Structure and Regulation of Army Ant Behavior
in SE-Asian Ponerines of the Genus Leptogenys

 

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My Ph.D. thesis is available in digitalized form here.





 

What is army ant behavior?

Many ant species are able to exploit rich food sources collectively by use of communication. There are many different ways how ants can transmit information; whenever nestmates are led to a certain place where work is required, this phenomenon is called recruitment (Wilson, 1971). Non-army ants may start recruitment after a scouting ant located a suitable food source. In contrast, army ants start food communication before prey contact. The result is a collective search pattern for (in most cases strictly carnivorous) food: a raid-formation. Such raids can expand over more than hundred meters and include more than a million ants e.g. in Dorylinae (the ant to the left).

The second characteristic of army ants is a frequent nest relocation. Temporary nests of these ants are referred to as bivouacs. Some species emigrate almost daily, others have nomadic and stationary phases, which are synchronized with the cycle of brood development (Schneirla, 1971).

Both behavioral patterns, group-raiding and nomadism, are not limited to army ants, but only here they are as tightly linked (Wilson, 1958). The characteristic combination of mass predation and nomadism was also named "the army ant adaptive syndrom" (Gotwald, 1982). In this sense, the term "army ant" has a functional meaning, describing a certain ecotype of ants. In the genus Leptogenys, which I studied, a variety of hunting strategies are realized, including true army ant behavior.

 

Why study army ants?

One of the most fascinating traits of social insects is their highly structured and organized behavior. It is of general interest, how complex structures arise through interactions of individual units, not only in social insects and not only in biology. Self-organization is a field of interdisciplinary interest, including physiology, neuroscience, ecology, evolution, economy, social sciences, mathematics, and computer sciences especially artificial intelligence (Bonabeau, 1998; Dooley, 1996; Gell-Mann, 1994). Army ants represent an impressive example of complex animal behavior, since there is no central controlling instance coordinating their raids and emigrations. "Intelligent" behavior forms out of the interactions of hundreds or thousands of individuals, each with very limited capabilities. In this respect, studying army ant behavior can be regarded as a study of the self-organization of a complex adaptive system.

A second fact should not be overlooked: Ants play very important roles in ecosystems all over the world. Many tropical biotopes, where army ants are mainly distributed, are dominated by arthropods which represent the main diet of army ants. In some tropical habitats ants can surpass vertebrates not only in number, but even in biomass.

 

Summarized results

I studied the coordination of collective behavior in seven species of the genus Leptogenys, based on the communication between the individuals of a colony. Influences of environmental factors were also considered, so that the coordination of behavior could be fully understood in the light of a species ecology.

 

The communication systems:

In all Leptogenys-species several pheromones, located in the pygidial and the poison gland, were involved in trail communication. In six species with army ant behavior (L. distinguenda, L. mutabilis, L. borneensis, L. myops, L. sp. 2 and L. sp. 3), pygidial gland components functioned in the initial forming of foraging formations in the absence of prey. These pheromones were also responsible for swarm cohesion during its further progress. "Slow" recruitments, which played a role in the exploration of new terrain, are coordinated with pygidial gland contents through summation of pheromone concentrations in the trails, supported by a medium-termed persistence. Furthermore, the ants used these substances as orientation cues to locate their nest. Furthermore, emigrations were guided exclusively by pygidial gland secretions (see below).

In species with army ant behavior, poison gland contents released intense prey recruitment and induced aggressiveness. Quick attacks were coordinated with signals of this gland, enabling the ants to overwhelm even large and mobile prey objects. The recruitment trails by these pheromones could sum up and become strong enough to influence the spatial development of entire raid formations. Furthermore, raid swarms reached a considerable flexibility to switch to new environmental conditions due to the extraordinary short persistence of this excitement pheromone. A second compound of the poison gland played an additional role in the cohesion of the swarm. This pheromone probably provides an extra orientation cue during and after prey-attack, when the pygidial gland might not find use.

In L. diminuta, a species not showing army ant behavior, the same pheromone glands were used in a different way. Pygidial gland contents were used in orientation and prey recruitment. The poison gland, however, had its primarily function in the orientation of scout ants. A second component released excitement in raid-groups and is assumed to coordinate the synchronized attack of prey.

In addition to chemical trails, which are by far the most important orientation cues in army ants, optical cues played a minor role in the orientation of L. distinguenda, although this species is strictly nocturnal. Within the extensive network of trails, individual ants integrate optical information to distinguish mayor directions.

 

The role of individual behavior:

In L. distinguenda, the most comprehensively studied species, individuals did not react equally to identical stimuli. Individual motivation was crucial for the behavioral response observed. The motivation itself depended not only on the age of workers, which led to temporal polyethism, but also on the current status of the colony like nutrition, humidity, temperature etc. as well as the actual task performed by an individual. Workers on their way to the swarm front, for instance, were highly sensitive to poison gland extracts if the colony was starved. This pheromone leads ants which are motivated to hunt to the actual raiding front. On the contrary, the same pheromone is ignored by ants retrieving booty to the nest. These ants reacted concentration-dependent to pygidial gland extracts, which guides them safely to the nestsite. Finally, workers participating in emigrations did not ignore rather than avoid poison gland extracts. This behavior enables the strict separation of raiding and emigration activities, both of which are often performed synchronously in this ant species. If, however, a colony was overfed, the reactions on pheromones were generally low and no raid formations were formed at all.

