Calibration processes in desert ant navigation: vector courses and systematic search

This study investigates the ability of desert ants to adapt their path integration system to an "open-jaw" training paradigm, in which the point of arrival (from the nest) does not coincide with the point of departure (to the nest). Upon departure the ants first run off their home vector and then start a systematic search for the nest. Even if they are subjected to this training-around-a-circuit procedure for more than 50 times in succession, they never adopt straight homeward courses towards the nest. Their path integration vector gets slightly recalibrated (pointing a bit closer to the nest), and their search pattern gets asymmetric (with its search density peak shifted towards the nest), but the bipartite structure of the inbound trajectory invariably remains. These results suggest (1). that the ants cannot learn separate inbound and outbound vectors (i.e. vectors that are not 180 degrees reversals of each other), (2). that the recalibrated vector is dominated by the ant's outbound course, (3). that the recalibration of the vector and the modification of the search geometry are fast and flexible processes occurring whenever the ant experiences a mismatch between the stored and actual states of its path integrator.     

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

Path integration is an ant's 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 ant's 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.     

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.     

Vector calibration in Australian desert ants,Melophorus bagoti: Effects of a delay after the acquisition of vector information

Desert ants navigate by using two chief strategies: path integration, keeping track of the straight‐line distance and direction to the starting point as they travel, and landmark guidance, orientation based on the visual panorama. Both Cataglyphis ants in North Africa and Melophorus bagoti in Central Australia are known to adjust their vectors derived from path integration to compensate for mismatches between their outbound direction of travel and (the reverse of) the inbound direction of travel that takes them home, a process known as vector calibration. We created mismatches of 90° between the outbound vector and the homebound direction by displacing ants from a feeder before their homebound run. We examined temporal factors in vector calibration by varying the delay (0, 1 or 3 hr) between the outbound run to the feeder and the homebound run from the displacement site. According to the temporal weighting rule, such a delay should decrease the weight given to the vector information obtained from the outbound run. This in turn should favour reliance on the visual panorama and thus speed up calibration. Results did not support this prediction. At the displacement site, a delay had little effect on the extent of calibration or the speed of calibration (the number of trials to reach maximum calibration). Just before being displaced, ants were also tested in a test ring surrounded by high walls that obliterated the visual scenery. In the test ring, a delay made the ants less likely to rely on their vector: ants were often not oriented as a group. Otherwise, the ants in the test ring also did not calibrate any more or any faster.     

Three-dimensional orientation in desert ants: context-independent memorisation and recall of sloped path segments

Desert ants (Cataglyphis fortis) navigate by a combination of path integration and landmark-based route memories. Their ability to correct sloped path segments to their ground distances enables them to orientate accurately even in undulating terrain. In this study, we tested whether or not ants incorporate vertical components of an itinerary into their route memory in similar ways as they do with visual landmarks and horizontal changes of direction. In two separate experiments, we trained desert ants to walk over artificial hills and later tested their acceptance of slopes within novel contexts. In the first paradigm, ants had to traverse a hill only on their outbound run, but not on their homebound trip. In a follow-up experiment, we confronted ramp-trained animals with descents in a completely new temporal and spatial context. The animals transferred their newly acquired acceptance of slopes from the outbound to the homebound run as well as to novel foraging trips. Cataglyphis obviously dissociates the experience of sloped path segments from the original context in which they appeared, thus reducing their significance as a navigational aid.     

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.     

How desert ants cope with enforced detours on their way home

Summary We ask whether desert ants (Cataglyphis fortis) perform path integration on their homeward as well as on their outward journey. If path integration does occur on the return journey, then, after an enforced detour, the ant's trajectory should point directly at its nest. To test whether this is so, ants were trained to forage at a spot 25 m from their nest. As an ant began its return journey to the nest, it was caught and transported to a test area where it was released either 2 m or 12 m from a wide barrier which obstructed its homeward path. The direction of the ants' trajectory after detouring around the barrier corresponded closely to that predicted on the assumption that the home vector is accurately updated during the detour.     

