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The biomechanics of load carriage has been studied extensively with regards to single individuals, yet not so much with regards to collective transport. We investigated the biomechanics of walking in 10 paired individuals carrying a load that represented 20%, 30%, or 40% of the aggregated body-masses. We computed the energy recovery rate at the center of mass of the system consisting of the two individuals plus the carried load in order to test to what extent the pendulum-like behavior and the economy of the gait were affected. Joint torque was also computed to investigate the intra- and inter-subject strategies occurring in response to this. The ability of the subjects to move the whole system like a pendulum appeared rendered obvious through shortened step length and lowered vertical displacements at the center of mass of the system, while energy recovery rate and total mechanical energy remained constant. In parallel, an asymmetry of joint moment vertical amplitude and coupling among individuals in all pairs suggested the emergence of a leader/follower schema. Beyond the 30% threshold of increased load mass, the constraints at the joint level were balanced among individuals leading to a degraded pendulum-like behavior. [ABSTRACT FROM AUTHOR]

Of particular importance in shaping species assemblages is the spatial heterogeneity of the environment. The aim of our study was to investigate the influence of spatial heterogeneity and environmental complexity on the distribution of ant functional groups and species diversity along altitudinal gradients in a temperate ecosystem (Pyrenees Mountains). During three summers, we sampled 20 sites distributed across two Pyrenean valleys ranging in altitude from 1,009 to 2,339 m by using pitfall traps and hand collection. The environment around each sampling points was characterized by using both physical and land-cover variables. We then used a self-organizing map algorithm (SOM, neural network) to detect and characterize the relationship between the spatial distribution of ant functional groups, species diversity, and the variables measured. The use of SOM allowed us to reduce the apparent complexity of the environment to five clusters that highlighted two main gradients: an altitudinal gradient and a gradient of environmental closure. The composition of ant functional groups and species diversity changed along both of these gradients and was differently affected by environmental variables. The SOM also allowed us to validate the contours of most ant functional groups by highlighting the response of these groups to the environmental and land-cover variables. [ABSTRACT FROM AUTHOR]

We investigated the effect of substrate roughness on load selection in the seed-harvester ant Messor barbarus. Ants were forced to travel either on sand or on gravel to reach a seed patch containing seed fragments of different weights. We hypothesized that foragers travelling on a rough substrate could either increase their load as a result of the increased distance travelled (due to a more sinuous path and an increase in the vertical component of the path) or decrease their load because of the anticipated difficulty of moving with a heavy load on a rough surface. Our results were consistent with neither of these hypotheses: Load selection by ants did not depend on the roughness of the substrate encountered during their outbound trip. The main effect of substrate roughness was to slow down the progression of the ants and increase their probability of dropping or transferring heavy seeds on their way back to the nest, thus resulting in an overall reduction of the rate of seed return to the nest. [ABSTRACT FROM AUTHOR]

We experimentally investigated both individual and collective behavior of the Argentine ant Linepithema humile as they crossed symmetrical and asymmetrical bifurcations in gallery networks. Ants preferentially followed the branch that deviated the least from their current direction and their probability to perform a U-turn after a bifurcation increased with the turning angle at the bifurcation. At the collective level, colonies were better able to find the shortest path that linked the nest to a food source in a polarized network where bifurcations were symmetrical from one direction and asymmetrical from the other than in a network where all bifurcations were symmetrical. We constructed a model of individual behavior and showed that an individual’s preference for the least deviating path will be amplified via the ants’ mass recruitment mechanism thus explaining the difference found between polarized and non-polarized networks. The foraging efficiency measured in the simulations was three times higher in polarized than in non-polarized networks after only 15 min. We conclude that measures of transport network efficiency must incorporate both the structural properties of the network and the behavior of the network users. [ABSTRACT FROM AUTHOR]

In this paper we investigate the flexibility of foraging behavior in the leaf-cutting ant Atta colombica, both at the individual and collective levels, following a change in the physical properties of their environment. We studied in laboratory conditions the changes occurring in foraging behavior when a height constraint was placed 1 cm above part of the trail linking the nest to the foraging area. We found that the size and shape of the fragments of foraging material brought back to the nest were significantly modified when the constraint was placed on the trail: independent of their size, forager ants cut smaller and rounder fragments in the presence of a height constraint than in its absence. This size adjustment does not require any direct sensory feedback because it occurred when the ants cut fragments in the foraging area; no further cutting was done when they encountered the constraint. This points to the existence of a template that ants store and use as a reference to adjust their reach while cutting. Remarkably, despite the decrease in the foraging material brought to the nest per capita the colony was still able to improve its foraging performance by doubling the number of transporters. This study illustrates the flexibility of foraging behavior exhibited by an ant colony. It provides a rare example of insects finding an intelligent solution to a problem occurring in a foraging context, at both the individual and collective levels. [ABSTRACT FROM AUTHOR]

