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Rüdiger Wehner

Publications and source records attributed to Rüdiger Wehner.

At least 19 recordsLinked to original sources
PubMed2007

The desert ant odometer: a stride integrator that accounts for stride length and walking speed.

Desert ants, Cataglyphis, use path integration as a major means of navigation. Path integration requires measurement of two parameters, namely, direction and distance of travel. Directional information is provided by a celestial compass, whereas distance measurement is accomplished by a stride integrator, or pedometer. Here we examine the recently demonstrated pedometer function in more detail. By manipulating leg lengths in foraging desert ants we could also change their stride lengths. Ants with elongated legs ('stilts') or shortened legs ('stumps') take larger or shorter strides, respectively, and misgauge travel distance. Travel distance is overestimated by experimental animals walking on stilts, and underestimated by animals walking on stumps - strongly indicative of stride integrator function in distance measurement. High-speed video analysis was used to examine the actual changes in stride length, stride frequency and walking speed caused by the manipulations of leg length. Unexpectedly, quantitative characteristics of walking behaviour remained almost unaffected by imposed changes in leg length, demonstrating remarkable robustness of leg coordination and walking performance. These data further allowed normalisation of homing distances displayed by manipulated animals with regard to scaling and speed effects. The predicted changes in homing distance are in quantitative agreement with the experimental data, further supporting the pedometer hypothesis.

Analysis of Variance↗
PubMed2006

Fundamental difference in life history traits of two species of Cataglyphis ants.

BACKGROUND: The two sympatric species of Tunisian desert ants, Cataglyphis bicolor and C. mauritanica, do not exhibit any differences in their foraging ecology, e.g. in food preferences and in their spatial and temporal activity patterns. Here we show that instead the two species markedly differ in their life histories. RESULTS: We analysed mtDNA of specimens that were collected along a 250-km transect. C. bicolor exhibited a genetically unstructured population (with the genetic and geographic distances among colonies not being correlated). On the contrary the populations of the polygynous C. mauritanica were clearly structured, i.e. exhibited a strong correlation between genetic and geographic distances. This difference is in accordance with large queen dispersal distances due to far-reaching mating flights in C. bicolor and small queen dispersal distances due to colony foundation by budding in C. mauritanica. Furthermore, wherever we found populations of both species to coexist within the same habitat, the habitat was used agriculturally. Mapping nest positions over periods of several years showed that plowing dramatically decreased the nest densities of either species. CONCLUSION: We conclude that owing to its greater queen dispersal potential C. bicolor might be more successful in quickly re-colonizing disturbed areas, while the slowly dispersing C. mauritanica could later out-compete C. bicolor by adopting its effective nest-budding strategy. According to this scenario the observed sympatry of the two species might be an intermediate stage in which faster colonization by one species and more powerful exploitation of space by the other species have somehow balanced each other out. In conclusion, C. bicolor and C. mauritanica represent an example where environmental disturbances in combination with different life histories might beget sympatry in congeneric species with overlapping niches.

Journal Article↗
PubMed2006

On being small: brain allometry in ants.

Comparative neurobiologists have provided ample evidence that in vertebrates small animals have proportionally larger brains: in a double-logarithmic plot of brain weight versus body weight all data points conform quite closely to a straight line with a slope of less than one. Hence vertebrate brains scale allometrically, rather than isometrically, with body size. Here we extend the phylogenetic scope of such studies and the size range of the brains under investigation to the insects, especially ants. We show that the principle of (negative) allometry applies as well, but that ants have considerably smaller brains than any ant-sized vertebrate would have, and that this result holds even if the relatively higher exoskeleton weights of ants (as compared to endoskeleton weights of mammals) are taken into account. Finally, interspecific comparisons within one genus of ants, Cataglyphis, show that species exhibiting small colony sizes (of a few hundred individuals) have significantly smaller brains than species in which colonies are composed of several thousand individuals.

Animals↗
PubMed2006

The significance of direct sunlight and polarized skylight in the ant's celestial system of navigation.

