Allometry in desert ant locomotion (Cataglyphis albicans and Cataglyphis bicolor) - does body size matter?

Desert ants show a large range of adaptations to their habitats. They can reach extremely high running speeds, for example, to shorten heat stress during foraging trips. It has recently been examined how fast walking speeds are achieved in different desert ant species. It is intriguing in this context that some species exhibit distinct intraspecific size differences. We therefore performed a complete locomotion analysis over the entire size spectrum of the species Cataglyphis bicolor, and we compared this intraspecific dataset with that of the allometrically similar species Cataglyphis albicans. Emphasis was on the allometry of locomotion: we considered the body size of each animal and analysed the data in terms of relative walking speed. Body size was observed to affect walking parameters, gait patterns and phase relationships in terms of absolute walking speed. Unexpectedly, on a relative scale, all ants tended to show the same overall locomotion strategy at low walking speeds, and significant differences occurred only between C. albicans and C. bicolor at high walking speeds. Our analysis revealed that C. bicolor ants use the same overall strategy across all body sizes, with small ants reaching the highest walking speeds (up to 80 body lengths s-1) by increasing their stride length and incorporating aerial phases. By comparison, C. albicans reached high walking speeds mainly by a high synchrony of leg movement, lower swing phase duration and higher stride frequency ranging up to 40 Hz.

The effect of spatially restricted experience on extrapolating learned views in desert ants, Melophorus bagoti.

Desert ants are known for learning walks at the beginning of their foraging life, during which they learn terrestrial cues of the panorama and surrounding landmarks around their nest. Foragers retain memories of the visual cues of the nest panorama learned during the pre-foraging trials. When away from the nest, they can compare these stored views with their current vision to return to their nest. In this study we investigated whether spatially restricted foraging ants can extrapolate their memory of visual cues to unexperienced sites. We carried out two conditions to examine whether desert ants extrapolate learned views. In the first condition, naïve ants of Melophorus bagoti were restricted to a nest arena 1 m in radius with a 10 cm high wall (wall condition) for 3 days, then released at distant locations on the fourth day and focal individuals return trips were recorded. In the second condition, a 10 cm sunken metallic barrier was constructed around the nest (moat condition) and the restricted foragers that viewed the unrestricted visual panorama around the 1 m-radius nest arena were then displaced away from the nest as in the wall condition. In the wall condition, most of the ants were unable to orient in the correct heading towards the home direction. In the moat condition ants were able to correctly orient to the nest from displacement sites up to 8 m from the nest. We conclude that while travelling to unfamiliar sites, M. bagoti ants can extrapolate views learned from foraging in a restricted area when given unrestricted views.

Running paths to nowhere: repetition of routes shows how navigating ants modulate online the weights accorded to cues.

Ants are expert navigators, keeping track of the vector to home as they travel, through path integration, and using terrestrial panoramas in view-based navigation. Although insect learning has been much studied, the learning processes in navigation have not received much attention. Here, we investigate in desert ants (Melophorus bagoti) the effects of repeating a well-travelled and familiar route segment without success. We find that re-running a homeward route without entering the nest impacted subsequent trips. Over trips, ants showed more meandering from side to side and more scanning behaviour, in which the ant stopped and turned, rotating to a range of directions. In repeatedly re-running their familiar route, ants eventually gave up heading in the nestward direction as defined by visual cues and turned to walk in the opposite direction. Further manipulations showed that the extent and rate of this path degradation depend on (1) the length of the vector accumulated in the direction opposite to the food-to-nest direction, (2) the specific visual experience of the repeated segment of the route that the ants were forced to re-run, and (3) the visual panorama: paths are more degraded in an open panorama, compared with a visually cluttered scene. The results show that ants dynamically modulate the weighting given to route memories, and that fits well with the recent models, suggesting that the mushroom bodies provide a substrate for the reinforcement learning of views for navigation.

Skyline retention and retroactive interference in the navigating Australian desert ant, Melophorus bagoti.

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.

Steering intermediate courses: desert ants combine information from various navigational routines.

A number of systems of navigation have been studied in some detail in insects. These include path integration, a system that keeps track of the straight-line distance and direction travelled on the current trip, the use of panoramic landmarks and scenery for orientation, and systematic searching. A traditional view is that only one navigational system is in operation at any one time, with different systems running in sequence depending on the context and conditions. We review selected data suggesting that often, different navigational cues (e.g., compass cues) and different systems of navigation are in operation simultaneously in desert ant navigation. The evidence suggests that all systems operate in parallel forming a heterarchical network. External and internal conditions determine the weights to be accorded to each cue and system. We also show that a model of independent modules feeding into a central summating device, the Navinet model, can in principle account for such data. No central executive processor is necessary aside from a weighted summation of the different cues and systems. Such a heterarchy of parallel systems all in operation represents a new view of insect navigation that has already been expressed informally by some authors.

How to find home backwards? Locomotion and inter-leg coordination during rearward walking of Cataglyphis fortis desert ants.

