I am very delighted to report the publication of our new paper about ant movement and pheromone trail formation in the latest edition of PLoS Computational Biology. In this article called “Individual Rules for Trail Pattern Formation in Argentine Ants (Linepithema humile)”, we analyse the movement of individual ants in order to understand how the presence of pheromone affect their displacement and ultimately lead them to form routes connecting their nest and various resources in the environment. Many ant species produce large dendritic networks of trails around their nest. These networks result from self-organized feedback mechanisms: ants leave small amounts of a chemical -a pheromone- as they move across space. In turn, they are attracted by this same pheromone so that eventually a trail is formed.
In our study, we introduce a new image analysis technique to estimate the concentrations of pheromone directly on the trails. This technique is illustrated in the following videos:
In both videos, we detected the presence of the ants and then colored the pixels covered by the ants with brighter colors as more ants passed over them. In this way, we can approximate the presence of the pheromone that ants of this particular species tend to lay on the ground almost all the time. The first video shows the process on a small portion of the experimental arena in real time while the second video shows the final result over the whole arena and a period of 60 minutes (here compressed in 15 seconds).
Using this technique, we can characterise the ingredients of the feedback loop that ultimately leads to the formation of trails. We show that the response to pheromone concentrations is linear: an ant will turn to the left with frequency proportional to the difference between the pheromone concentrations on its left and right sides. Such a linear individual response was rejected by previous literature, as it would be incompatible with the results of a large number of experiments: trails can only be reinforced if the ants have a disproportionally higher probability to select the trail with higher pheromone concentration. However, we show that the required non-linearity does not reside in the perceptual response of the ants, but in the noise associated with their movement.
Links and references
2012
- Perna, Andrea; Granovskiy, Boris; Garnier, Simon; Nicolis, Stamatios; Labédan, Marjorie; Theraulaz, Guy; Fourcassié, Vincent; Sumpter, David J.T(2012): Individual rules for trail pattern formation in Argentine ants (Linepithema humile). PLoS Computational Biology, 8, 7, Page(s): e1002592.Abstract | Links & PDF | BibTeX
@article{Perna-2012,
name = {Individual rules for trail pattern formation in Argentine ants (Linepithema humile)},
author = {Perna, Andrea and Granovskiy, Boris and Garnier, Simon and Nicolis, Stamatios and Labédan, Marjorie and Theraulaz, Guy and Fourcassié, Vincent and Sumpter, David J.T},
url = {http://www.ploscompbiol.org/article/fetchObjectAttachment.action?uri=info%3Adoi%2F10.1371%2Fjournal.pcbi.1002592&representation=PDF},
year = {2012},
date = {2012-07-19},
journal = {PLoS Computational Biology},
volume = {8},
number = {7},
pages = {e1002592},
abstract = {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.},
}
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.