As textbook knowledge has it, bees and ants use polarized skylight as a backup cue whenever the main compass cue, the sun, is obscured by clouds. Here we show, by employing a unique experimental paradigm, that the celestial compass system of desert ants, Cataglyphis, relies predominantly on polarized skylight. If ants experience only parts of the polarization pattern during training but the full pattern in a subsequent test situation, they systematically deviate from their true homeward courses, with the systematics depending on what parts of the skylight patterns have been presented during training. This "signature" of the polarization compass remains unaltered, even if the ants can simultaneously experience the sun, which, if presented alone, enables the ants to select their true homeward courses. Information provided by direct sunlight and polarized skylight is picked up by different parts of the ant's compound eyes and is channeled into two rather separate systems of navigation.

Animals↗
PubMed2006

Desert ants: is active locomotion a prerequisite for path integration?

Desert ants Cataglyphis fortis have been shown to be able to employ two mechanisms of distance estimation: exploiting both optic flow and proprioceptive information. This study aims at understanding possible interactions between the two possibly redundant mechanisms of distance estimation. We ask whether in Cataglyphis the obviously minor contribution of optic flow would increase or even take over completely if the ants were deprived of reliable proprioceptive information. In various experimental paradigms ants were subjected to passive horizontal displacements during which they perceived optic flow, but were prohibited from active locomotion. The results show that in desert ants active locomotion is essential for providing the ants' odometer and hence its path integrator with the necessary information.

Animals↗
PubMed2006

The ant odometer: stepping on stilts and stumps.

Desert ants, Cataglyphis, navigate in their vast desert habitat by path integration. They continuously integrate directions steered (as determined by their celestial compass) and distances traveled, gauged by as-yet-unknown mechanisms. Here we test the hypothesis that navigating ants measure distances traveled by using some kind of step integrator, or "step counter." We manipulated the lengths of the legs and, hence, the stride lengths, in freely walking ants. Animals with elongated ("stilts") or shortened legs ("stumps") take larger or shorter strides, respectively, and concomitantly misgauge travel distance. Travel distance is overestimated by experimental animals walking on stilts and underestimated by animals walking on stumps.

Animals↗
PubMed2006

Ant navigation: one-way routes rather than maps.

In recent years, there has been an upsurge of interest and debate about whether social insects-central-place foragers such as bees and ants-acquire and use cognitive maps, which enable the animal to steer novel courses between familiar sites . Especially in honey bees, it has been claimed that these insects indeed possess such "general landscape memories" and use them in a "map-like" way . Here, we address this question in Australian desert ants, Melophorus bagoti, which forage within cluttered environments full of nearby and more distant landmarks. Within these environments, the ants establish landmark-based idiosyncratic routes from the nest to their feeding sites and select different one-way routes for their outbound and inbound journeys. Various types of displacement experiments show that inbound ants when hitting their inbound routes at any particular place immediately channel in and follow these routes until they reach the nest, but that they behave as though lost when hitting their habitual outbound routes. Hence, familiar landmarks are not decoupled from the context within which they have been acquired and are not knitted together in a more general and potentially map-like way. They instruct the ants when to do what rather than provide them with map-like information about their position in space.

Animals↗
PubMed2006

Age-dependent and task-related volume changes in the mushroom bodies of visually guided desert ants, Cataglyphis bicolor.

Desert ants of the genus Cataglyphis are skillful long-distance navigators employing a variety of visual navigational tools such as skylight compasses and landmark guidance mechanisms. However, the time during which this navigational toolkit comes into play is extremely short, as the average lifetime of a Cataglyphis forager lasts for only about 6 days. Here we show, by using immunohistochemistry, confocal microscopy, and three-dimensional reconstruction software, that even during this short period of adult life, Cataglyphis exhibits a remarkable increase in the size of its mushroom bodies, especially of the visual input region, the collar, if compared to age-matched dark-reared animals. This task-related increase rides on a much smaller age-dependent increase of the size of the mushroom bodies. Due to the variation in body size exhibited by Cataglyphis workers we use allometric analyses throughout and show that small animals exhibit considerably larger task-related increases in the sizes of their mushroom bodies than larger animals do. It is as if there were an upper limit of mushroom body size required for accomplishing the ant's navigational tasks.