Path integration controls nest-plume following in desert ants

The desert ant Cataglyphis fortis is equipped with sophisticated navigational skills for returning to its nest after foraging. The ant's primary means for long-distance navigation is path integration, which provides a continuous readout of the ant's approximate distance and direction from the nest. The nest is pinpointed with the aid of visual and olfactory landmarks. Similar landmark cues help ants locate familiar food sites. Ants on their outward trip will position themselves so that they can move upwind using odor cues to find food. Here we show that homing ants also move upwind along nest-derived odor plumes to approach their nest. The ants only respond to odor plumes if the state of their path integrator tells them that they are near the nest. This influence of path integration is important because we could experimentally provoke ants to follow odor plumes from a foreign, conspecific nest and enter that nest. We identified CO(2) as one nest-plume component that can by itself induce plume following in homing ants. Taken together, the results suggest that path-integration information enables ants to avoid entering the wrong nest, where they would inevitably be killed by resident ants.     

Use of Long-Term Stored Vector Information in the Neotropical Ant <Emphasis Type="BoldItalic">Gigantiops destructor</Emphasis>

We investigated how the formicine ant Gigantiops destructor can use vector information to navigate within the cluttered environment of the rain forest. Displaced foragers use skylight information to move in the theoretical feeder-to-nest direction, whether they are prevented from updating their path-integrator during foraging or captured at the departure from their nest, i.e. with a current accumulator state very close to zero. Only ants that have collected food are able to download a long-term stored reference vector pointing in the nest direction, irrespective of the current accumulator state of their path-integrator stored in a working memory and independent of familiar landmarks. Depending on the release sites, ants that became lost at a maximum distance of 50 cm could still hit and recognize their familiar route, or they engaged in a systematic search for it centered on the release sites. In contrast to Cataglyphis desert ants, Gigantiops ants do not rely primarily on the current accumulator state of their egocentric path integrator. Such a long-term vector-based navigation primed by food capture is well adapted for a tropical ant foraging during periods spanning several hours. This could prevent the numerous cumulative errors in the evaluation of the angles steered that might result from a continuously running path-integrator operating during complex foraging patterns performed at ground or arboreal levels and during passive displacement in response to heavy rain.     

Calibration of vector navigation in desert ants.

Desert ants (Cataglyphis sp.) monitor their position relative to the nest using a form of dead reckoning [1] [2] [3] known as path integration (PI) [4]. They do this with a sun compass and an odometer to update an accumulator that records their current position [1]. Ants can use PI to return to the nest [2] [3]. Here, we report that desert ants, like honeybees [5] and hamsters [6], can also use PI to approach a previously visited food source. To navigate to a goal using only PI information, a forager must recall a previous state of the accumulator specifying the goal, and compare it with the accumulator's current state [4]. The comparison - essentially vector subtraction - gives the direction to the goal. This whole process, which we call vector navigation, was found to be calibrated at recognised sites, such as the nest and a familiar feeder, throughout the life of a forager. If a forager was trained around a one-way circuit in which the result of PI on the return route did not match the result on the outward route, calibration caused the ant's trajectories to be misdirected. We propose a model of vector navigation to suggest how calibration could produce such trajectories.     

A new navigational mechanism mediated by ant ocelli

Many animals rely on path integration for navigation and desert ants are the champions. On leaving the nest, ants continuously integrate their distance and direction of travel so that they always know their current distance and direction from the nest and can take a direct path to home. Distance information originates from a step-counter and directional information is based on a celestial compass. So far, it has been assumed that the directional information obtained from ocelli contribute to a single global path integrator, together with directional information from the dorsal rim area (DRA) of the compound eyes and distance information from the step-counter. Here, we show that ocelli mediate a distinct compass from that mediated by the compound eyes. After travelling a two-leg outbound route, untreated foragers headed towards the nest direction, showing that both legs of the route had been integrated. In contrast, foragers with covered compound eyes but uncovered ocelli steered in the direction opposite to the last leg of the outbound route. Our findings suggest that, unlike the DRA, ocelli cannot by themselves mediate path integration. Instead, ocelli mediate a distinct directional system, which buffers the most recent leg of a journey.     

