The Army Ant Entourage

Think, for a moment, about the animals involved with human civilization.

For food we keep chickens, cows, goats, pigs, turkeys, sheep, bees, and others. Our pets include cats, dogs, ferrets, goldfish, parakeets, and many more. Then there are the pests that live off us, like bed bugs and lice, and off our crops, like the bollworms, rootworms, aphids, and armyworms. And don’t forget the scores of animals that shadow our homes and cities, taking advantage of byproducts of civilization. These are the rats, cockroaches, mice, and pigeons.

The sheer number of animals that have insinuated themselves into our societies is a testament to our enormous global footprint. We must have hundreds of species thriving in our wake. The size of the entourage can be viewed, perhaps, as a measure of our dominance.

You might think that we humans are unique in this manner. But we’ve got at least one rival: the swarm-raiding army ant Eciton burchellii.

The late army ant biologist Carl Rettenmeyer (1931-2009) spent much of his career documenting the associates of this tropical species, found in forests from Mexico to Argentina. A compendium of Rettenmeyer’s studies has just been published by Insectes Sociaux, and the species count is astounding:

The 557 recorded associates range from birds to insects and mites and comprise the largest described animal association centering around one particular species.

At least 300 of the associates are obligate, solely dependent on army ants for their survival.

Army ant colonies travel with a veritable bestiary. There are beetles, bristletails, and silverfish that run in the columns, feasting on leftovers. There are mites that ride on the ants- and estimated 20,000 per colony. There are birds and parasitic flies that live off the scrambling mass of arthropods spooked up by ant raids. There are scores of arthropods living in the refuse heaps. There are insects that prey on the ants themselves.

As a measure of the significance of army ants in tropical forests, it’s hard to beat a pages-long account of species that have evolved to live with Eciton burchellii. In some respects, the swarm-raiding army ants are like humans in the pressures they exert on their environment. For their associates as for ours, adapting to benefit from a dominant species is a good long-term survival strategy.

update: here’s a summary of the porportions of army ant associates by major taxonomic group

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source: Rettenmeyer, C. W., Rettenmeyer, M. E., Joseph, J., Berghoff, S. M. 2010. The largest animal association centered on one species: the army ant Eciton burchellii and its more than 300 associates. Insectes Sociaux online early DOI: 10.1007/s00040-010-0128-8.