Outbound Ants Research Articles

Self-organized traffic via priority rules in leaf-cutting ants.

Ants, termites and humans often form well-organized and highly efficient trails between different locations. Yet the microscopic traffic rules responsible for this organization and efficiency are not fully understood. In previous experimental studies with leaf-cutting ants (Atta colombica), a set of local priority rules were isolated and it was proposed that these rules govern the temporal and spatial organization of the traffic on the trails. Here we introduce a model based on these priority rules to investigate whether they are sufficient to produce traffic similar to that observed in the experiments on both a narrow and a wider trail. We establish that the model is able to reproduce key characteristics of the traffic on the trails. In particular, we show that the proposed priority rules induce de-synchronization into clusters of inbound and outbound ants on a narrow trail, and that priority-type dependent segregated traffic emerges on a wider trail. Due to the generic nature of the proposed priority rules we speculate that they may be used to model traffic organization in a variety of other ant species.

Polarized light use in the nocturnal bull ant, Myrmecia midas.

Solitary foraging ants have a navigational toolkit, which includes the use of both terrestrial and celestial visual cues, allowing individuals to successfully pilot between food sources and their nest. One such celestial cue is the polarization pattern in the overhead sky. Here, we explore the use of polarized light during outbound and inbound journeys and with different home vectors in the nocturnal bull ant, Myrmecia midas. We tested foragers on both portions of the foraging trip by rotating the overhead polarization pattern by ±45°. Both outbound and inbound foragers responded to the polarized light change, but the extent to which they responded to the rotation varied. Outbound ants, both close to and further from the nest, compensated for the change in the overhead e-vector by about half of the manipulation, suggesting that outbound ants choose a compromise heading between the celestial and terrestrial compass cues. However, ants returning home compensated for the change in the e-vector by about half of the manipulation when the remaining home vector was short (1−2 m) and by more than half of the manipulation when the remaining vector was long (more than 4 m). We report these findings and discuss why weighting on polarization cues change in different contexts.

Effect of density on traffic and velocity on trunk trails of Formica pratensis.

The allocation of large numbers of workers facilitates the swift intake of locally available resources which is essential for ant colony survival. To organise the traffic between nest and food source, the black-meadow ant Formica pratensis establishes permanent trunk trails, which are maintained by the ants. To unravel the ant organisation and potential traffic rules on these trails, we analysed velocity and lane segregation under various densities by experimentally changing feeding regimes. Even under the highest ant densities achieved, we never observed any traffic jams. On the contrary, velocity increased after supplementary feeding despite an enhanced density. Furthermore, inbound ants returning to the nest had a higher velocity than those leaving the colony. Whilst at low and medium density the ants used the centre of the trail, they used the full width of the trail at high density. Outbound ants also showed some degree of lane segregation which contributes to traffic organisation.

Information transfer in head-on encounters between leaf-cutting ant workers: food, trail condition or orientation cues?

Living in large societies involves costs associated with high density of individuals, but being near others includes the benefit of access to conspecifics' information. High densities of workers in ant colonies impose traffic congestion costs on foraging trails. It has been postulated that crowding also increases foraging efficiency by facilitating information transfer between workers in head-on encounters. However, this hypothesis remains untested. Here we assessed, in 24 field nests of the leaf-cutting ant Atta cephalotes, whether head-on encounters between workers facilitate information transfer about trail condition, orientation and food. Several experimental manipulations failed to fit predictions of certain types of communication. (1) Trail disturbance (and thus potential need for information transfer) did not affect the rate of head-on encounters, (2) head-on encounters did not decrease the time required for laden ants to properly orient when entering a trail, and (3) ants that had been experimentally disoriented did not increase the number of head-on encounters when they returned to the trail. Nevertheless, one experiment strongly suggested information acquisition: (4) outbound ants were more likely to find and collect food after a head-on encounter with an ant carrying the same kind of food. These results do not support the hypotheses that workers exchange information about trail condition and orientation in head-on encounters, but suggest that workers acquire food information. The information transferred in head-on encounters could thus increase foraging efficiency under crowded conditions. The cost of the reduced speed due to worker collisions might be outweighed by the benefits of information acquisition, and could explain why leaf-cutting ants do not form distinct lanes of outbound and returning workers. Our results reinforce the key role of information use in the adaptive behaviour of animals and in the maintenance of group living.

