R. Wehner

The hidden spiral: systematic search and path integration in desert ants, Cataglyphis fortis

The main navigational mechanism used by foraging desert ants of the genus Cataglyphis is path integration (dead reckoning). Any such egocentric system of navigation is prone to cumulative navigational errors. Hence, while homing Cataglyphis might have reset its path integration system and yet not arrived at the start of its foraging excursion, the nest entrance. Then it resorts to piloting or performs a systematic search for the nest. The search pattern consists of a system of loops of ever increasing size centred about the origin, i.e. the start of the search. Here we show that underlying the system of loops is a spiral search programme that gets transformed into the observed pattern of loops by the ants idiosyncratic path-integration algorithm. The ant starts to follow a spiral course, then breaks off this course and walks towards the centre, i.e. to what its path-integration system has computed to be the origin of the search. This reset episode is followed by another spiral course, which is terminated by the next reset, and so forth. After each reset, the spiral gets wider, so that the whole pattern expands. Futhermore, every now and then the spiral might change its sign. Computer simulations based on these simple rules lead to search patterns of the kind actually recorded in Cataglyphis ants. These patterns ensure that those parts of the area in which the target (nest entrance) is most likely to be located are searched most heavily; in other words: the search density profile is adapted to the probability density function of the target.

Desert ants Cataglyphis fortis use self-induced optic flow to measure distances travelled

While foraging, desert ants of the genus Cataglyphis use a vector navigation (route integration) system for homing. Any vector navigation system requires that the animal is able to evaluate the angles steered and the distances travelled. Here we investigate whether the ants acquire the latter information by monitoring self-induced optic flow. To answer this question, the animals were trained and tested within perspex channels in which patterns were presented underneath a transparent walking platform. The patterns could be moved at different velocities (up to > 0.5 the ants walking speed) in the same or in the opposite direction relative to the direction in which the animal walked. Experimental manipulations of the optic flow influenced the ants homing distances (Figs. 2 and 4). Distance estimation depends on the speed of self-induced image motion rather than on the contrast frequency, indicating that the motion sensitive mechanism involved is different from mechanisms mediating the optomotor response. Experiments in which the ants walked on a featureless floor, or in which they wore eye covers (Fig. 6), show that they are able also to use additional (probably kinesthetic) cues for assessing their travel distance. Hence, even though optic flow cues are not the only ones used by the ants, the experiments show that ants are obviously able to exploit such cues for estimation of travel distance.

The ant's estimation of distance travelled: experiments with desert ants, Cataglyphis fortis.

Foraging desert ants, Cataglyphis fortis, monitor their position relative to the nest by path integration. They continually update the direction and distance to the nest by employing a celestial compass and an odometer. In the present account we addressed the question of how the precision of the ant’s estimate of its homing distance depends on the distance travelled. We trained ants to forage at different distances in linear channels comprising a nest entrance and a feeder. For testing we caught ants at the feeder and released them in a parallel channel. The results show that ants tend to underestimate their distances travelled. This underestimation is the more pronounced, the larger the foraging distance gets. The quantitative relationship between training distance and the ant’s estimate of this distance can be described by a logarithmic and an exponential model. The ant’s odometric undershooting could be adaptive during natural foraging trips insofar as it leads the homing ant to concentrate the major part of its nest-search behaviour on the base of its individual foraging sector, i.e. on its familiar landmark corridor.

Path integration in desert ants, Cataglyphis: how to make a homing ant run away from home

Path integration is an ans lifeline on any of its foraging journeys. It results in a homebound global vector that continually informs the animal about its position relative to its starting point. Here, we use a particular (repeated training and displacement) paradigm, in which homebound ants are made to follow a familiar landmark route repeatedly from the feeder to the nest, even after they have arrived at the nest. The results show that during the repeated landmark–guided home runs the ans path integrator runs continually, so that the current state of the homebound vector increasingly exceeds the reference state. The dramatic result is that the homing ants run away from home. This finding implies that the ants do not rely on cartographic information about the locations of nest and feeder (e.g. that the nest is always south of the feeder), but just behave according to what the state of their egocentric path integrator tells them.

