Merkle, Tobias Friedrich Christian: Orientation and Search Strategies of Desert Arthropods : Path Integration Models and Experiments with Desert Ants, Cataglyphis fortis (Forel 1902). - Bonn, 2007. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
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author = {{Tobias Friedrich Christian Merkle}},
title = {Orientation and Search Strategies of Desert Arthropods : Path Integration Models and Experiments with Desert Ants, Cataglyphis fortis (Forel 1902)},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2007,
note = {Path integration enables desert arthropods to find back to their nest on the shortest track from any position. To perform path integration successfully, speeds and turning angles along the preceding outbound path have to be measured continuously and combined to determine an internal global vector leading back home at any time. A number of experiments have given an idea how arthropods might use allothetic or idiothetic signals to perceive their orientation and moving speed. When the global vector has been run off but the nest has not yet been reached, the arthropods engage in systematic search behavior. This behavior consists of a series of search loops of ever increasing size and finally leads to a search density profile peaking at the starting location.
In the theoretical part of this work, the model descriptions of mathematically precise path integration that have been developed so far are reviewed, and the hitherto not used variant of egocentric cartesian coordinates is proposed and explained. Its simple and intuitive structure is demonstrated in comparison to the previous path integration models. Measuring two quantities, forward moving speed and angular turning rate, and implementing them into a linear system of differential equations provides the necessary information during foraging run, reorientation process (e.g. at a feeding site) and return path to the nest. In addition, several possible types of systematic errors that can cause deviations from the correct homeward course are easily implemented and illustrated by means of the model. Such deviations have been observed for several species of desert arthropods in different experiments, but their origin is still under debate. The two most important error mechanisms in this respect are the Müller-Wehner-error, an approximative path integration model that accumulates systematic miscalculations in path integration whenever the animal walks different from the correct inbound and outbound direction, and the leaky integrator, a mechanism that predicts a linear underestimation of the distance to the nest with an exponential rate; both error types have been shown to occur in specific experimental paradigms with desert ants Cataglyphis fortis. Using the egocentric path integration model, simple indices are proposed that might allow to rule out or corroborate certain error types by conducting experiments. Experiments were conducted with desert ants C. fortis. Those experiments, in which natural outbound runs as well as the following inbound runs and systematic search behaviors were observed and analyzed, revealed that natural outbound runs do not differ remarkably among different ants. This holds true for their spatial conformation as well as for overall path length and distance covered during foraging. Consequently, no significant correlations between all factors determining the shape of the outbound runs and the errors that were measured via different variables for inbound run as well as systematic search were found. Besides, the extension of the systematic search does not differ remarkably. However, due to the only slight differences of the natural outbound runs, such correlations cannot be totally excluded.
The error postulated by Müller and Wehner seems to be of no or minor importance during natural foraging excursions; the principle of the leaky integrator, on the other hand, might be able to explain some shortcomings of the path integration mechanism with respect to distance estimation. Repeated training increases the straightness of outbound runs. In experiments, where desert ants were trained to different distances, it became obvious that the longer the distances of foraging excursions, the larger the errors occurring during path integration (again measured via home run and systematic search), and that the ants adapt their systematic search strategy to their increasing uncertainty by extending the search pattern.
Additional experiments, during which the distance was kept constant, revealed that not only the characteristics of the foraging trip influence the accuracy of path integrator and systematic search behavior, but that also nest- or route specific cues have an impact on the orientation and the systematic search patterns of desert ants. If desert ants are disturbed during their outbound runs, most of them immediately set out in direction back to the nest, even without having food in their mandibles. External cues, in the respective experiment huge landmarks placed on the route between nest and feeder, increased the number of ants that continued its preceding foraging run; but still the majority headed back towards the nest. For a number of ants successive outbound and inbound runs (ontogeny-experiment) were recorded and analyzed. As a result, their outbound runs to a known feeding site get straighter over time, whereas the inbound runs are very straight from the very beginning and no increase of their straightness could be observed. For both outbound and inbound runs also no improvement in terms of accuracy of the path integrator was found; obviously the ants perform path integration in the same fashion all the time. Even if trained to a feeder for a long time in an area free of landmarks, desert ants do not develop specific paths, as they have been observed for other species of desert arthropods.},

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