Aging↗
PubMed2006

Ant navigation: resetting the path integrator.

Desert ants use path integration as their predominant system of long-distance navigation, but they also make use of route-defining and nest-defining visual landmarks. Such landmark-gained information might override the information provided by the path integrator, but nevertheless the path integrator keeps running. Here we show that only cues that are associated with the ant being inside the nest are able to reset the path integrator to zero state. Ants were captured at a feeder, i.e. without having run off their home vector, and were forced to enter the nest. On their next outbound run their walking direction differed by 180 degrees from that of ants that had also been captured at the feeder but instead of having been forced to enter the interior of the nest were released at its entrance. Whereas these latter ants still ran off their home vector pointing in the feeder-nest direction (and by this departed from the nest in a direction opposite to the feeder direction) the former ants had reset their home vector to zero state, and had therefore been able to reload their learned feeder vector, and consequently departed from the nest in the feeder direction. Owing to its egocentric nature the path-integrator is error prone. Hence, it is a suitable strategy to reset the path integrator if the ant has appeared at its final goal, the nest. Otherwise during consecutive foraging runs navigational errors would steadily increase.

Animals↗
PubMed2006

Visual and tactile learning of ground structures in desert ants.

Place defining landmarks that have been studied intensively in insect navigation are large, voluminous objects visible to the insect from quite some distance. Here, we show that in desert ants, Cataglyphis fortis, local variations in ground properties can also serve as landmarks. The ants were trained to forage within a linear channel, in which the floor adjacent to the nest entrance was altered in optical and tactile properties. When ants were later tested within a test channel that ran parallel to the training channel, they recognized this landmark and centred their search on the part of the ground structure during training that was closest to the nest entrance. Hence, physical properties of the ground can be learnt and used as cues defining, for example, the position of the nest. In a second series of experiments the ants were presented with ground structures that differed in their visual and tactile properties from the training structure. We show that the absence of either the correct tactile properties or the correct optical properties of the ground structure make the ants reject the previously accepted structure. Hence small ground structures are recognized by the ants as familiar landmarks only if both visual and tactile information coincides with what the ants have experienced during training.

Animals↗
PubMed2006

Uncertainty about nest position influences systematic search strategies in desert ants.

Foraging desert ants return to their starting point, the nest, by means of path integration. If the path-integration vector has been run off but the nest has not yet been reached, the ants engage in systematic search behavior. This behavior results in a system of search loops of ever increasing size and finally leads to a search density profile peaking at the location where the path integration system has been reset to zero. In this study we investigate whether this systematic search behavior is adapted to the uncertainty resulting from the preceding foraging run. We show first that the longer the distances of the foraging excursions, the larger the errors occurring during path integration, and second that the ants adapt their systematic search strategy to their increasing uncertainty by extending their search pattern. Hence, the density of the systematic search pattern is correlated with the ants' confidence in their path integrator. This confidence decreases with increasing foraging distances.

Animals↗
PubMed2005

The night-time temporal window of locomotor activity in the Namib Desert long-distance wandering spider, Leucorchestris arenicola.

Even though being active exclusively after sunset, the male Leucorchestris arenicola spiders are able to return to their point of departure by following bee-line routes of up to several hundreds of meters in length. While performing this kind of long-distance path integration they must rely on external cues to adjust for navigational errors. Many external cues which could be used by the spiders change dramatically or disappear altogether in the transition period from day to night. Hence, it is therefore imperative to know exactly when after sunset the spiders navigate in order to find out how they do it. To explore this question, we monitored their locomotor activity with data loggers equipped with infrared beam sensors. Our results show that the male spiders are most active in the period between the end and the beginning of the astronomical twilight period. Moreover, they prefer the moonless, i.e. darkest times at night. Hence, we conclude that the males are truly-and extremely-nocturnal. We further show that they are able to navigate under the very dim light conditions prevailing on moonless nights, and thus do not have to rely on the moon or on moon-related patterns of polarised light as potential compass cues.