Honeybees learn the colours of landmarks

Summary1.To discover whether bees learn the colours of landmarks, individually marked foragers were trained to collect sucrose from a small reservoir on the floor of a room. The reservoir was placed at one of two sites each defined by its position relative to one of two different arrays of cylindrical landmarks. On each foraging trip, a bee encountered one of the two arrays. Once a bee was trained to both arrays, its pattern of search was occasionally recorded on videotape during test trials in which one array of landmarks was present and the sucrose absent.2.Both training arrays were composed of two dark blue and two light yellow landmarks placed at the corners of a square. The arrays differed only in the arrangement of coloured landmarks. When bees were tested separately with each array, they searched close to the reward-site defined by that array (Figs. 1 and 2). They behaved similarly on tests in which dark yellow and light blue landmarks replaced the dark blue and light yellow landmarks respectively (Fig. 3). To distinguish between the two arrays, the bees must have used the arrangement of colours.

Time-courses of memory decay in vector-based and landmark-based systems of navigation in desert ants, Cataglyphis fortis

Abstract In foraging and homing, desert ants of the genus Cataglyphis employ two different systems of navigation: a vector-based or dead-reckoning mechanism, depending on angles steered and distances travelled, and a landmark-based piloting mechanism. In these systems the ants use either celestial or terrestrial visual information, respectively. In behavioural experiments we investigated how long these types of information are preserved in the ants memory, i.e. how long the ants are able to orient properly in either way. To answer this question, ants were tested in specific dead-reckoning and piloting situations, whereby the two vector components, direction and distance, were examined separately. The ability to follow a particular vector course vanishes rapidly. Information about a given homing direction is lost from the 6th day on (the time constant of the exponential memory decay function is τ = 4.5 days). The homing distances show a significantly higher dispersion from the 4th day on (τ = 2.5 days). Having learned a constellation of landmarks positioned at the corners of an equidistant triangle all ants were oriented properly after 10 days in captivity, and 64% of the ants exhibited extremely precise orientation performances even when tested after 20 days. Thus, the memory decay functions have about the same short time-course for information on distance and direction, i.e. information used for dead-reckoning. In contrast, landmark-based information used in pinpointing the nest entrance is stored over the entire lifetime of a Cataglyphis forager.

Local vectors in desert ants: context-dependent landmark learning during outbound and homebound runs.

Desert ants, Cataglyphis fortis, associate nestward-directed vector memories (local vectors) with the sight of landmarks along a familiar route. This view-based navigational strategy works in parallel to the self-centred path integration system. In the present study we ask at what temporal stage during a foraging journey does the ant acquire nestward-directed local vector information from feeder-associated landmarks: during its outbound run to a feeding site or during its homebound run to the nest. Tests performed after two reversed-image training paradigms revealed that the ants associated such vectors exclusively with landmarks present during their homebound runs.

Calculation of visual receptor spacing inDrosophila melanogaster by pattern recognition experiments

Summary1.Drosophila flies show a very pronounced tendency to move towards vertical black stripes when confronted with both vertically and horizontally striped patterns.2.Within the range 4.8°<λ<9.5° of pattern wavelengths horizontal stripes are preferred. No pattern preferences are found at the zero-points λ01=9.5° and λ02 = 4.8°. For a constant pattern wavelength these zero-points are not affected by the ratio of the widths of black and white stripes. Therefore, the sign of the reaction frequency to vertical black stripes depends on the pattern wavelength and not on the width of the black stripes.3.The vertical stripe preference is discussed to be a mechanism of loeomotor course control. Thus referring to movement perception concepts a receptor spacing of Δϕ=4.8° can be deduced from the pattern recognition tests.4.Receptor spacing ofTenebrio molitor beetles is calculated for comparison (λ01=19°).5.No pattern preferences are found inDrosophila melanogaster, when the vertical and horizontal stripes are offered on ground glass screens (u.v. free and u.v. containing light), being the only light sources within the whole apparatus. In that situation the flies show a mere phototactic behavior.