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Ant species exhibit behavioural commonalities when solving navigational challenges for successful orientation and to reach goal locations. These behaviours rely on a shared toolbox of navigational strategies that guide individuals under an array of motivational contexts. The mechanisms that support these behaviours, however, are tuned to each species' habitat and ecology with some exhibiting unique navigational behaviours. This leads to clear differences in how ant navigators rely on this shared toolbox to reach goals. Species with hybrid foraging structures, which navigate partially upon a pheromone-marked column, express distinct differences in their toolbox, compared to solitary foragers. Here, we explore the navigational abilities of the Western Thatching ant (Formica obscuripes), a hybrid foraging species whose navigational mechanisms have not been studied. We characterise their reliance on both the visual panorama and a path integrator for orientation, with the pheromone's presence acting as a non-directional reassurance cue, promoting continued orientation based on other strategies. This species also displays backtracking behaviour, which occurs with a combination of unfamiliar terrestrial cues and the absence of the pheromone, thus operating based upon a combination of the individual mechanisms observed in solitarily and socially foraging species. We also characterise a new form of goalless orientation in these ants, an initial retreating behaviour that is modulated by the forager's path integration system. The behaviour directs disturbed inbound foragers back along their outbound path for a short distance before recovering and reorienting back to the nest., (© 2023. The Author(s).)

The behaviours and cognitive mechanisms animals use to orient, navigate, and remember spatial locations exemplify how cognitive abilities have evolved to suit a number of different mobile lifestyles and habitats. While spatial cognition observed in vertebrates has been well characterised in recent decades, of no less interest are the great strides that have also been made in characterizing and understanding the behavioural and cognitive basis of orientation and navigation in invertebrate models and in particular insects. Insects are known to exhibit remarkable spatial cognitive abilities and are able to successfully migrate over long distances or pinpoint known locations relying on multiple navigational strategies similar to those found in vertebrate models-all while operating under the constraint of relatively limited neural architectures. Insect orientation and navigation systems are often tailored to each species' ecology, yet common mechanistic principles can be observed repeatedly. Of these, reliance on visual cues is observed across a wide number of insect groups. In this review, we characterise some of the behavioural strategies used by insects to solve navigational problems, including orientation over short-distances, migratory heading maintenance over long distances, and homing behaviours to known locations. We describe behavioural research using examples from a few well-studied insect species to illustrate how visual cues are used in navigation and how they interact with non-visual cues and strategies., (© 2022. The Author(s).)

We propose an expansion of neuroecological comparisons to include the capabilities of brainless and non-neural organisms. We begin this enterprise by conducting a systematic search for studies on learning in echinoderms. Echinodermata are marine invertebrates comprising starfish, brittle stars, sea cucumbers, sea urchins, and sea lilies. Animals in this phylum lack any centralized brain and instead possess diffuse neural networks known as nerve nets. The learning abilities of these animals are of particular interest as, within the bilaterian clade, they are close evolutionary neighbors to chordates, a phylum whose members exhibit complex feats in learning and contain highly specialized brains. The learning capacities and limitations of echinoderms can inform the evolution of nervous systems and learning in Bilateria. We find evidence of both non-associative and associative learning (in the form of classical conditioning) in echinoderms, which was primarily focused on starfish. Additional evidence of learning is documented in brittle stars, sand dollars, and sea urchins. We then discuss the evolutionary significance of learning capabilities without a brain, the presence of embodied cognition across multiple groups, and compare the learning present in echinoderms with the impressive cognitive abilities documented in the oldest linage group within vertebrates (the major group within the phylum of chordates), fish., (© 2021. The Psychonomic Society, Inc.)

Navigation is comprised of a variety of strategies which rely on multiple external cues to shape a navigator's behavioral output. Here, we explored in the ant Veromessor pergandei, the interactions between the information provided by the pheromone trail and the home vector guided by the celestial compass. We found that a cross sensory interaction between the pheromone cue and the path integrator underlies correct orientation during the inbound journey. The celestial compass provides directional information, while the presence of the trail pheromone acts as a critical context cue, triggering distinct behaviors (vector orientation, search, and backtracking). While exposed to the pheromone, foragers orient to the vector direction regardless of vector state, while in the pheromone's absence, the current remaining vector determines the forager's navigational behavior. This interaction also occurs in foragers with no remaining path integrator, relying on the activation of a celestial compass-based memory of the previous trip. Such cue interactions maximize the foragers' return to the nest and inhibit movement off the pheromone trail. Finally, our manipulations continuously rotated foragers away from their desired heading, yet foragers were proficient at counteracting these changes, steering to maintain a correct compass heading even at rotational speeds of ~ 40°/s.

Nocturnal insects have remarkable visual capacities in dim light. They can navigate using both the surrounding panorama and celestial cues. Individual foraging ants are efficient navigators, able to accurately reach a variety of goal locations. During navigation, foragers compare the current panoramic view to previously learnt views. In this natural experiment, we observed the effects of large panorama changes, the addition of a fence and the removal of several trees near the nest site, on the navigation of the nocturnal bull ant Myrmecia midas. We examined how the ants' navigational efficiency and behaviour changed in response to changes in ~ 30% of the surrounding skyline, following them over multiple nights. Foragers were displaced locally off-route where we collected initial orientations and homing paths both before and after large panorama changes. We found that immediately after these changes, foragers were unable to initially orient correctly to the nest direction and foragers' return paths were less straight, suggesting increased navigational uncertainty. Continued testing showed rapid recovery in both initial orientation and path straightness.