Different ways of orientation on individual level were also found in L. diminuta, a member of the genus displaying scout-induced group raiding. Only high concentrations of both poison and pygidial gland extracts led entire raid-groups safely to a prey object. However, when the trails were low concentrated (or aged), they were followed only by the scouts, which took over the leadership of the group under these conditions. On their way back to the nest, all ants followed all trail concentrations equally.

 

Pheromone properties and ecology:

Physical properties of pheromone components were also investigated and discussed in an ecological context. Comparing different species revealed that physical differences of components correlated with differences in ecology. Without exception, the persistences of pheromone trails were short in species with army ant behavior (poison max. 5 min and pygidial max. 30 min). The high dynamics of raiding formations typical for these ants are clearly based on the use of volatile communication substances. On the contrary, trails of L. diminuta persist considerably longer (poison max. 24 h and pygidial max. 1,5 h). Singly foraging scouts are able to orientate on these stable trails during long lasting foraging excursions.

 

Self-organized swarming:

Some of the ecological differences are not correlated to different pheromone properties. The structure of raiding formations e.g. is highly dependent on the type of prey and its distribution in space. Species which are partly or completely specialized in locally concentrated food resources like social insect colonies (e.g. L. mutabilis), forage preferably in columns in contrast to swarm hunters, which accept a wide range of arthropod prey or even small vertebrates (e.g. L. distinguenda). In this respect, the spatial development of army ant raids is self-organizing due to interactions between individual ants on the one hand and their environment on the one hand. An army ant colony can be viewed as a complex adaptive system with regard to its adaptation to outer influences, its self-organizing structure and the ability to change system properties through evolution. I visualized the raiding and emigration behavior of army ants as a complex system in form of a schematic illustration. Furthermore, I modeled a possible evolutionary scenario of the main communication systems and the corresponding predatory behavior in the genus Leptogenys. This approach contributes to hypotheses existing in literature.

 

Characteristics of army ant behavior:

By use of a theoretical model, I studied the role of different pheromone components for the recruitment efficiency of L. distinguenda. In this model, a communication-system of at least two pheromone signals meets the requirement for a trail system like it is found in army ants best. Such a system includes not only a highly efficient recruitment but also a remarkable flexibility to switch behavior according to changing conditions. Other models of swarming systems in ants based on one single pheromone cannot satisfactorily explain this phenomenon.

In a comprehensive survey including numerous ant species, I established basic principles, which are important for the coordination of army ant behavior. Some behavioral patterns, which are displayed by army ants, are also found in ants of other ecotypes and only the combination of all patterns is unique to army ants. The classical definition of the army ant syndrome is not completely unambiguous, resulting in a confusion in the literature with ant species, which do not show true army ant behavior. For this reason I suggest a modified and unequivocal definition: The expressions "group-predation" or "group-raiding" should not be used anymore for army ant like hunting strategies, rather than exclusively for scout-induced foraging. If raiding behavior of army ants is consequently referred to as "mass-predation" or "mass-raiding", any misunderstanding is excluded. Furthermore, these definitions refer better to the communication systems realized in each group and consequently transfer the expressions "group-recruitment" and "mass-recruitment" to the corresponding foraging behavior.

 

Ant-guests:

I also took the interesting biology of myrmecophiles into consideration, which are extraordinarily abundant in army ants, by examining their integration into the host colonies. This was done with special regard to the chemical communication. In L. distinguenda, part of the guest fauna is able to detect pheromone trails of their hosts and follow emigrations actively (springtails, a bristletail, a spider, staphylinid beetles, phorid flies). Other species are not able to detect any pheromone trails but they nevertheless participate in emigrations, riding on the brood which is carried by workers to the new nest (Acari, a woodlouse, nitidulid beetles). Regularly, part of these myrmecophiles stay behind when they miss an emigration of their host colony.

 

Literature:

Bonabeau, E., 1998. Social insect colonies as complex adaptive systems. Ecosystems 1: 437-443.

Dooley, K., 1996. A nominal definition of complex adaptive systems. The Chaos Network 8: 2-3.

Gell-Mann, M., 1994. The Quark and the Jaguar. Freeman & Co, New York, NY. 392 pp.

Gotwald, W. H. J., 1982. Army Ants. In: Social insects (H. R. Hermann, Ed.), Academic Press, New York. pp. 157-254.

Schneirla, T. C., 1971. Army Ants: A Study in Social Organization. W. H. Freeman, San Francisco. 349 pp.

Wilson, E. O., 1958. The beginnings of nomadic and group-predatory behavior in the ponerine ants. Evolution 12: 24-31.

Wilson, E. O., 1971. The Insect Societies. The Belknap Press of Harvard University Press, Cambridge, Mass. 548 pp.

 

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