Animals↗
PubMed2005

Vector navigation in desert ants, Cataglyphis fortis: celestial compass cues are essential for the proper use of distance information.

Foraging desert ants navigate primarily by path integration. They continually update homing direction and distance by employing a celestial compass and an odometer. Here we address the question of whether information about travel distance is correctly used in the absence of directional information. By using linear channels that were partly covered to exclude celestial compass cues, we were able to test the distance component of the path-integration process while suppressing the directional information. Our results suggest that the path integrator cannot process the distance information accumulated by the odometer while ants are deprived of celestial compass information. Hence, during path integration directional cues are a prerequisite for the proper use of travel-distance information by ants.

Adaptation, Physiological↗
PubMed2005

Idiosyncratic route-based memories in desert ants, Melophorus bagoti: how do they interact with path-integration vectors?

Individually foraging desert ants of central Australia, Melophorus bagoti, exhibit amazingly precise mechanisms of visual landmark guidance when navigating through cluttered environments. If trained to shuttle back and forth between the nest and a feeder, they establish habitual outbound and inbound routes, which guide them idiosyncratically across the natural maze of extended arrays of grass tussocks covering their foraging areas. The route-based memories that usually differ between outbound and inbound runs are acquired already during the first runs to the nest and feeder. If the ants are displaced sideways of their habitual routes, they can enter their stereotyped routes at any place and then follow these routes with the same accuracy as if they had started at the usual point of departure. Furthermore, the accuracy of maintaining a route does not depend on whether homebound ants have been captured at the feeder shortly before starting their home run and, hence, with their home vector still fully available (full-vector ants), or whether they have been captured at the nest after they had already completed their home run (zero-vector ants). Hence, individual landmark memories can be retrieved independently of the state of the path-integration vector with which they have been associated during the acquisition phase of learning. However, the ants display their path-integration vector when displaced from the feeder to unfamiliar territory.

Animals↗
PubMed2005

Phylogeny of three parapatric species of desert ants, Cataglyphis bicolor, C. viatica, and C. savignyi: a comparison of mitochondrial DNA, nuclear DNA, and morphological data.

Due to morphological comparisons the Tunisian desert ant species Cataglyphis bicolor has been divided into three parapatric species: C. bicolor, C. viatica, and C. savignyi. The species status of the latter is supported by sequence analyses of the mitochondrial CO1 and CO2 region, while analyses of the same mitochondrial region lacked resolution for the separation of C. bicolor and C. viatica. However, the geographic distribution of mtDNA haplotypes points to different population viscosities with C. bicolor queens having longer migration distances than queens of C. viatica. Furthermore, by the use of microsatellites we excluded ongoing gene flow between geographically overlapping populations of C. bicolor and C. viatica, and hence support the morphology-based three-species hypothesis. Concerning the ongoing discussion on the future roles of morphology and molecular biology in systematics we call for a combination of both whenever possible.

Animals↗
PubMed2005

Do monarch butterflies use polarized skylight for migratory orientation?

To test if migratory monarch butterflies use polarized light patterns as part of their time-compensated sun compass, we recorded their virtual flight paths in a flight simulator while the butterflies were exposed to patches of naturally polarized blue sky, artificial polarizers or a sunny sky. In addition, we tested butterflies with and without the polarized light detectors of their compound eye being occluded. The monarchs' orientation responses suggested that the butterflies did not use the polarized light patterns as a compass cue, nor did they exhibit a specific alignment response towards the axis of polarized light. When given direct view of the sun, migratory monarchs with their polarized light detectors painted out were still able to use their time-compensated compass: non-clockshifted butterflies, with their dorsal rim area occluded, oriented in their typical south-southwesterly migratory direction. Furthermore, they shifted their flight course clockwise by the predicted approximately 90 degrees after being advance clockshifted 6 h. We conclude that in migratory monarch butterflies, polarized light cues are not necessary for a time-compensated celestial compass to work and that the azimuthal position of the sun disc and/or the associated light-intensity and spectral gradients seem to be the migrants' major compass cue.

Animal Migration↗