The desert harvester ant (Veromessor pergandei) employs a mixture of social and individual navigational strategies at separate stages of their foraging trip. Individuals leave the nest along a pheromone-based column, travelling 3-40 m before spreading out to forage individually in a fan. Foragers use path integration while in this fan, accumulating a direction and distance estimate (vector) to return to the end of the column (column head), yet foragers' potential use of path integration in the pheromone-based column is less understood. Here we show foragers rely on path integration both in the foraging fan and while in the column to return to the nest, using separate vectors depending on their current foraging stage in the fan or column. Returning foragers displaced while in the fan oriented and travelled to the column head location while those displaced after reaching the column travel in the nest direction, signifying the maintenance of a two-vector system with separate fan and column vectors directing a forager to two separate spatial locations. Interestingly, the trail pheromone and not the surrounding terrestrial cues mediate use of these distinct vectors, as fan foragers briefly exposed to the pheromone cues of the column in isolation altered their paths to a combination of the fan and column vectors. The pheromone acts as a contextual cue triggering both the retrieval of the column-vector memory and its integration with the forager's current fan-vector.

The polarisation pattern of skylight serves as an orientation cue for many invertebrates. Solitary foraging ants, in particular, rely on polarised light to orient along with a number of other visual cues. Yet it is unknown, if this cue is actively used in socially foraging species that use pheromone trails to navigate. Here, we explore the use of polarised light in the presence of the pheromone cues of the foraging trail. The desert harvester ant, Veromessor pergandei, relies on pheromone cues and path integration in separate stages of their foraging ecology (column and fan, respectively). Here, we show that foragers actively orient to an altered overhead polarisation pattern, both while navigating individually in the fan and while on the pheromone-based column. These heading changes occurred during twilight, as well as in the early morning and late afternoon before sunset. Differences in shift size indicate that foragers attend to both the polarisation pattern and the sun's position when available, yet during twilight, headings are dominated by the polarisation pattern. Finally, when the sun's position was experimentally blocked before sunset, shift sizes increased similar to twilight testing. These findings show that celestial cues provide directional information on the pheromone trail.

Foraging ants are able to acquire and retain long-term memories of panorama cues around the nest and along known routes. Here we explore foragers' ability to learn and retain skyline cues experienced on only the outbound or inbound portion of the foraging trip. Foragers exposed to the skyline on the outbound portion showed single trial learning of these cues. Furthermore, the navigational performance of these "Outbound-Only" foragers was on par with foragers that experienced the full route. In contrast, foragers experiencing the skyline only on the inbound portion, "Inbound-Only" foragers, took 5 trips to successfully learn these cues. These performance differences persisted for long-term memory retention. Outbound-Only foragers successfully oriented after a 3-day delay and showed similar performance to the full route control, whereas Inbound-Only foragers were no longer able to orient successfully to these cues after 3 days. Additionally, long-term memory formation of skyline cues appears to require multiple presentations, as foragers with only one outbound experience of the skyline could not successfully orient after the delay. Our results suggest that terrestrial cue learning and retention is more robust when cues are experienced on the outbound segment of the foraging trip.

Visual cues commonly aid solitary foraging ants. Specifically, foragers can use the skyline where terrestrial landmarks meet the sky. Foraging ants show a remarkable affinity to retain these terrestrial cues, developing lifelong memories of the nest site panorama. Here we explore foragers' ability to retain skyline cues of resource locations at some distance from the nest through experiments with artificial skylines erected around a resource location. We also tested the foragers' memories of one skyline at several time points after the skyline was replaced by a different one. During retention testing, foragers appear able to retain robust memories of these skylines over periods (5 days) that surpass their average life span. Exposure to the nest panorama during these periods did not interfere with navigational performance at the distant skyline. Foragers in the replacement experiment initially oriented correctly to both skylines. Thereafter, the foragers' headings in tests with the first skyline gradually shifted away from the correct homeward direction. We argue that new skyline memories cause retroactive interference in the retention of previously learned skylines. Skyline memories may compete during memory retrieval, or may be retrieved in association with context cues present in the current testing paradigm such as vector length.

Rüdiger Wehner's work on insect orientation and navigation has influenced many scientists studying navigation, not only in ants and bees, but in other animals as well. We review the scientific legacy of six topics arising from Wehner's work on navigation. The polarisation compass is a chapter with a lot of behavioural and neurobiological detail. It has influenced the study of polarisation vision in other systems, and led Wehner to formulate the concept of a matched filter. The matched filter has probably had earlier formulations, but Wehner's paper on it has been much cited in studies on navigation and in other fields. The polarisation compass serves the task of path integration in insects. Work on path integration took off in the 1980s with work on desert ants and rodents. The use of terrestrial visual cues, landmarks or the panorama in view-based matching is another major theme of navigational research today. Search strategies were also well described in desert ants, and this line of research helped to launch theoretical and empirical developments in searching behaviour, now a lively area of research. Finally, robotic work has often drawn inspiration from work on insect navigation. We end with some discussion of current research directions.