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.