The ability to orient and navigate in space is essential for all animals whose home range is organized around a central point. Because of their small home range compared to vertebrates, central place foraging insects such as ants have for a long time provided a choice model for the study of orientation mechanisms. In many ant species, the movement of individuals on their colony home range is achieved essentially collectively, on the chemical trails laid down by their nest mates. In the initial stage of food recruitment, these trails can cross each other and thus form a network of interconnected paths in which ants have to orient. Previous simulation studies have shown that ants can find the shortest path between their nest and a food source in such a network only if there is a bias in the branch they choose when they reach an asymmetrical bifurcation. In this paper, we studied the choice of ants when facing either a symmetrical or an asymmetrical bifurcation between two tunnels. Ants were tested either on their way to a food source or when coming back to their nest, and either in the presence or in the absence of a chemical trail. Overall, our results show that the choice of an ant at a tunnel bifurcation depends more on the presence/absence of a trail pheromone than on the geometry of the bifurcation itself. [ABSTRACT FROM AUTHOR]

In this paper we examine the effect of crowding on the selection of a path in the mass-recruiting ant Lasius niger. In our experiment, ants had to go from their nest to a food source by crossing a diamond-shaped bridge, giving the choice between two paths. Two types of bridges were used: the first had two branches of equal length but different width while the second had two branches of different length and width. Experiments at high traffic volume always ended up with the selection of the wider branch, even if it was longer. This result shows that overcrowding on the narrow branch plays an essential role in the mechanism underlying the choice of route in ants. A mathematical model was developed to evaluate the importance of two mechanisms that could account for this result. The first is based on the difference in travel duration between the two paths. The second is based on the repulsive interactions between workers making head-on encounters. The model shows that travel duration per se is not sufficient to explain path choice. Rather, it is the interplay between trail following behaviour and repulsive interactions that allows ants to choose the path that minimizes their travel time. When choosing a path ants thus prefer to trade time against energy. Our results demonstrate that any environmental constraint that alters the dynamics of trail recruitment can lead to the emergence of adaptive foraging decisions without any explicit coding of information by the foragers at the individual level. [ABSTRACT FROM AUTHOR]

Efficient transportation, a hot topic in nonlinear science, is essential for modern societies and the survival of biological species. Biological evolution has generated a rich variety of successful solutions, which have inspired engineers to design optimized artificial systems. Foraging ants, for example, form attractive trails that support the exploitation of initially unknown food sources in almost the minimum possible time. However, can this strategy cope with bottleneck situations, when interactions cause delays that reduce the overall flow? Here, we present an experimental study of ants confronted with two alternative routes. We find that pheromone-based attraction generates one trail at low densities, whereas at a high level of crowding, another trail is established before traffic volume is affected, which guarantees that an optimal rate of food return is maintained. This bifurcation phenomenon is explained by a nonlinear modelling approach. Surprisingly, the underlying mechanism is based on inhibitory interactions. It points to capacity reserves, a limitation of the density-induced speed reduction, and a sufficient pheromone concentration for reliable trail perception. The balancing mechanism between cohesive and dispersive forces appears to be generic in natural, urban and transportation systems. [ABSTRACT FROM AUTHOR]

Division of labor in social insects has been explained by response threshold models which are based on differential responses to task-specific stimuli. In the present study, we argue that other types of stimuli, such as location-related cues, which are correlated with but not directly linked to task performance, may be significant. Using the black garden ant Lasius niger as a model, we focused on three groups of workers that perform extranidal tasks: (1) scouts, which explore new and unmarked areas remote from the nest; (2) patrollers, which are not recruited by other ants, but nonetheless walk outside in the nest vicinity; and (3) recruits, which are temporary foragers whose exit is triggered by recruitment. We used standardized tests to investigate, in a context-independent way, whether differences in task performance by these three groups could be correlated to intrinsic differences in their responsiveness to trail pheromone or to nest-related stimuli such as the presence of nestmates or colony odor. Overall, we found that the task profile of workers was correlated neither with their tendency to explore unmarked areas nor with their social attraction to nestmates. Scouts showed a lower attraction to colony odor and lower scores of trail following than recruits. Conversely, recruits were more attracted to colony chemical cues and showed lower response threshold to trail pheromone. Patrollers displayed behaviors between those of recruits and scouts. Our study thus shows that differences in ant responsiveness to location-related cues and recruitment trails contribute to the regulation of extranidal tasks in ants. Significance statement: Work organization in insect societies has been explained by models of differential response thresholds to stimuli that are directly linked to the performance of specific tasks. However, the task performance of individuals could also depend on their responsiveness to social and location-related cues that are correlated but not directly linked to these tasks. Using standardized context-independent tests, we found that scout ants, which explore unknown areas, display a lower responsiveness to colony area marking and trail pheromone. Conversely, recruits, which forage outside the nest only during food exploitation, are the most attracted to nestmates, colony area marking and trail pheromone. Patrollers display intermediate levels of attraction. Location-related cues could therefore play a role in determining the level of task performance by ants and should be taken into consideration in threshold models of task allocation. [ABSTRACT FROM AUTHOR]