Israeli scientific discovery: fruit bats navigate using a "cognitive map" based on vision

The research was carried out using the smallest GPS devices in the world, which allowed the bats to be tracked in homecoming experiments after being transferred to the desert 

GPS devices allow us to navigate efficiently anywhere, and have already become an integral part of the comfortable modern life, which relies on advanced technologies. But the ability to navigate and orient oneself in space is not just a matter of convenience. In fact, it is essential for the survival of both humans and animals. The fruit bats, for example, fly tens of kilometers every night to the permanent fruit tree from which they feed, and return to the cave. How do they do it?

In a first-of-its-kind field study, conducted using unique miniaturized GPS devices, scientists followed the movement of fruit bats, collecting accurate data on their flight habits and navigational skills. The results of the study show that the bats navigate according to a comprehensive "cognitive map", which includes the area known to them by sight, and position themselves according to lights or other prominent landmarks in the area. The study, published this week on the website of the journal "Records of the Academy of Sciences of the USA" (PNAS), provides for the first time evidence of the navigation method used by mammals in the wild.

The question of how animals manage to find their way - whether it is the homing pigeons returning to the thicket, or the salmon returning from the sea to lay eggs in the stream where they were born - has occupied scientists for a long time. Attempts to investigate this question through field experiments in nature have encountered technical limitations, and so far have produced results for birds, fish, insects, lobsters, sea turtles and more - but not mammals. Laboratory experiments, which enable more precise tracking of the movement of mammals, are done on small scales of a few meters, and in conditions that do not simulate what is done in real navigation in the field. A new method that makes it possible to enjoy both worlds was developed in collaboration between neuroscientist Dr. Nahum Ulanovsky, from the Weizmann Institute of Science, ecologist Prof. Ran Natan, head of the Institute of Life Sciences at the Hebrew University of Jerusalem, and the research student in his group (from the Movement Ecology Laboratory), Assaf Tzuar. Scientists from Italy and Switzerland also participated in the study. The scientists developed tiny devices - weighing about ten grams - that include the smallest GPS device in the world (among the devices used today to track wild animals), as well as a memory unit and battery.

In a series of field experiments, the scientists used these devices to follow the movements of common fruit bats (Rousettus aegyptiacus) for several consecutive nights.

In the first phase, the scientists collected data on the bats' routine night flights from their cave in the Yehuda Lowlands, near Beit Shemesh. It turned out that they fly in a straight line, at high speed (on average about 35 km/h and in some cases even more than 60 km/h), and at a high altitude (hundreds of meters), for a distance of up to about 30 km in each direction - to a tree their favorite fruit, and that they return to the same tree for several nights, while ignoring seemingly identical fruit trees closer to the cave. The data collected by Prof. Natan, Dr. Ulanovsky, Assaf Tzuar and their research partners show that bats have an impressive navigational ability, comparable to that of pigeons. The loyalty of bats to their favorite trees also explains why it is so difficult to get them to leave a particular site where they are not welcome.

The bats' ability to "home" on a particular tree while ignoring identical fruit trees suggests that navigation is not based on the tree's scents. In addition, the route analysis showed that they are not helped by the road route. What, then, is the explanation for this impressive navigational ability? To solve the mystery, the scientists collected the bats in their familiar environment and moved them to the Givat Gorel area, 44 km south of their permanent base. Some of them were released from there at nightfall, and some were given food during the night and released only at the end. The bats that were released at the beginning of the night, being hungry, returned on a straight course to their favorite fruit tree, and then returned to the cave. In contrast, the seventy bats, which were released at the end of the night, returned directly to the permanent roosting cave.

At this stage, the research team consulted the pilots and ran a computer model. This is how it turned out that from the Givat Gorel area prominent objects such as the chimneys of the power plant in Ashkelon and central intersections can be seen from a distance. It is therefore possible that the bats, who have large eyes and an excellent sense of sight, know these landmarks from their daily flight in their habitat, and that they navigate with their help. To prevent the bats from relying on familiar visual landmarks, the researchers used a natural "pit", from which the field of vision is limited: this time the bats were taken to the Great Crater, 84 km away from the cave. Half of them were released from the bottom of the crater and the other half from the summit of Mount Avnon, a high mountain standing on the edge of the crater. While the bats released from the top of the mountain flew directly into the cave, the bats released inside the crater wandered and moved inside it for a long time in a winding and multi-directional route, but eventually managed to get out of it in the right direction and fly back to the cave.
These findings confirm that the bats navigate according to a "cognitive map", which includes an extensive area known to them most likely by sight. Navigation is mainly guided by distant landmarks, such as mountains and hills, or settlement lights, which allow bats to position themselves in relation to them - through triangulation. In addition, in most cases, the bats left from the northern part of the crater, that is, in the general direction they wanted to go.

Prof. Natan, Dr. Ulanovsky and Assaf Tzoer believe that this fact points to the existence of another navigational mechanism - based, apparently, on the magnetic field or on the smells carried with the breeze coming from the Mediterranean Sea - a "backup mechanism" used by the bats in cases where They cannot rely on the cognitive map.

Map-based navigation has so far been demonstrated in mammals on a scale of one or two meters, under laboratory conditions only. The new findings from the fruit bat study constitute the first example in mammals of map-based navigation on a large scale, about 100 kilometers.