Patrick Schultheiss

Views, landmarks, and routes : how do desert ants negotiate an obstacle course?

The Australian desert ant Melophorus bagoti often follows stereotypical routes through a cluttered landscape containing both distant panoramic views and obstacles (plants) to navigate around. We created an artificial obstacle course for the ants between a feeder and their nest. Landmarks comprised natural objects in the landscape such as logs, branches, and tussocks. Many ants travelled stereotypical routes home through the obstacle course in training, threading repeatedly the same gaps in the landmarks. Manipulations altering the relations between the landmarks and the surrounding panorama, however, affected the routes in two major ways. Both interchanging the positions of landmarks (transpositions) and displacing the entire landmark set along with the starting position of the ants (translations) (1) reduced the stereotypicality of the route, and (2) increased turns and meanders during travel. The ants might have used the entire panorama in view-based travel, or the distal panorama might prime the identification and use of landmarks en route. Despite the large data set, both options (not mutually exclusive) remain viable.

Finding the nest: inbound searching behaviour in the Australian desert ant, Melophorus bagoti

Australian desert ants, Melophorus bagoti , return home after foraging by means of path integration and visual navigation. If these mechanisms do not deliver them exactly at the inconspicuous nest entrance, they engage in a systematic search. Here we describe the structure of this search pattern in detail. Trained ants ran home from a feeder in the natural visual setting where they navigated very accurately, and were then captured for tests on a distant test field after they had almost reached their nest. The search pattern consisted of loops and was centred on the position where the nest was most likely to be located. At first, it covered a rather small area, but then gradually extended outwards to cover a larger area. The search density was also adapted to the preceding outbound foraging distance, with longer distances leading to flatter, wider search distributions. Since the visual surround at the time of capture was similar for ants with all outbound distances, we suggest this is an adaptation to the cumulative error of the homing vector. The frequency distribution of segment lengths in the paths of searching ants does not show characteristics of a Levy walk strategy. Instead, it is well described by a double exponential model, lending support to a theoretically optimal strategy that consists of a mixture of two random walks, as in the composite Brownian walk strategy.

Information content of visual scenes influences systematic search of desert ants

SUMMARY Many animals – including insects – navigate visually through their environment. Solitary foraging desert ants are known to acquire visual information from the surrounding panorama and use it to navigate along habitual routes or to pinpoint a goal such as the nest. Returning foragers that fail to find the nest entrance engage in searching behaviour, during which they continue to use vision. The characteristics of searching behaviour have typically been investigated in unfamiliar environments. Here we investigated in detail the nest-searching behaviour of Melophorus bagoti foragers within the familiar visual environment of their nest. First, by relating search behaviour to the information content of panoramic (360 deg) images, we found that searches were more accurate in visually cluttered environments. Second, as observed in unfamiliar visual surrounds, searches were dynamic and gradually expanded with time, showing that nest pinpointing is not rigidly controlled by vision. Third, contrary to searches displayed in unfamiliar environments, searches observed here could be modelled as a single exponential search strategy, which is similar to a Brownian walk, and there was no evidence of a Lévy walk. Overall, our results revealed that searching behaviour is remarkably flexible and varies according to the relevance of information provided by the surrounding visual scenery.

Does the Australian desert ant Melophorus bagoti approximate a Lévy search by an intrinsic bi-modal walk?

We suggest that the Australian desert ant Melophorus bagoti approximates a Lévy search pattern by using an intrinsic bi-exponential walk and does so when a Lévy search pattern is advantageous. When attempting to locate its nest, M. bagoti adopt a stereotypical search pattern. These searches begin at the location where the ant expects to find the nest, and comprise loops that start and end at this location, and are directed in different azimuthal directions. Loop lengths are exponentially distributed when searches are in visually familiar surroundings and are well described by a mixture of two exponentials when searches are in unfamiliar landscapes. The latter approximates a power-law distribution, the hallmark of a Lévy search. With the aid of a simple analytically tractable theory, we show that an exponential loop-length distribution is advantageous when the distance to the nest can be estimated with some certainty and that a bi-exponential distribution is advantageous when there is considerable uncertainty regarding the nest location. The best bi-exponential search patterns are shown to be those that come closest to approximating advantageous Lévy looping searches. The bi-exponential search patterns of M. bagoti are found to approximate advantageous Lévy search patterns.