Desert ants do not acquire and use a three-dimensional global vector

Background

Desert ants (Cataglyphis fortis) are central place foragers that navigate by means of path integration. This mechanism remains accurate even on three-dimensional itineraries. In this study, we tested three hypotheses concerning the underlying principles of Cataglyphis' orientation in 3-D: (1) Do the ants employ a strictly two-dimensional representation of their itineraries, (2) do they link additional information about ascents and descents to their 2-D home vector, or (3) do they use true 3-D vector navigation?

Results

We trained ants to walk routes within channels that included ascents and descents. In choice tests, ants walked on ramps more frequently and at greater lengths if their preceding journey also included vertical components. However, the sequence of ascents and descents, as well as their distance from nest and feeder, were not retraced. Importantly, the animals did not compensate for an enforced vertical deviation from the home vector.

Conclusion

We conclude that Cataglyphis fortis essentially represents its environment in a simplified, two-dimensional fashion, with information about vertical path segments being learnt, but independently from their congruence with the actual three-dimensional configuration of the environment. Our findings render the existence of a path integration mechanism that is functional in all three dimensions highly unlikely.     

Distance estimation in the third dimension in desert ants

Desert ants of the genus Cataglyphis perform large-scale foraging excursions from which they return to their nest by path integration. They do so by integrating courses steered and the distances travelled into a continually updated home vector. While it is known that the angular orientation is based on skylight cues, it still is largely enigmatic how the ants measure distances travelled. We extended the ants' task into the third dimension by training them to walk within an array of uphill and downhill channels, and later testing them on flat terrain, or vice versa. In these tests the ants indicated homing distances that did not correspond to the distances actually travelled, but to the ground distances; that is, to the sum of the horizontal projections of the uphill and downhill segments of the ants' paths. These results suggest a much more sophisticated mechanism of distance estimation than hitherto thought. The ants must be able to measure the slopes of undulating terrain and to integrate this information into their "odometer" for the distance estimation process.     

Navigating desert ants (Cataglyphis fortis) learn to alter their search patterns on their homebound journey

In navigating home, desert ants first run off a global vector estimated on their outbound journey, and then engage in systematic search consisting of ever‐increasing loops interrupted by returns to the starting point of search. Desert ants (Cataglyphis fortis; Wehner, 1983 ) were trained to travel 6 m down a channel to a food source. Different groups of ants were trained to return home in another channel, from distances of 6 m (control), 9 m or 12 m. Ants at the feeder were then tested in a long test channel. The measure of where the ants first turned back on a test gave an estimate of the length of the global vector calculated on their outbound trip. The median distance of search on a 5‐min test gave an estimate of the centre of the search pattern. Relative to controls, the experimental ants did not increase their estimated length of global vector, but changed their search patterns, searching on average further from the start than the controls. Tests of the outbound journey, however, revealed no differences between groups. Desert ants can learn to modify their search pattern based on experience.     

Tandem Recruitment and Foraging in the Ponerine Ant <Emphasis Type="Italic">Pachycondyla harpax</Emphasis> (Fabricius)

