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ABSTRACT Dynamics of the restoration of physical trails in the grass-cutting ant Atta capiguara. Leaf-cutting ants of the genus Atta build long physical trails by cutting the vegetation growing on the soil surface and removing the small objects they find across their path. Little is known on the dynamics of trail construction in these ants. How much time do they need to build a trail? To answer this question we selected six trails belonging to two different nests of A. capiguara and removed on each trail a block of soil of 20 cm × 15 cm that included a portion of the physical trail. This block was then replaced by a new block of the same size that was removed in the pasture near the trail and that was uniformly covered by the same type of vegetation as that found on the block of soil that was removed. The time required to restore the trail was then evaluated by the length of the grass blades found along the former location of the trail. The results show that ants rapidly restore the portion of the physical trail that was interrupted, which suggests that they could also do the same after their trails have been recolonized by the vegetation.

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Foraging networks are a key element for ant colonies because they facilitate the flow of resources from the environment to the nest and they allow the sharing of information among individuals. Here we report the results of an 8-month survey, extending from November 2009 to June 2010, of the foraging networks of four mature colonies of Atta bisphaerica, a species of grass-cutting ant which is considered as a pest in Brazil. We found that the distribution of foraging effort was strongly influenced by the landscape features around the nests, in particular by the permanently wet parts of the pasture in which the nests were located. The foraging networks consisted of underground tunnels which opened on average at 21.5m from the nests and of above-ground physical trails that reached on average 4.70m in length. The use of the foraging networks was highly dynamic, with few sections of the networks used for long periods of time. Three different phases, which could be linked to the seasonal change in the local rainfall regime, could be identified in the construction and use of the foraging networks. The first phase corresponded to the beginning of the rainy season and was characterized by a low foraging activity, as well as a low excavation and physical trail construction effort. The second phase, which began in February and extended up to the end of the humid season at the end of March, was characterized by an intense excavation and trail construction effort, resulting in an expansion of the foraging networks. Finally, in the third phase, which corresponded to the beginning of the dry season, the excavation and trail construction effort leveled off or decreased while foraging activity kept increasing. Our hypothesis is that ants could benefit from the underground tunnels and physical trails built during the humid season to maintain their foraging activity at a high level.

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We studied the formation of trail patterns by Argentine ants exploring an empty arena. Using a novel imaging and analysis technique we estimated pheromone concentrations at all spatial positions in the experimental arena and at different times. Then we derived the response function of individual ants to pheromone concentrations by looking at correlations between concentrations and changes in speed or direction of the ants. Ants were found to turn in response to local pheromone concentrations, while their speed was largely unaffected by these concentrations. Ants did not integrate pheromone concentrations over time, with the concentration of pheromone in a 1 cm radius in front of the ant determining the turning angle. The response to pheromone was found to follow a Weber's Law, such that the difference between quantities of pheromone on the two sides of the ant divided by their sum determines the magnitude of the turning angle. This proportional response is in apparent contradiction with the well-established non-linear choice function used in the literature to model the results of binary bridge experiments in ant colonies (Deneubourg et al. 1990). However, agent based simulations implementing the Weber's Law response function led to the formation of trails and reproduced results reported in the literature. We show analytically that a sigmoidal response, analogous to that in the classical Deneubourg model for collective decision making, can be derived from the individual Weber-type response to pheromone concentrations that we have established in our experiments when directional noise around the preferred direction of movement of the ants is assumed.

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Operárias de Formica rufa utilizam estimulação química e visual para se orientar espacialmente. Objetivou-se verificar a contribuição dessas modalidades sensoriais em situações experimentais nas quais um tipo de estimulação pertinente era retirado ou modifi cado. Na primeira experiência, que serviu de comparação para as próximas, as formigas foram liberadas em um ambiente inalterado com relação à situação familiar. Na segunda experiência, uma cortina foi colocada entre o dispositivo experimental e a estimulação visual da sala. Na terceira, todo o dispositivo sofreu rotação de 180°. Cada situação dessas foi seguida por uma semelhante em todos os parâmetros, exceto pelo da pista química que foi interrompida em determinado ponto. Concluiu-se que as operárias puderam se orientar corretamente utilizando apenas uma das modalidades; que não utilizaram informações visuais situadas no teto e que aprenderam a se deslocar seguindo orientação contextualizada: em direção ao alimento ou em direção ao ninho.

Formica rufa worker ants use chemical and visual stimulation for spatial orientation. This work aimed to verify the contribution of these sensorial modalities in experimental situations in which one kind of pertinent stimulation was removed or modified. In the first experience, which served as comparison to the next ones, the ants were released in an unchanged environment concerning to the familiar situation. In the second experience, a curtain was placed between the experimental apparatus and the visual stimulation in the room. In the third experience, the whole apparatus was rotated 180 degrees. Each of these experiences was followed by one similar experience with respect to all parameters, except the chemical clue, which was discontinued in a certain point. We conclude that (1) the worker ants could orientate correctly using only one of the sensorial modalities; (2) they did not use visual information from the ceiling, and (3) they have learned to move using contextualized orientation: towards food or towards the nest.

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Operárias de Formica rufa utilizam estimulação química e visual para se orientar espacialmente. Objetivou-se verificar a contribuição dessas modalidades sensoriais em situações experimentais nas quais um tipo de estimulação pertinente era retirado ou modifi cado. Na primeira experiência, que serviu de comparação para as próximas, as formigas foram liberadas em um ambiente inalterado com relação à situação familiar. Na segunda experiência, uma cortina foi colocada entre o dispositivo experimental e a estimulação visual da sala. Na terceira, todo o dispositivo sofreu rotação de 180°. Cada situação dessas foi seguida por uma semelhante em todos os parâmetros, exceto pelo da pista química que foi interrompida em determinado ponto. Concluiu-se que as operárias puderam se orientar corretamente utilizando apenas uma das modalidades; que não utilizaram informações visuais situadas no teto e que aprenderam a se deslocar seguindo orientação contextualizada: em direção ao alimento ou em direção ao ninho.(AU)

Formica rufa worker ants use chemical and visual stimulation for spatial orientation. This work aimed to verify the contribution of these sensorial modalities in experimental situations in which one kind of pertinent stimulation was removed or modified. In the first experience, which served as comparison to the next ones, the ants were released in an unchanged environment concerning to the familiar situation. In the second experience, a curtain was placed between the experimental apparatus and the visual stimulation in the room. In the third experience, the whole apparatus was rotated 180 degrees. Each of these experiences was followed by one similar experience with respect to all parameters, except the chemical clue, which was discontinued in a certain point. We conclude that (1) the worker ants could orientate correctly using only one of the sensorial modalities; (2) they did not use visual information from the ceiling, and (3) they have learned to move using contextualized orientation: towards food or towards the nest.(AU)

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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.

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