Multiple sources of celestial compass information in the Central Australian desert ant Melophorus bagoti

The Central Australian desert ant Melophorus bagoti is known to use celestial cues for compass orientation. We manipulated the available celestial cues for compass orientation for ants that had arrived at a feeder, were captured and then released at a distant test site that had no useful terrestrial panoramic cues. When tested in an enclosed transparent box that blocked some or most of the ultraviolet light, the ants were still well oriented homewards. The ants were again significantly oriented homewards when most of the ultraviolet light as well as the sun was blocked, or when the box was covered with tracing paper that eliminated the pattern of polarised light, although in the latter case, their headings were more scattered than in control (full-cue) conditions. When the position of the sun was reflected 180° by a mirror, the ants headed off in an intermediate direction between the dictates of the sun and the dictates of unrotated cues. We conclude that M. bagoti uses all available celestial compass cues, including the pattern of polarised light, the position of the sun, and spectral and intensity gradients. They average multiple cues in a weighted fashion when these cues conflict.

Nest Relocation and Colony Founding in the Australian Desert Ant, Melophorus bagoti Lubbock (Hymenoptera: Formicidae)

Even after years of research on navigation in the Red Honey Ant, Melophorus bagoti, much of its life history remains elusive. Here, we present observations on nest relocation and the reproductive and founding stages of colonies. Nest relocation is possibly aided by trail laying behaviour, which is highly unusual for solitary foraging desert ants. Reproduction occurs in synchronised mating flights, which are probably triggered by rain. Queens may engage in multiple matings, and there is circumstantial evidence that males are chemically attracted to queens. After the mating flight, the queens found new colonies independently and singly. Excavation of these founding colonies reveals first insights into their structure.

Are Lévy flight patterns derived from the Weber–Fechner law in distance estimation?

Honeybees (Apis mellifera) are regularly faced with the task of navigating back to their hives from remote food sources. They have evolved several methods to do this, including compass-directed “vector” flights and the use of landmarks. If these hive-centered mechanisms are disrupted, bees revert to searching for the hive, using an optimal Lévy flight searching strategy. The same strategy is adopted when a food source at a known location ceases to be available. Here, we show that the programming for this Lévy strategy does not need to be very sophisticated or clever on the bee’s part, as Lévy flight patterns can be derived from the Weber–Fechner law in a bee’s odometer. Odometry errors of a different kind occur in desert ants (Cataglyphis spp., Melophorus bagoti). The searching behaviors of these ants are very similar in overall structure to that of honeybees but do not display any Lévy flight characteristics. We suggest that errors in the estimation of distance can be implicitly involved in shaping the structure of systematic search behavior and should not be regarded as merely deficiencies in the odometer.

Foraging ecology of an Australian salt-pan desert ant (genus Melophorus)

Abstract. Over the expanse of the salt lakes of the Lake Eyre basin lives a species of ant of the genus Melophorus (as yet unnamed), both on the edges of the salt-pan and on the salt-pan itself. Most of the foragers are small (∼3.0–5.0 mm), but ∼10% of the ants seen foraging are much larger (∼7.0 mm) and may form a soldier caste. Foragers are thermophilic and show diurnal activity, displaying a single-peaked activity profile across the day, with activity time limited to 2–6 h at most each day (during the late-summer study period, 28 February to 28 March 2012). They forage largely for dead arthropods, but also occasionally bring home plant materials. Foraging success (not considering possible liquid food intake) is ∼20%, resembling the success rate of their congener Melophorus bagoti, which inhabits cluttered environments. When displaced with food from a feeder, the ants head systematically and precisely in the feeder-to-nest direction, thus exhibiting path integration abilities involving celestial compass cues. The study of this species provides an interesting comparative perspective in contrasting desert ants of the same genus and thus genetic heritage inhabiting habitats differing in complexity of panoramic terrestrial cues as well as comparing ecologically similar species inhabiting the same type of habitat (in the present case, salt-pans) but differing in their phylogenetic relationships.

How to Navigate in Different Environments and Situations: Lessons From Ants

Ants are a globally distributed insect family whose members have adapted to live in a wide range of different environments and ecological niches. Foraging ants everywhere face the recurring challenge of navigating to find food and to bring it back to the nest. More than a century of research has led to the identification of some key navigational strategies, such as compass navigation, path integration, and route following. Ants have been shown to rely on visual, olfactory, and idiothetic cues for navigational guidance. Here, we summarize recent behavioral work, focusing on how these cues are learned and stored as well as how different navigational cues are integrated, often between strategies and even across sensory modalities. Information can also be communicated between different navigational routines. In this way, a shared toolkit of fundamental navigational strategies can lead to substantial flexibility in behavioral outcomes. This allows individual ants to tune their behavioral repertoire to different tasks (e.g., foraging and homing), lifestyles (e.g., diurnal and nocturnal), or environments, depending on the availability and reliability of different guidance cues. We also review recent anatomical and physiological studies in ants and other insects that have started to reveal neural correlates for specific navigational strategies, and which may provide the beginnings of a truly mechanistic understanding of navigation behavior.