SUMMARYIndividual foraging ants are known to rely on views of their surroundings for route learning and for pinpointing goals. Different strategies have been proposed to explain how ants might process visual information for navigation, but little is known about the actual development and nature of the view-based strategies used by ants in complex natural environments. Here, we constrained the knowledge of Melophorus bagoti ants to either the nest vicinity or a curved route (length 10m) and analysed their initial direction when released at both novel and familiar locations. In parallel, we used 360deg pictures of the scene as a basis for modelling different navigational strategies. We propose here a new hypothesis based on skyline height comparison to explain how ants home from novel locations. Interestingly, this strategy succeeded well at novel locations but failed on familiar terrain. By contrast, the use of a visual compass strategy failed at novel locations but could explain the results on familiar routes. We suggest that ants might switch between skyline height comparison and a visual compass strategy, depending on whether they are on familiar terrain or not. How ants could switch between strategies and how their memories develop are discussed in turn.

Rats trained to relocate a particular corner in a rectangular arena systematically confound the correct corner and the diametrically opposite one--this rotational error demonstrates the use of the geometry of space (i.e., the spatial arrangement of the different components of a visual scene). In many cases, geometric information is preferentially used over other spatial cues, suggesting the presence of a dedicated geometric module located in the parahippocampus and processing only geometric information. Since rotational errors were first demonstrated in 1986, the use of the geometry of space has attracted great interest and now seems to be widespread in vertebrate species, including humans. Until now, rotational errors have only been considered in vertebrate species. Here, for the first time, rotational errors are demonstrated in an insect. Our results, similar to those obtained with vertebrates, can be parsimoniously explained by a view-based matching strategy well known in insects, thereby challenging the hypothesis of a "geometric module" located in the animal's brain. While introducing a new concept of flexibility in the view-based matching theory, this study creates a link between two major topics of animal navigation: rotational errors in vertebrates and view-based navigation in insects.

BackgroundInsects are known to rely on terrestrial landmarks for navigation. Landmarks are used to chart a route or pinpoint a goal. The distant panorama, however, is often thought not to guide navigation directly during a familiar journey, but to act as a contextual cue that primes the correct memory of the landmarks.ResultsWe provided Melophorus bagoti ants with a huge artificial landmark located right near the nest entrance to find out whether navigating ants focus on such a prominent visual landmark for homing guidance. When the landmark was displaced by small or large distances, ant routes were affected differently. Certain behaviours appeared inconsistent with the hypothesis that guidance was based on the landmark only. Instead, comparisons of panoramic images recorded on the field, encompassing both landmark and distal panorama, could explain most aspects of the ant behaviours.ConclusionAnts navigating along a familiar route do not focus on obvious landmarks or filter out distal panoramic cues, but appear to be guided by cues covering a large area of their panoramic visual field, including both landmarks and distal panorama. Using panoramic views seems an appropriate strategy to cope with the complexity of natural scenes and the poor resolution of insects' eyes. The ability to isolate landmarks from the rest of a scene may be beyond the capacity of animals that do not possess a dedicated object-perception visual stream like primates.