BACKGROUND

A massive swarm-raid by an army ant colony is one of the most impressive behaviors by social insects. Thousands of ants advance in a swarm, scouring the forest for food. Foraging workers carrying off small insects and gang up on and overwhelm larger animals by sheer numbers. Many insects and other animals flee ahead of the advancing swarm. Workers use their bodies to create living bridges to overcome obstacles, and at night, form a living shelter for the queen and brood.

As these huge colonies move through the forest, they are often accompanied by a community of other species, including birds, other insects, lizards, and even mammals (Schneirla 1971). But what exactly is the nature of the relationship between the ants and their followers? Do they help or hinder the ants?

"Army ant" is both a taxonomic designation and a description of a lifestyle. Most army ants are in one of two genera: Dorylus (paleotropics) and Eciton (neotropics). The army ant lifestyle includes nomadism (usually the whole colony moves), group predation (raids), and, often, above-ground nesting. Although ants in other genera have these behaviors, the extent to which they are developed and combined is unique to the "true" army ants (Gotwald 1995).

An army ant colony typically cycles between two behavioral phases: nomadic and statary (Schneirla 1971). During the nomadic phase, a colony moves daily, housing the queen and brood in a temporary bivouac, and conducts large foraging raids. During the statary phase, the colony stays in one place for several weeks and conducts small-scale raids during the day. The main prey of a raiding army ant colony are leaf-litter invertebrates and the brood of other social hymenoptera (ants, bees, and wasps). Workers usually aggregate on prey, collectively subduing and dismembering it (Gotwald 1995). As a result, army ants can capture prey many times larger than an individual worker. Because army ants will attack almost anything, many animals flee the oncoming swarm. After an army ant swarm raid has passed, arthropod densities in the leaf litter may be reduced by as much as 50% (Gotwald 1995). Because of their large colony sizes (up to 1.5 x 10⁶ ants/colony) and mobility (raids cover >1000 m² of forest per day), army ants have a significant impact on tropical forest communities.

Army ants also have indirect effects by supporting a community of ant followers, especially birds. In the New World topics, over 50 bird species regularly follow army ant raids, many of them classed as "professionals" in that they get >50% of their food at army ant swarms. Common ant-following birds include members of the antbird family (Formicariidae) as well as woodcreepers, cuckoos, and tanagers. The number of birds following a particular ant swarm varies considerably from day to day, but large colonies of regularly-swarming army ants are frequently accompanied by flocks of over 20 individuals of several bird species (Willis and Oniki 1978).

Ant-following birds stay just ahead of the advancing army ant swarm or perch on branches just above it, capturing animals, mostly larger arthropods and small vertebrates, disturbed by and fleeing from the army ant raid. Although they may occasionally incidentally consume army ant workers that are attached to something else they are eating, studies reveal that the birds do not deliberately eat army ant workers (Willis and Oniki 1978). By getting food that they otherwise would not, the birds clearly benefit from the association. Early studies suggested that the birds might flush prey back to the ants, making the relationship mutualistic (Willis and Oniki 1978). However, later studies do not specifically describe the nature of the interaction, and it is possible that birds remove prey that the army ants would otherwise capture (Wrege et al. 2005). The balance between mutualism, commensalism, and parasitism is often a fine one, and determining the exact nature of this type of interspecific relationship often takes careful measurement of the costs and benefits to each species.

References

• Gotwald, W. H., Jr. 1995. Army Ants: The Biology of Social Predation. Cornell University Press, Ithaca, NY.
• Schneirla, T. C. 1971. Army Ants: a Study of Social Organization. W. H. Freeman and Company, San Francisco, CA.
• Willis, E. O., and Y. Oniki. 1978. Birds and army ants. Annual Review of Ecology and Systematics 9:243-263.
• Wrege, P. H., M. Wikelski, J. T. Mandel, T. Rassweiler, and I. D. Couzin. 2005. Antbirds parasitize foraging army ants. Ecology 86:555-559.