How desert ants use a visual landmark for guidance along a habitual route

Abstract

Many animals learn to follow habitual routes between important locations, but how they encode their routes is still largely unknown. Desert ants traveling between their nest and a food site develop stable, visually guided routes that can wind through desert scrub without the use of trail pheromones. Their route memories are sufficiently robust that if a nest-bound ant is caught at the end of its route and replaced somewhere earlier along it, the ant will recapitulate the route from the release site. Insects appear to use panoramas to recognize when they are on a familiar route. I examine here the cues then used for their guidance. Several mechanisms are known for straight segments of a route; but how does an ant encode a curved route along which both the views it sees, and the directions it takes, are constantly changing? The results here suggest that when an ant travels past a landmark on a familiar route, it uses the gradually changing direction of the landmark to trigger a set of associated learned heading directions. A route through a complex 2D environment could thus be encoded and followed economically if it is divided into panorama-defined segments, with each segment controlled by such a 1D mapping. The solution proposed for the ants would be simple to implement in an autonomous robot.

Scene perception and the visual control of travel direction in navigating wood ants

“…This ability also requires elaborate innate computational mechanisms, but has the extra complication of extracting, storing and then using the specific visual information that is needed to follow a habitual route. Ants are turning out to be particularly informative, because the precision and stereotypy of their routes [17][18][19][20] make it possible to analyse the underlying strategies and mechanisms [21,22]. Ants are also interesting because of the multiple sensory cues that contribute to their navigation, particularly olfaction, with long range cues from wind-borne volatiles [23][24][25] and short-range cues from pheromone trails on the ground [26].…”

Image-matching during ant navigation occurs through saccade-like body turns controlled by learned visual features

“…Landmarks both set the shape of the route that an ant follows and specify the position of the route's endpoint. Ants, far from the goal, approach landmarks as beacons, or detour round them, or use them for directional information (5)(6)(7)(8). When close to the goal, ants and other insects move as if they are trying to improve the fit of their current 2D image of the goal's surroundings to their stored view of those surroundings and search for the goal where the two images coincide (9)(10)(11)(12)(13)(14).…”

A desert ant's memory of recent visual experience and the control of route guidance

“…By rotating so as to align the current retinal view with retinotopic visual memories acquired on previous trips, an individual will automatically travel in the habitual directions from points along a route [8][9][10][11][12][13][14][15]. This widely used mechanism can be termed 'alignment image-matching' [11,16]. It allows individuals to follow stereotyped and often idiosyncratic routes [17][18][19].…”

“…If an individual travels a path repeatedly, then it acquires visual-based route memories of its habitual heading directions along the path [8][9][10][11][12]. By rotating so as to align the current retinal view with retinotopic visual memories acquired on previous trips, an individual will automatically travel in the habitual directions from points along a route [8][9][10][11][12][13][14][15].…”

“…This means that once the ant has been displaced, the PI output vector will no longer point towards the nest: on release, the ant is said to have a 'zero vector' [20]. Nonetheless, as long as it retains its homeward motivation (which it generally does so long as it has food to take home), a zero-vector ant will use its visual memories to recapitulate the route it just took [11,[17][18][19]. In the current study, ants were trained along a route that passed a single dominating landmark in an otherwise largely bare environment (figure 1a and the electronic supplementary material, figure S1).…”