Priority rules govern the organization of traffic on foraging trails under crowding conditions in the leaf-cutting ant Atta colombica

Foraging in leaf-cutting ants is generally organized along well-defined recruitment trails supporting a bi-directional flow of outbound and nestbound individuals. This study attempts to reveal the priority rules governing the organization of traffic on these trails. Ants were forced to move on a narrow trail, allowing the passage of only two individuals at a time. In this condition, a desynchronization of inbound and outbound traffic was observed, involving the formation of alternating clusters of inbound and outbound ants. Most clusters of inbound ants were headed by laden ants followed by unladen ants. This occurred because inbound unladen ants did not attempt to overtake the laden ants in front of them. As unladen ants move on average faster than laden ants, these ants were thus forced to decrease their speed. By contrast, this decrease was counterbalanced by the fact that, by staying in a cluster instead of moving in isolation, inbound unladen ants limit the number of head-on encounters with outbound ants. Our analysis shows that the delay induced by these head-on encounters would actually be twice as high as the delay induced by the forced decrease in speed incurred by ants staying in a cluster. The cluster organization also promotes information transfer about the level of food availability by increasing the number of contacts between outbound and inbound laden ants, which could possibly stimulate these former to cut and retrieve leaf fragments when reaching the end of the trail.

Path efficiency of ant foraging trails in an artificial network

In this paper we present an individual-based model describing the foraging behavior of ants moving in an artificial network of tunnels in which several interconnected paths can be used to reach a single food source. Ants lay a trail pheromone while moving in the network and this pheromone acts as a system of mass recruitment that attracts other ants in the network. The rules implemented in the model are based on measures of the decisions taken by ants at tunnel bifurcations during real experiments. The collective choice of the ants is estimated by measuring their probability to take a given path in the network. Overall, we found a good agreement between the results of the simulations and those of the experiments, showing that simple behavioral rules can lead ants to find the shortest paths in the network. The match between the experiments and the model, however, was better for nestbound than for outbound ants. A sensitivity study of the model suggests that the bias observed in the choice of the ants at asymmetrical bifurcations is a key behavior to reproduce the collective choice observed in the experiments.

CAN ANTS PREDICT EARTHQUAKES?

At 5:58 a.m. on the 28th of June 1992, the ground began to tremble in the Mojave Desert, California. John Lighton and Frances Duncan, in the midst of a study on ant locomotion energetics, were sitting near a creosote bush collecting data on their computer when the magnitude 7.4 earthquake struck. It was an impressive event: `our car was bouncing on its springs', Lighton recalls. The startled researchers had the presence of mind to key a time stamp into their computer and continue their study. Lighton and Duncan realised that they had a perfect opportunity to test the anecdotal reports that animals can predict earthquakes, which have crept into the popular press in recent years(p. 3103).Lighton and Duncan had been measuring desert harvester ants' metabolic rate to calculate their cost of transport. Every day at the crack of dawn, as the ants began to forage, the pair laid a compressed-fibre board in the ants' path to separate the ant trail from the CO2 coming from the ground. The ants happily marched through a respirometry chamber nestled on the board, so the researchers could measure the ants' CO2 production. A video trained on the ant trail allowed the pair to count the number of nest-bound and outbound ants and measure each ant's size. Finally, they measured air temperature at `ant height' by placing a thermocouple 3 mm above the ground.With this suite of measurements taken before, during and after the earthquake, Lighton and Duncan could test whether the ants reacted to the multiple quakes, aftershocks and possible precursors on E-day – the day the earthquake struck – relative to other, earthquake-free days. First,they tested anecdotal reports of ant nest evacuations during earthquakes. To detect any mass exodus, they measured the ratio of nest-bound ants to total traffic (nest-bound plus outbound foragers) on E-day and subsequent days. To their surprise, the earthquake and its aftershocks had no effect on ant traffic dynamics. Next, they examined whether ants ran slower or faster during the earthquake than on other days. They found that temperature accounted for 87% of the ants' running speed, and the relationship between running speed and temperature on E-day was identical to the relationship on subsequent earthquake-free days. Reasoning that seismic activity might cause ants to reallocate tasks and keep ants of certain size inside the nest, Lighton and Duncan looked for differences in foraging ants' body size. Again, there was no difference between E-day and other days. In a last-ditch attempt to find some sign that the ants weren't completely oblivious to the shaking earth, they looked for anomalies in metabolic rate on E-day – and failed to find any. `We were convinced that the ants would show some abnormal behaviour, so we were amazed to find that the ants kept slogging on as usual', Lighton says.Lighton and Duncan concluded that ants can't predict – and apparently don't even react to – earthquakes. But their study highlights an important issue: what to do if you realise during data collection that you can answer a different hypothesis than the one you set out to test. To test the untestable, like the claim that animals can predict earthquakes, we have little choice but to rely on serendipitous events. After all, how likely is it that you'll get funding to scrutinize animals while patiently waiting for an earthquake to strike?

