Modeling Army Ants

Introduction
Modeling work
Data Collection
Download Models
References

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Introduction:

Mathematical and computer models allow researchers to manipulate systems and ask questions in ways that would be difficult or impossible to do in the field. Building a model requires you to create a precise description of the processes that are going on in the system which in turn helps you to realize very quickly what you do and don't know about a system. My ongoing research is focused on building an individual-based model of swarming behavior in the army ant Eciton burchelli.

Nearly 70 years of field work on Eciton has greatly advanced our understanding of the life-history and biology of army ants (Schneirla, 1970, Franks, 1980, Gotwald, 1995, Rettenmeyer, 1963). But no one has yet quantitatively measured the behavioral rules through which Eciton burchelli army ants organize their raiding behavior.

Only two individual-based models of army ant swarm behavior have been created (Deneubourg et al., 1989; Sole, et al. 2000). Both of these models were generated to examine emergent patterns in army ant swarms and as such only incorporated very basic army ant biology. The Deneubourg et al. (1989) Sole demonstrated that raiding patterns which were visually similar to characteristic swarm patterns of existing army ant species, could be generated through variations in resource distribution, without changes to the behavioral parameter values. These results suggest that differing patterns of prey distribution in the wild may play an important role in generating species-specific differences in raiding patterns. Sole et al. (2000) re-analyzed the Deneubourg et al. model using a simplified genetic algorithm approach to find optimal parameter values which maximized fitness under different food distributions. Their work also indicated that resource distribution may strongly influence raiding patterns, but their results yielded very different parameter values from those chosen by Deneubourg et al. (more information).

Field tests by Franks et al. (1991) had moderate success modifying E. burchelli foraging patterns in the field by changing resource distribution. However, other than these experiments, the Deneubourg et al. model results have only been qualitatively correlated with real army ant behavior. Army ant foraging behavior in the field has not yet been described in enough quantitative detail to permit rigorous testing of model results.

To examine the behavioral rules involved in the organization of E. burchelli swarms, I am creating a detailed individual-based computer simulation of swarm behavior based on parameters determined through my field work and from the literature. Predictions of the model will be tested in the field to verify the accuracy of the model and the modeling work will inform and shape the direction of my field work and allow me to investigate questions that are not easily addressed in the field. (more information)

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The Model:

In the Spring of 2002 I created an initial model reproducing the work of Deneubourg et al. (1989) and Solé et al. (2000). I am now extending the model to include more realistic descriptions of army ant behavior. Data is currently being collected on ant search behavior, turn-angle, velocity and ant-density from digital video of swarms taken in the field at Corcovado, Costa Rica. Data I generate through video analysis will be used to develop a more realistic ant movement algorithm which integrates the effects on turning behavior of both pheromone and density. The model will also include improved spatial and temporal scales and incorporate resource distribution and capture success data from my field work and from the literature (Franks, 1985, Franks, 1982, Franks and Bossert, 1983, Otis et al. 1986, Rettenmeyer, 1963, Rettenmeyer et al., 1983). Although the Solé et al. model (2000) contained a measure of fitness (number of prey items per ant per unit time), actual measurements of energy use in army ants have been made (Bartholomew et al., 1988, Feener et al. 1988a,b) and will be included in the new model to permit a more accurate measurement of fitness.

The data I collected at La Selva in 2001 on swarm movement, trunk trail composition and foraging success will be used to ground-truth the model. Model results will also be compared with more qualitative descriptions of distinct swarm behaviors recorded throughout my three field seasons and in the literature. With the inclusion of energetics as a measure of fitness, it will be possible to measure rigorously the impact of variations in parameter values against foraging success and real-world data on swarm behavior.

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Data Collection and Analysis:

Digital videos of E. burchelli swarms taken in Corcovado are being analyzed using a software system I developed that permits manual tracking of individual ants in video clips and the measurement of density.

Density and pheromone concentration: The head and abdomen positions of every ant in a given frame are being recorded for every 100th frame in each video clip. This enables measurement of the distribution of size-classes and density of ants, and their angle relative to each other and to other ants that have passed through that area. Pheromone is also being measured by assuming that number of ants passing a given point provides a reasonable metric for pheromone concentration in that area.

