Abstract: The Australian desert ant Melophorus bagoti shows remarkable visual navigational skills relying on visual rather than on chemical cues during their foraging trips. M. bagoti ants travel individually through a visually cluttered environment guided by landmarks as well as path integration. An examination of their visual system is hence of special interest and we address this here. Workers exhibit distinct size polymorphism and their eye and ocelli size increases with head size. The ants possess typical apposition eyes with about 420-590 ommatidia per eye, a horizontal visual field of approximately 150° and facet lens diameters between 8 and 19 μm, depending on body size, with frontal facets being largest. The average interommatidial angle ÎÏ is 3.7°, the average acceptance angle of the rhabdom ÎÏ(rh) is 2.9°, with average rhabdom diameter of 1.6 μm and the average lens blur at half-width ÎÏ(l) is 2.3°. With a ÎÏ(rh)/ÎÏ ratio of much less than 2, the eyes undersample the visual scene but provide high contrast, and surprising detail of the landmark panorama that has been shown to be used for navigation.
Abstract: Animals are active at different times of the day and their activity schedules are shaped by competition, time-limited food resources and predators. Different temporal niches provide different light conditions, which affect the quality of visual information available to animals, in particular for navigation. We analysed caste-specific differences in compound eyes and ocelli in four congeneric sympatric species of Myrmecia ants, with emphasis on within-species adaptive flexibility and daily activity rhythms. Each caste has its own lifestyle: workers are exclusively pedestrian; alate females lead a brief life on the wing before becoming pedestrian; alate males lead a life exclusively on the wing. While workers of the four species range from diurnal, diurnal-crepuscular, crepuscular-nocturnal to nocturnal, the activity times of conspecific alates do not match in all cases. Even within a single species, we found eye area, facet numbers, facet sizes, rhabdom diameters and ocelli size to be tuned to the distinct temporal niche each caste occupies. We discuss these visual adaptations in relation to ambient light levels, visual tasks and mode of locomotion.
Abstract: Many animals become active during twilight, a narrow time window where the properties of the visual environment are dramatically different from both day and night. Despite the fact that many animals including mammals, reptiles, birds and insects become active in this specific temporal niche, we do not know what cues trigger this activity. To identify the onset of specific temporal niches, animals could anticipate the timing of regular events or directly measure environmental variables. We show that the Australian bull ant, Myrmecia pyriformis, starts foraging only during evening twilight throughout the year. The onset occurs neither at a specific temperature nor at a specific time relative to sunset, but at a specific ambient light intensity. Foraging onset occurs later when light intensities at sunset are brighter than normal or earlier when light intensities at sunset are darker than normal. By modifying ambient light intensity experimentally, we provide clear evidence that ants indeed measure light levels and do not rely on an internal rhythm to begin foraging. We suggest that the reason for restricting the foraging onset to twilight and measuring light intensity to trigger activity is to optimize the trade-off between predation risk and ease of navigation.
Abstract: The Central Australian desert ant Melophorus bagoti is the most thermophilic ant on the continent. It comes out to forage during the hottest part of the day in the summer months. The ant shares a cluttered, plant-filled habitat with other arthropods and uses a range of navigational strategies. We review recent studies on this species concerning its use of habitual routes, distant landmarks, landmarks around the nest, and path integration, which is keeping track of the distance and direction traveled from one's starting point. Functional predictions concerning the acquisition, retention, and integration of memories of distances and of landmarks are also reviewed, illuminating the behavioral ecology of spatial cognition.
Abstract: This study in Western Ghats, India, investigates the relation between nesting sites of ants and a single remotely sensed variable: the Normalised Difference Vegetation Index (NDVI). We carried out sampling in 60 plots each measuring 30 x 30 m and recorded nest sites of 13 ant species. We found that NDVI values at the nesting sites varied considerably between individual species and also between the six functional groups the ants belong to. The functional groups Cryptic Species, Tropical Climate Specialists and Specialist Predators were present in regions with high NDVI whereas Hot Climate Specialists and Opportunists were found in sites with low NDVI. As expected we found that low NDVI values were associated with scrub jungles and high NDVI values with evergreen forests. Interestingly, we found that Pachycondyla rufipes, an ant species found only in deciduous and evergreen forests, established nests only in sites with low NDVI (range = 0.015 - 0.1779). Our results show that these low NDVI values in deciduous and evergreen forests correspond to canopy gaps in otherwise closed deciduous and evergreen forests. Subsequent fieldwork confirmed the observed high prevalence of P. rufipes in these NDVI-constrained areas. We discuss the value of using NDVI for the remote detection and distinction of ant nest sites.
Abstract: Estimation of distance travelled (odometry) forms a vital part of navigation for solitarily foraging ants. In this study we investigated the properties of odometric memory in the Australian desert ant Melophorus bagoti. Ants were trained to travel in linear channels to a feeder placed at 6 m or 12 m from the nest. We determined if the ability to estimate distances accurately increased with experience. We also determined the delay at which the odometric memory started to decay at both these distances. Ants with six trials of experience did not get better at estimating distances and the odometric memory decayed after a 24 h delay. We then determined if ants integrated their latest odometric memory with their previous memories. We did this by training two groups of ants for five trials, one group to 6 m and another to 12 m, and then halving or doubling the outbound distance on their sixth trip, respectively. The ant's estimate was noted when the ants were released either immediately or after a 24 h delay. Ants always estimated their last outbound distance when released immediately. However, they switched to route-based navigation rather than estimating distances after a 24 h delay (at which time their odometric memory would have decayed).