Temporal organization of bi-directional traffic in the antLasius niger(L.)

Foraging in ants is generally organized along well-defined trails supporting a bi-directional flow of outbound and nestbound individuals and one can hypothesize that this flow is maximized to ensure a high rate of food return to the nest. In this paper we examine the effect of bottlenecks on the temporal organization of ant flow. In our experiments ants had to cross a bridge to go from their nest to a food source. Two types of bridges were used: one with and one without bottlenecks. Traffic counts show that, in spite of the bottlenecks and the reduction of path width, the volume of traffic and the rate of food return were the same on both bridges. This was due to a change in the temporal organization of the flow: when path width decreases alternating clusters of inbound and outbound ants were observed crossing the bridge. This organization limits the number of head-on encounters and thus allows to maintain the same travel duration as on the wide bridge. A model is proposed to assess in various conditions the importance of the behavioural rules observed at the individual level for the regulation of traffic flow. It highlights how the interplay between the value of the flow and cooperative behaviours governs the formation and size of the clusters observed on the bridge.

Reduction in the foraging activity of the leaf‐cutting ant Atta sexdens caused by the phorid Neodohrniphora sp.

AbstractFemales of the parasitic phorid Neodohrniphora sp. were collected in the field and released singly inside an observation chamber placed between a laboratory colony of Atta sexdens (L.) and its foraging arena. The number and speed of loaded and unloaded ants returning to the nest, the weight of foragers and their loads, the number of leaf fragments abandoned by ants, and the number of small workers ‘hitchhiking’ on leaf fragments were measured before phorids were released, while they were in the observation chamber, and after they were removed. Relatively few ants were attacked by Neodohrniphora sp., but the presence of flies prompted outbound ants to return to the nest and caused a significant reduction on the number and mass of foragers. Additionally, the weight of leaf fragments transported by ants was reduced and the number of abandoned fragments increased in response to Neodohrniphora sp. Presence of the parasitoid caused no significant changes in the number of hitchhiking ants. The regular ants' traffic was resumed after phorids were removed, but foraging activity remained below normal for up to three hours. In the field A. sexdens forages mostly at night, but colonies undergo periods of diurnal foraging during which ants are subject to parasitism from several species of phorid flies. Considering that daytime foraging may be necessary for nutritional or metabolical needs, phorids may have a significant impact on their hosts by altering their foraging behavior regardless of the numerical values of parasitism.

The dynamics of collective exploration and trail‐formation in Monomorium pharaonis: experiments and model

Abstract. When exploring a chemically unmarked area devoid of food sources, workers of the pest ant Monomorium pharaonis L. (Formicidae, Myrmicinae) leave scent marks on the ground and after 30–60min a network of diverging exploratory trails begins to emerge.Exploratory activity is affected by the nutritional state of the colony and a period of food deprivation induces a dramatic increase in the number of workers leaving the nest. A mathematical model based on a logistic growth equation is proposed to describe the exploratory recruitment observed. When travelling along exploratory trails the proportion of ants displaying trail‐laying behaviour is higher for outbound than for nestbound workers. Outbound ants also show a greater propensity than nestbound ants to follow the scent marks of their nestmates. The chemical used to mark a novel area does not appear to be colony‐specific and thus does not have a territorial function sensu stricto. The adaptive value of the collective exploratory behaviour observed in this study is discussed in relation to the common features of other pest ant species described in the literature.