The more scientists discover about desert ants, the more impressive they seem. Decades of research have established that ants use path integration - an innate form of mental trigonometry - in order to navigate the visually featureless environments that are the salt pans of Tunisia. They do this by calibrating a mental clock based on the motion of the sun, which they combine with a "mental chronometer," a step counter. Together, this allows a desert ant to estimate both the distance and direction they must travel to make it back home. It also turns out that they represent their location in three dimensions; they account for hills and valleys in their mental calculations.
But desert ants are able to use other cues as well to help them get home. Variations in soil composition, breaks in the salt, and dead plants, all contribute to the creation of an odor gradient across the landscape. Ants who have had one or both of their antennae removed were less successful in some navigation tasks, suggesting that they are able to use smell in guiding their navigation as well. And since they needed both antennae to perform optimally, it seems as if ants smell in stereo.
New research just published in the journal PLoS ONE has added ground vibrations and magnetic fields to the list of cues desert ants can possibly use as aids to navigation. For this experiment, rather than using Cataglyphis fortis, the ants of Tunisia, they used a related species, Cataglyphis noda, of Cirali, Turkey.
The set-up was actually very simple. First, Cornelia Buehlmann and her colleagues from the Max Planck Institute for Chemical Ecology placed some food in a long channel near a nest entrance, and trained some ants to locate the bait. The nest entrance was outfitted with one of four different types of landmarks: visual, olfactory, magnetic, or vibrational. A fifth group of ants served as a control group: they were trained to find the food just as the other four groups, but their nest entrance was not matched with any type of landmark.
Once the ants learned the route from their nest to the feeder, they would be relocated to a new channel. In the new channel, the landmark was placed somewhere different from the nest entrance. If the ants relied on path integration to navigate back to their nests, they should not have been fooled. However, if ants were using external cues such as visual, olfactory, vibrational, or magnetic landmarks, then they should navigate to the landmark's new location, completely bypassing the actual nest's entrance.
The control group, for whom the nest was not paired with a landmark, searched for their nest as expected, based on the output of the path integration system. The same was true for ants whose nests were not matched with a landmark, but for whom a landmark was introduced during testing. The mere presence of a visual, olfactory, vibrational, or magnetic cue was not enough to impair the path integration system: all of these ants found their way home.
The same could not be said of the four groups of ants who were trained while their nests were paired with a landmark. These ants walked right on past the place where their actual nest would have been, and instead searched for their nest at the spot where the landmark had been placed. In addition to visual and olfactory landmarks, desert ants can apparently use vibrational and magnetic landmarks as homing beacons. That they were able to learn an association between the landmark and the location of their nests is itself not that surprising. Associating two different stimuli is one of the most basic forms of learning.
More bewildering, however, was that these ants ignored the information from the path integration system, and instead relied on those external cues to find their way home - a strategy that would have proven disastrous if the researchers had not been there to return them safely to their nests afterwards!
The path integration system is not a perfect system: it is subject to cumulative error equaling 10 percent of distance and two degrees of direction. For this reason, when an ant believes it is near the nest's location, it switches strategies and walks back and forth until it finds the nest. This is why, in the diagram above, the paths are indicated by zig-zagging lines of decreasing length.
The researchers point out that despite the ants' ability to learn an association between magnetic cues and their nests' location, it is not clear whether enough magnetic variation exists for magnetic fields to be useful homing cues for ants under normal circumstances. In addition, the ability to pair a magnetic field with their nest's location does not by itself indicate the presence of some sort of magnetic organ within ants. Instead, it could be that the magnetic fields created some generic sort of change in the way the ants' neurons fired near their nest's entrance. If this is the case, then the ants' association of the magnet with their nest may not be due to the magnetic field, per se. As ever, more research will be required to investigate this possibility.
As for vibration, there is more research indicating that some ant species are able to perceive vibrations in the ground. Leaf-cutter ants, for example, even use vibrations to communicate with nestmates, through up to several centimeters of dirt. As with magnetic fields it is unknown if enough vibrational landmarks exist naturally to be useful for navigation and homing in desert ants.
What does this mean for path integration? Probably not much.
While impressive, all this research seems to indicate is that under experimental conditions, desert ants are able to use associative learning to match two additional types of external landmarks to nest locations. Vibrational and magnetic cues can be added to visual and olfactory cues as features that desert ants can perceive and use under some circumstances for navigation.
Until researchers are able to demonstrate that variations in magnetic fields and ground vibrations exist at small enough spatial scales (on the order of centimeters to meters) in these environments, and that desert ants use them spontaneously rather than under controlled experimental designs, path integration will remain the desert ants' main tool for navigation. The movement of the sun above and of the feet below are still the best tools for the job.
Cornelia Buehlmann, Bill S. Hansson, & Markus Knaden (2012). Desert Ants Learn Vibration and Magnetic Landmarks PLoS ONE, 7 (3) : 10.1371/ journal.pone.0033117
Images: Header image via Google Maps. Ant on stilts origin unknown. Buehlmann image via Max Planck Institute for Chemical Ecology/Badeke.