Abstract: Highly evolved eusocial insects such as ants return from a food source to their nest by the shortest possible distance. This form of navigation, called path-integration, involves keeping track of the distance travelled and the angles steered on the outbound journey, which then aids in the computation of the shortest return distance. In featureless terrain, ants rely on the path integrator to travel the entire distance to return to the nest, whereas in landmark-rich habitats ants are guided by visual cues and in the absence of the visual cues homing ants rely on the path integrator to travel only an initial 10-60 cm of the homebound distance. The functioning of the path integrator in a habitat of intermediate landmark density is unknown. The findings reported here show that when the outward journey is on a familiar foraging area, and the inward journey is on an unfamiliar area, the Australian route-following desert ant Melophorus bagoti relies on the path integrator and consistently travels half the distance of the outward trip. However, when both the outward and inward trips are performed in plain and featureless channels, which blocks the distinct terrestrial visual cues, ants travel the entire distance accurately. A similar half-way abbreviation of the home vector occurs when the ant's outward trip is in an L-shaped channel and the homeward trip is over an open and unfamiliar region. The ecological significance of these new findings is discussed.
Abstract: Individually foraging ants are known to return to their nest by using path-integration and recording visual information present in the environment. The interaction between the path integrator and the information provided by the visual cues in an Australian desert ant are reported here. Ants were trained to travel in a 1-m wide and 20-m long corridor of cylinders. Homeward paths of trained ants were recorded in the presence and absence of vector information and route cues in both the familiar training field and in an unfamiliar test field. Homing ants used route cue information only in a familiar context. The route cues were not essential but served to reduce the deviation of the homing trajectory from the nest-feeder line. When displaced locally, homebound ants initially oriented towards the nest using distant cues and then headed in a direction intermediate between that dictated by the path integrator and the distant cues. If in the course of travel ants encountered the familiar path they adhered to it. If not, they travelled on average half the distance of the outbound journey and initiated a search directed towards the nest. Following the search, ants headed in a direction intermediate between that dictated by the route cues and the distant cues. In an unfamiliar context neither vector nor route cue information could steer a homing ant towards the nest. The dominance of distant cues, the importance of familiar context and the interaction between different navigation strategies are discussed here.
Abstract: A variety of social insects use visual cues for homing. In this study, we examine the possible factors affecting the learning and retention of nest-associated visual cues by the Australian desert ant Melophorus bagoti and the manner in which such cues are encoded by foraging ants. We placed four prominent cylindrical landmarks around a nest and trained foragers from that nest to a food source. Ants were tested with the landmark array in a distant testing field after (1) a known number of exposures to the landmarks (1, 3, 7 or 15 trials, spread over a period of 1 day, 2 days or â¥3 days) and (2) after a known period of delay (0, 24, 48, 96 or 192 h). The results show that a combination of an increase in training trials and an increase in number of training days affected the acquisition of landmark memory. Moreover, once the landmarks were learnt, they became a part of long-term memory and lasted throughout the antsâ foraging lifetime. To examine visual cue encoding behaviour, ants trained under similar conditions for 4 days were tested with (1) an identical landmark array, (2) landmarks of the same size used in training, but placed at twice the distance from each other, and (3) landmarks whose dimensions were doubled and placed at twice the distance from each other. In conditions (1) and (3), the ants searched extensively at the centre of the four landmarks, suggesting that, similar to the Saharan ant (genus Cataglyphis) and the honeybee, M. bagoti too uses a snapshot to match the view of the landmarks around the nest. But contrary to the snapshot model, in condition (2), the ants did not search extensively at the centre of the landmarks, but searched primarily 0.5 m from the landmark, the distance from each landmark to the nest during training. We discuss how various search models fare in accounting for these findings.
Abstract: Assuming that the acquisition and retention of memories have costs, properties of memories should fit the functional requirements for the system of memory. Based on a functional analysis of what path integration is meant to do, we predicted that odometric memories in desert ants should show (1) little improvement with repeated training: performance should be as good after one training trial as after six training trials, (2) decay of memory after 24 h, and (3) performance based solely on the most recent outbound trip, with no integration over multiple memories. Desert ants (Cataglyphis fortis) traveled in narrow straight plastic channels to forage for cookie crumbs in a feeder at 6- or 12-m distance. Each ant was tested once by being taken from the feeder and released 2 m from the end of a 32-m channel to run home. The distance at which the ant first turned back (first turn) constituted the data. In acquisition, groups trained one or six times before being tested had unsystematic scatter that did not differ significantly. In retention, ants tested after a 24-h delay showed larger unsystematic scatter than control animals tested after no delay. In integration, ants were trained five times at 6 or 12 m and then tested at 12 or 6 m, respectively. No evidence of integration of multiple odometric memories was found. The results show that the properties of odometric memories are indeed tailored to what the memory system is used for.
