 km from their cave. Interestingly, the bats flew to the same trees every night, bypassing apparently identical trees closer to their home cave. 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 kilometers south of their normal range (Figure 2). Some bats were released at dusk without being fed; others were fed in the new area and released just before dawn. As predicted, the unfed bats navigating to their favorite fruit trees, returning straight back to their caves afterward. Those who were fed prior to release made a beeline back to the cave once they were released.

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Figure 2. (A) a map of the homing experiments, where bats were released at a remote site far from their normal foraging area. The red lines indicate travel paths from the release site to their roost cave. (B) Flight parameters of all relocated bats. (From Tsoar et al. 2011)
The scientists suspected that the bats were using visual landmarks (hills or lights from towns). To prevent the bats from using visual landmarks to guide them, the researchers removed the bats to a natural crater located some 84 km south of their cave; the crater walls limited the bat’s field of vision. Here, some of the bats were released from a hilltop at the top of the crater and others were let go at the crater’s bottom (Figure 3).

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Figure 3. (B) One example of bat 259,released inside the crater bottom. Without familiar landmarks the bat flew a tortuous 33.9 km path before it left the crater and turned northward toward home. (C) Homing flight of two bats (green lines) released inside the crater and two bats (blue lines) released high on the crater rim. The bats released at the crater rim flew north much straighter than bats released inside the crater. (From Tsoar et al. 2011)
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 to construct a cognitive map of a wide area.
Most of the bats released in the crater bottom, when they finally left, exited to the north (the direction of home), Tsoar and colleagues 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 Desert.
The results of this study showed that the bats carry around an internal, cognitive map of their home range, based on visual landmarks, such as lights or hills, but the study also suggests an additional, large-scale navigational mechanism. The study reveals for the first time how free-ranging mammals find their way around their natural environment.

Story Source:
This story is adapted from materials provided by Hebrew University of Jerusalem, via AlphaGalileo.

References
Tsoar, A., Nathan, R., Bartan, Y., Vyssotski, A., Dell"Omo, G., and N. Ulanovsky. (2011). Large-scale navigational map in a mammal Proceedings of the National Academy of Science : 10.1073/pnas.1107365108