Desert ants, Cataglyphis fortis, perform large-scale foraging trips in their featureless habitat using path integration as their main navigation tool. To determine their walking direction they use primarily celestial cues, the sky's polarization pattern and the sun position. To examine the relative importance of these two celestial cues, we performed cue conflict experiments. We manipulated the polarization pattern experienced by the ants during their outbound foraging excursions, reducing it to a single electric field (e-)vector direction with a linear polarization filter. The simultaneous view of the sun created situations in which the directional information of the sun and the polarization compass disagreed. The heading directions of the homebound runs recorded on a test field with full view of the natural sky demonstrate that none of both compasses completely dominated over the other. Rather the ants seemed to compute an intermediate homing direction to which both compass systems contributed roughly equally. Direct sunlight and polarized light are detected in different regions of the ant's compound eye, suggesting two separate pathways for obtaining directional information. In the experimental paradigm applied here, these two pathways seem to feed into the path integrator with similar weights.

Color vision relies on the ability to discriminate different wavelengths and is often improved in insects that inhabit well-lit, spectrally rich environments. Although the Opsin proteins themselves are sensitive to specific wavelength ranges, other factors can alter and further restrict the sensitivity of photoreceptors to allow for finer color discrimination and thereby more informed decisions while interacting with the environment. The ability to discriminate colors differs between insects that exhibit different life styles, between female and male eyes of the same species, and between regions of the same eye, depending on the requirements of intraspecific communication and ecological demands.

Desert ants, Cataglyphis fortis, perform large foraging excursions during which they continuously compute a home vector that allows them to return to the nest on the shortest way. This type of navigation, termed path integration, needs a compass system and an odometer. Ants use several cues to determine their walking direction, two of the most important ones being the sun position and the polarization pattern of the sky. We tested whether an information transfer is possible from one compass system to the other, which depend on different anatomical substrates. Since the sky's polarization pattern is detected by UV-photoreceptors located in the dorsal rim area (DRA), we used an orange Perspex filter that eliminated the UV part of the spectrum to prevent the use of the polarization compass. The use of the sun compass could be excluded by appropriate screens. In the critical tests the ants had learned a nest-feeder direction with e.g. the sun compass only, and were later tested with the polarization compass, or vice versa. The results show that a transfer is possible in both directions.

SUMMARYDesert ants, Cataglyphis, use the skyʼs pattern of polarized light as a compass reference for navigation. However, they do not fully exploit the complexity of this pattern, rather -as proposed previously -they assess their walking direction by means of an approximate solution based on a simplified internal template. Approximate rules are error-prone. We therefore asked whether the ants use additional cues to improve the accuracy of directional decisions, and focused on ʻidiotheticʼ cues, i.e. cues based on information from proprioceptors. We trained ants in a channel system that was covered with a polarization filter, providing only a single e-vector direction as a directional ʻcelestialʼ cue. Then we observed their homebound runs on a test field, allowing full view of the sky. In crucial experiments, the ants were exposed to a cue conflict, in which sky compass and idiothetic information disagreed, by training them in a straight channel that provided a change in e-vector direction. The results indicated that the polarization information completely dominates over idiothetic cues. Two path segments with different e-vector orientations are combined linearly to a summed home vector. Our data provide additional evidence that Cataglyphis uses a simplified internal template to derive directional information from the skyʼs polarization pattern.

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