Ant trajectories: At standardized time-points in each clip, a representative selection of individual ants are being tracked as long as they are present in the area videotaped. From these data, average turn angle and velocity can be measured. The effects of density and pheromone concentration on turn angle and velocity can then be calculated.

References:

Bartholomew, G.A., J.R.B. Lighton, and D.H. Feener, Jr. 1988. Energetics of trail-running, load carriage, and emigration in the column-raiding army ant eciton hamatum. Physiol Zool. 61: p. 57-68.
Deneubourg, J. L., Goss, S., Franks, N., & Pasteels, J. M. (1989). The blind leading the blind: modeling chemically mediated army ant raid patterns. J. Insect Behav., 2, 719-725.
Feener, D.H., Jr., J.R.B. Lighton, and G.A. Bartholomew. 1988a. Curvilinear allometry, energetics and foraging ecology: A comparison of leaf-cutting ants and army ants. Functional Ecology. 2: p. 509-520.
Feener, D.H., Jr., J.R.B. Lighton, and G.A. Bartholomew. 1988b. Energetic costs of foraging: A comparison of leaf-cutting ants and army ants (hymenoptera: Formicidae). Proceedings of the 18th International Congress of Entomology, p. 233
Franks, N. R. 1980. The evolutionary ecology of the army ant Eciton burchelli on Barros Colorado Island, Panama. Ph.D. dissert., The University of Leeds, Leeds, England.
Franks, N.R. 1982. Social insects in the aftermath of swarm raids of the army ant eciton burchelli [ethology, escape and foraging behaviors], in The biology of social insects: Proceedings, ninth congress, international union for the study of social insects, boulder, colorado, august 1982, H.E. Evans, Editor. Westview Press: Boulder. p. 275-279.
Franks, N.R.. 1985. Reproduction, foraging efficiency and worker polymorphism in army ants, in Experimental behavioral ecology and sociobiology: In memoriam karl von frisch,1886-1982, M. Lindauer, Editor. Sinauer Associates: Sunderland, Mass. p. 91-107.
Franks, N.R. and W.H. Bossert. 1983. The influence of swarm raiding army ants on the patchiness and diversity of a tropical leaf litter ant community, in Tropical rain forest: Ecology and management. Special publication of the british ecological society no. 2, A.C. Chadwick, Editor. 1983, Blackwell: Oxford. xii + 498 p. p. 151-163.Franks, N. R. 1989. Army ants: a collective intelligence. Am. Scient., 77, 139-145.
Gotwald, W. H., Jr. 1995. Army ants: the biology of social predation. Ithaca, New York: Cornell University Press.
Hölldobler, B., & Wilson, E. O. 1990. The ants. Cambridge, Mass.: Harvard University Press.
Otis, G.W., et al. 1986. The effect of foraging army ants on leaf-litter arthropods. Biotropica, 1986. 18: p. 56-61.
Rettenmeyer, C. W. 1963. Behavioral studies of army ants. Univ. Kans. Sci. Bull., 44, 281-465.
Rettenmeyer, C.W., et al. 1983. Comparative foraging by neotropical army ants, in Social insects in the tropics. Volume 2, P. Jaisson, Editor. Université Paris-Nord: Paris. 252 p. p. 59-73.
Schneirla, T. C. 1971. Army ants. A study in social organization. (Edited by H. R. Topoff.). San Francisco: W. H. Freeman & Co.
Sole, R. V., Bonabeau, E., Delgado, J., Fernandez, P., & Marin, J. 2000. Pattern formation and optimization in army ant raids. Artificial Life, 6(3), 219-226.
Swartz, M. B. 1997. Behavioral and population ecology of the army ant Eciton burchelli and ant-following birds. Unpublished Ph.D. Dissertation. University of Texas, Austin.
Topoff, H. 1984. Social organization of raiding and emigrations in army ants. Advanced Studies in Behavior 14: p. 81-126.
Watmough, J., & Edelstein-Keshet, L. (1995). A one-dimensional model of trail propagation by army ants. J. Math. Biol., 33, 459-476.