Tandem running is a common recruitment strategy in ant species with small colony sizes. During a tandem run, an informed leader guides a usually naïve nestmate to a food source or a nest site. Some species perform tandem runs only during house hunting, suggesting that tandem running does not always improve foraging success in species known to use tandem running as a recruitment strategy, but more natural history information on tandem running under natural conditions is needed to better understand the adaptive significance of tandem recruitment in foraging. Studying wild colonies in Brazil, we for the first time describe tandem running in the ponerine ant Pachycondyla harpax (Fabricius). We asked if foragers perform tandem runs to carbohydrate- (honey) and protein-rich (cheese) food items. Furthermore, we tested whether the speed and success rate of tandem runs depend on the foraging distance. Foragers performed tandem runs to both carbohydrate food sources and protein-rich food items that exceed a certain size. The probability to perform a tandem run and the travelling speed increase with increasing foraging distances, which could help colonies monopolize more distant food sources in a competitive environment. Guiding a recruit to a food source is costly for leaders as ants are ~66% faster when travelling alone. If tandem runs break up (~23% of all tandem runs), followers do not usually discover the food source on their own but return to the nest. Our results show that tandem running to food sources is common in P. harpax, but that foragers modify their behaviour according to the type of food and its distance from the nest. Competition with other ants was intense and we discuss how tandem running in P. harpax might help colonies to build-up a critical number of ants at large food items that can then defend the food source against competitors.     

Eco-ethological Factors Affecting the Scouting and Raiding Behaviour of the Slave-making Ant,Polyergus rufescens Latr. (Hymenoptera,Formicidae)

A dulotic colony of the obligatory slave-making ant Polyergus rufescens was monitored daily in the field (Parma, Italy) from 1100–2000 h over the summers of 1991 and 1992. The first P. rufescens workers to emerge from the nest each day were individually marked and their activity was accurately recorded. The path of 47 such individuals (21 over a complete trip) was followed in detail and mapped. Results confirm a clear scouting activity: the route of these workers away from the nest is generally tortuous and time consuming and is followed by a return trip along a different and straighter path. Observations also showed that these scouts recruit nestmates and lead raiding columns towards target nests, confirming their important role in the organization of slave raids. The similarity between routes taken by scouts and raiders during outbound trips indicates the close connection between scouting and raiding activity, which was also recorded in detail since 40 raids were observed. Some experiments analysing the orientation behaviour of inbound columns showed that raiders use a chemical trail (deposited during the outbound run) to lead them back home. The hypothesis of a multiple strategy for the location of host colonies operated by this slave-making species is also discussed.     

Interactions of the polarization and the sun compass in path integration of desert ants

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.     

Optimal cue integration in ants

In situations with redundant or competing sensory information, humans have been shown to perform cue integration, weighting different cues according to their certainty in a quantifiably optimal manner. Ants have been shown to merge the directional information available from their path integration (PI) and visual memory, but as yet it is not clear that they do so in a way that reflects the relative certainty of the cues. In this study, we manipulate the variance of the PI home vector by allowing ants (Cataglyphis velox) to run different distances and testing their directional choice when the PI vector direction is put in competition with visual memory. Ants show progressively stronger weighting of their PI direction as PI length increases. The weighting is quantitatively predicted by modelling the expected directional variance of home vectors of different lengths and assuming optimal cue integration. However, a subsequent experiment suggests ants may not actually compute an internal estimate of the PI certainty, but are using the PI home vector length as a proxy.     

How do insects represent familiar terrain?

We argue here that ants and bees have a piecemeal representation of familiar terrain. These insects remember no more than what is needed to sustain the separate and parallel strategies that they employ when travelling between their nest and foraging sites. One major strategy is path integration. The insect keeps a running tally of its distance and direction from the nest and so can always return home. This global path integration is enhanced by long-term memories of significant sites that insects store in terms of the coordinates (direction and distance) of these sites relative to the nest. With these memories insects can plan routes that are steered by path integration to such sites. Quite distinct from global path integration are memories associated with familiar routes. Route memories include stored views of landmarks along the route with, in some cases, local vectors linked to them. Local vectors by encoding the direction and/or distance from one landmark to the next, or from one landmark to a goal, help an insect keep to a defined route. We review experiments showing that although local vectors can be recalled by recognising landmarks, the global path integration system is independent of landmark information and that landmarks do not have positional coordinates associated with them. The major function of route landmarks is thus procedural, telling an insect what action to perform next, rather than its location relative to the nest.