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These Ants Don’t Follow the Sun

Sep 2006
Caren B. Les, Associate News Editor

Desert ants (Cataglyphis fortis) don’t necessarily look to the sun for directional cues. To find their way, they typically navigate by the global pattern of polarized skylight, a property of light normally invisible to the human eye. Although it has been known that ants, as well as bees, have used polarized skylight as a compass cue, it was thought that they used it only as a backup to sunlight, as on a cloudy day. Experiments conducted in plains near Maharès, Tunisia, by Rüdiger Wehner and Martin Müller, scientists at the University of Zürich in Switzerland, have now revealed the extent to which the ants rely on polarized light. Details of the investigations are reported in the Aug. 15 issue of PNAS.

This experimental trolley was moved along over an ant as it charted its course. The screen shields the sun. The circular apertures are equipped with polarization filters to induce changes in the pattern of polarized light in the sky. Courtesy of Rüdiger Wehner.

To determine whether the ants used skylight polarization the scientists trained them in narrow channels so that the ants could see only a part of the whole pattern of polarization and then tested them in the open, where they could see the full pattern.

The insects were trained to run from the nest to the feeder in the channels, where they were captured and allowed to run in the open test field, which was covered with a gridwork of white lines. Upon release, they ran in their home direction — or what they thought was their home direction, Wehner said. When the ants were trained with a partial view of the sky, and tested with a full view, they systematically deviated by a certain angular amount to the left or right from their true homeward course.

The ants were trained in plastic channels where they could see only a part of the whole pattern of polarization, and then tested in the open, where they could see the full pattern. Courtesy of Rüdiger Wehner.

The researchers refer to these systematic errors as the “signature” of the ants’ polarization compass. Through further experimentation, the scientists determined that when the ants had access to both forms of light simultaneously, they favored the polarization compass. The ants make the same induced navigational errors as they do with the polarized light alone, as if the sun were not present, according to Wehner.

A researcher is moving an optical trolley under which an ant is running. The device could be equipped with optical filters to manipulate the ant’s visual surroundings, particularly its view of the sky. Courtesy of James Amos.

The findings also suggest that the sun’s spectral characteristics in relation to the remainder of the sky are important in the ant’s navigational system.

Two forms of light are picked up by different parts of the ants’ eye and processed by separate parts of its nervous system. Polarized light is acquired by a tiny part of the eye that is located at the upper dorsal rim area, which comprises only 5 percent of the eye’s photoreceptors. Sunlight is picked up by the remainder of the dorsal rim area.

Long-distance navigators, desert ants (Cataglyphis fortis) were shown to rely more heavily onpolarized skylight than on direct sunlight to find their way home. They were tested under a variety of light conditions, including setups where polarized light was blocked by filmy paper or where sunlight was blocked but polarized light was allowed through. Courtesy of Rüdiger Wehner.

Wehner said that, as a real-world application, they have built a robot that refers to polarized light in much the same way as the ants do. They are now testing how the ants acquire information about the daily rotation of the sky, following the movement of the pattern of polarization along with the sun during the course of the day.

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