Many of us, at one time or another, will have paused to admire ants scurrying up and down the cracks between our patio slabs or among the blades of grass in our lawns, working efficiently in their co-operative societies to build nests, retrieve food and feed their young. In the UK, the species you’re most likely to encounter is the common black garden ant, which lives in colonies of up to a few thousand individuals.
But further afield, in rainforests around the world, there are ants that take co-operation and efficiency to another level. They move as a single giant battalion, overwhelm prey animals many times their own size, and create ‘buildings’ out of their own bodies. These are the army ants, whose spectacular behavior] and biology has inspired the writings of early explorer-naturalists as well as novels and horror films.
The name ‘army ant’ refers to a group of ants that comprises some 400 species, most of which occur in tropical Africa, Asia, and Central and South America. I have studied these ants for the past 17 years, following their colonies for weeks at a time and enduring their bites and stings – and their fascinating adaptations never cease to amaze me.
An army ant hunting swarm is one of the greatest spectacles of the natural world. Many times, during fieldwork in the tropical rainforests of Costa Rica, Kenya, and Venezuela, I have been startled to see a patch of rainforest floor up to 20m wide suddenly come alive with a surging flood of hundreds of thousands of ants, streaming across the leaf-litter and up into the vegetation in their search for prey.
When one of these tiny warriors encounters a suitable meal – perhaps a katydid or earthworm – it emits a chemical signal that attracts an entire platoon. The ants are ruthless in their efficiency, quickly overwhelming their target, pinning it down, and neatly severing it into small pieces suitable for transport. In just one day’s raid, they will capture tens of thousands of arthropods and other invertebrates.
Army ants are also known as ‘driver’ ants because their swarms drive out a wave of fleeing animals. Even if a fugitive escape the intruders, it is nowhere close to safety. Dozens of birds gather at army ant swarms, ready to snack on the escapees. Some species are so specialized that they keep track of the various colonies in a particular region of forest, feeding almost exclusively at the incursions.
Alongside the birds are scores of flies. Some hover above the advancing front like miniature helicopters; others perch on twigs and leaves, darting down to a fleeing arthropod when the opportunity arises. These flies are parasitic, and their maggots develop inside the bodies of insects and spiders. It has been estimated that more than half of the animals that escape are ultimately killed by being eaten from the inside.
Army ant societies are among the largest on the planet, with estimates of 20 million individuals for some African species. One I have got to know particularly well is Eciton burchellii, which lives in colonies of approximately 500,000 and inhabits tropical and subtropical forests from Mexico to southern Brazil. This species has a broad prey spectrum that includes katydids, cockroaches, centipedes and scorpions, and many other large arthropods.
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