A number of animal species are capable of astonishing navigational feats. This ability appears to be widespread, with groups as diverse as birds, turtles, insects, and fish all showing navigational skills. Now, we can apparently add bats to the list of species that can manage to find their way, even after researchers have played a variety of tricks on their homing systems. Those tricks weren't just cruel, however, as the researchers' work showed that the bats probably use at least two systems to orient themselves and navigate using a three-dimensional representation of their usual surroundings.

The species in question is the Egyptian fruit bat (Rousettus aegyptiacus), which is native to Israel's Negev Desert and, conveniently, large enough to wear a GPS tracking device. When released near their cave, tagged bats went straight to a small collection of fruit trees about 15km away, typically at speeds of over 35km an hour. And when we say straight, we mean it: the bats passed by other fruit trees on the way, and deviated by less than 3 percent of the total distance traveled. Most bats returned straight to the same trees on consecutive nights. So, from both the consistency and directness perspectives, these bats are superb navigators.

The researchers then started messing with the bats. For their first text, they took a number of them 44km south of their cave and released them at the start of the evening. Again, the bats zeroed in on their preferred feeding trees, and did so remarkably smoothly, traveling at similar speeds and deviating from a straight line by only 15 percent.

This might suggest that the bats knew how to find their tree of choice, so the authors performed a second experiment in which they fed the bats overnight and released them closer to morning. Under these conditions, the bats didn't bother going to the trees, but instead headed directly home to the caves where they slept (five of six of them did, at least). The authors conclude that the navigation skill is far more general than simply an ability to remember the location of a food source.

For the final experiment, the bats were taken to an eroded crater that was about 84 km away from their normal caves. Some of the bats were released at the peak of the crater rim, which allowed them to rapidly scan the surroundings for landmarks. These bats quickly oriented themselves and headed directly off in the expected direction. Those released at the bottom of the crater had a harder time. They did manage to head roughly in the right direction (north, in this case), but their paths showed a lot of deviations as they wandered around inside the crater. Once they hit the rim, however, the bats quickly behaved as their peers did, and headed off in the right direction on a far more direct route.

To the authors, this suggests that the bats have two distinct methods of navigation: one that helps them figure out the right general direction to their destination, and a second that helps them navigate once familiar landmarks are available. Because they manage to fly directly to a destination even if approaching it from a different direction, the authors conclude that the bats have an internal map that they can use. Once a sufficient number of landmarks are recognized, the bats can figure out where they are on the map, and use it to fly in a roughly straight path to their intended destination.

The authors are fairly well convinced that the crater experiments suggest that visual cues help provide the bats with their map-based navigation. Unlike many species that feed on insects, the Egyptian fruit bat has excellent vision, so this seems to be a fairly plausible explanation.

Figuring out how they identified the correct direction in the bottom of the crater, however, is much harder. The distance chosen—84km—is distant enough to be challenging, but not so distant that a whole lot will have changed. The difference in celestial and magnetic cues, for example, is going to be pretty minimal at that distance, and the authors don't think there's likely to be a significant difference in the olfactory environment, so the bats probably couldn't have figured out which way, say, the ocean was relative to their altered starting location.

For the time being, part of the bats' skill set will remain a mystery. But the experiments as a whole clearly show that bats have very advanced navigational skills. This sort of ability has been seen in mammals before—the traditional rats in a maze, for example—but the authors claim that this is the first time that this level of orientation has been demonstrated in a free-ranging mammal that's traveling a significant distance.

PNAS, 2011. DOI: 10.1073/pnas.1107365108 (About DOIs).