For insects, flexibility in the performance of terrestrial locomotion is a vital part of facing the challenges of their often unpredictable environment. Arthropods such as scorpions and crustaceans can switch readily from forward to backward locomotion, but in insects this behaviour seems to be less common and, therefore, is only poorly understood. Here we present an example of spontaneous and persistent backward walking in Cataglyphis desert ants that allows us to investigate rearward locomotion within a natural context. When ants find a food item that is too large to be lifted up and to be carried in a normal forward-faced orientation, they will drag the load walking backwards to their home nest. A detailed examination of this behaviour reveals a surprising flexibility of the locomotor output. Compared with forward walks with regular tripod coordination, no main coordination pattern can be assigned to rearward walks. However, we often observed leg-pair-specific stepping patterns. The front legs frequently step with small stride lengths, while the middle and the hind legs are characterized by less numerous but larger strides. But still, these specializations show no rigidly fixed leg coupling, nor are they strictly embedded within a temporal context; therefore, they do not result in a repetitive coordination pattern. The individual legs act as separate units, most likely to better maintain stability during backward dragging.

Combining sky and earth: desert ants (Melophorus bagoti) show weighted integration of celestial and terrestrial cues.

Insects typically use celestial sources of directional information for path integration, and terrestrial panoramic information for view-based navigation. Here we set celestial and terrestrial sources of directional information in conflict for homing desert ants (Melophorus bagoti). In the first experiment, ants learned to navigate out of a round experimental arena with a distinctive artificial panorama. On crucial tests, we rotated the arena to create a conflict between the artificial panorama and celestial information. In a second experiment, ants at a feeder in their natural visually-cluttered habitat were displaced prior to their homing journey so that the dictates of path integration (feeder to nest direction) based on a celestial compass conflicted with the dictates of view-based navigation (release point to nest direction) based on the natural terrestrial panorama. In both experiments, ants generally headed in a direction intermediate to the dictates of celestial and terrestrial information. In the second experiment, the ants put more weight on the terrestrial cues when they provided better directional information. We conclude that desert ants weight and integrate the dictates of celestial and terrestrial information in determining their initial heading, even when the two directional cues are highly discrepant.

The Geomagnetic Field Is a Compass Cue in Cataglyphis Ant Navigation.

Desert ants (Cataglyphis) are famous insect navigators. During their foraging lives, the ants leave their underground colonies for long distances and return to their starting point with fair accuracy [1, 2]. Their incessantly running path integrator provides them with a continually updated home vector [3-5]. Directional input to their path integrator is provided by a visual compass based on celestial cues [6, 7]. However, as path integration is prone to cumulative errors, the ants additionally employ landmark guidance routines [8-11]. At the start of their foraging lives, they acquire the necessary landmark information by performing well-structured learning walks [12, 13], including turns about their vertical body axes [14]. When Cataglyphis noda performs these pirouettes, it always gazes at the nest entrance during the longest of several short stopping phases [14]. As the small nest entrance is not visible, the ants can adjust their gaze direction only by reading out their path integrator. However, recent experiments have shown that, for adjusting the goal-centered gaze directions during learning walks, skylight cues are not required [15]. A most promising remaining compass cue is the geomagnetic field, which is used for orientation in one way or the other by a variety of animal species [16-25]. Here, we show that the gaze directions during the look-back-to-the-nest behavior change in a predictable way to alterations of the horizontal component of the magnetic field. This is the first demonstration that, in insects, a geomagnetic compass cue is both necessary and sufficient for accomplishing a well-defined navigational task.

Beginnings of a synthetic approach to desert ant navigation.

In a synthetic approach to studying navigational abilities in desert ants, we review recent work comparing ants living in different visual ecologies. Those living in a visually rich habitat strewn with tussocks, bushes, and trees are compared to those living in visually barren salt pans, as exemplified by the Central Australian Melophorus bagoti and the North African Cataglyphis fortis, respectively. In bare habitats the navigator must rely primarily on path integration, keeping track of the distance and direction in which it has travelled, while in visually rich habitats the navigator can rely more on guidance by the visual panorama. Consistent with these expectations, C. fortis performs better than M. bagoti on various measures of precision at path integration. In contrast, M. bagoti learned a visually based associative task better than C. fortis, the latter generally failing at the task. Both these ants, however, exhibit a similar pattern of systematic search as a 'back up' strategy when other navigational strategies fail. A newly investigated salt-pan species of Melophorus (as yet unnamed) resembles C. fortis more, and its congener M. bagoti less, in its path integration. The synthetic approach would benefit from comparing more species chosen to address evolutionary questions. This article is part of a Special Issue entitled: CO3 2013.

Re-visiting of plentiful food sources and food search strategies in desert ants.

North African desert ants, Cataglyphis fortis, are established model organisms in animal navigation research. Cataglyphis re-visit plentiful feeding sites, but their decision to return to a feeder and the organization of food searches has been little studied. Here we provide a review of recent advances regarding this topic. At least two parameters determine the ants' assessment of site quality, namely, amount of food available and reliability of food encounter on subsequent visits. The amount of food appears to be judged by the concentration of items at the food uptake site. Initially the amount of food in a feeder dominates the foragers' decision to return, whereas learning about reliability takes precedence in the course of a few visits. The location of a worthwhile site is determined by the animals' path integration system. In particular, the distance of the feeding site is memorized as the arithmetic average of the distances covered during the previous outbound and homebound journeys. Feeding sites that are small and inconspicuous cannot be approached directly with sufficient certainty, due to inevitable inaccuracies of the path integrator. Instead, desert ants steer downwind of the goal to encounter the odor plume emanating from the food and they follow this plume to the feeder. The angle steered downwind reflects the animals' maximal navigation error and is adjusted according to experience. In summary, food searches of desert ants provide an unexpected wealth of features that may advance our understanding of search, navigation, and decision strategies. There are several aspects that warrant further scrutiny.