Eciton army ant

Eciton army ant, (Eciton burchellii), species of aggressive army ant found in heavily forested, low-elevation regions of Central and South America. Eciton army ants are key predators and live in large, typically nomadic colonies that move hunting grounds frequently. Given that numerous vertebrate and invertebrate species depend on Eciton colonies for survival, the ants are keystone species in Neotropical forests and disproportionately influence the biodiversity of their communities. The 11 other species of the genus Eciton are also army ants, though E. burchellii is the most visible and well studied of the group.

Colony castes and physical characteristics

Eciton army ants are highly evolved social insects. Like other ants, each Eciton army ant colony has three basic castes: a fertile queen, infertile female workers, and male drones. Eciton army ants are polymorphic, meaning individuals of the different castes and types of workers have different physical characteristics. The minor workers, or minims, are the smallest and tend to the brood and the queen in the nest. They are 3–7 mm (0.12–0.28 inch) long and are dark in color with orange abdomens. The largest workers are the fearsome soldier ants, or majors, which play a defensive role and have a strong bite. They are 10–12 mm (0.39–0.47 inch) long with pale orange heads, dark orange legs, and extraordinarily large mandibles. Media and submajors are of an intermediate size and carry out a variety of tasks. Army ant colonies can have from 100,000 to 2 million individual ants.

Each colony has one large queen that is born without wings and can live several years. A queen mates one or possibly more times with numerous males, many of whom are from other colonies, and then spends the rest of her life devoted to laying eggs; males die shortly after mating. One queen can lay 100,000 eggs during each 20-day fertile period. Upon reaching sexual maturity, new queens born in the colony take a significant portion of the workers with them to start a new colony.

Natural history

Unlike most other ant species, Eciton army ants are nomadic and do not construct permanent nests. They are diurnal and spend their nights resting in temporary nests called bivouacs. Made up of a mass of worker ants, a bivouac is generally in a sheltered location at ground level, such as under fallen trees, though the ants will sometimes create their living nests in trees during periods of heavy rain. The queen and immature ants are protected inside a cylindrical structure formed from the bodies of the worker ants, which use the claws on their front legs to link themselves (a strategy the ants also use to form living bridges). For hunting, the foraging castes leave the bivouac and march en masse as a fanning stream across the shady forest floor, returning each night. During the queen’s egg-laying phase, the colony typically spends a few weeks hunting in the same area before the bivouac is disassembled and moved to a new location. The developing larvae are carried along each time the colony moves. When the new adults have emerged, the colony becomes more nomadic and moves to a new site every day until the queen begins laying eggs again.

Eciton army ants have limited vision and largely use touch and pheromones for communication. They use pheromones released from the tips of their abdomens to create chemical trails for the horde to follow when marching to a new bivouac site or during hunting raids. Alarm pheromones are released when an ant’s head is crushed, such as when the colony is under attack, and serve as a defense mechanism to attract soldiers and other workers to the site.

Eciton army ants also release pheromones when they open their mandibles to bite prey, attracting swarms of thousands of their sisters to help bite, sting, and tear apart the conquest. The ants are predatory and carnivorous, with a varied diet that can include cockroaches, crickets, katydids, tarantulas, and scorpions, as well as such vertebrate animals as frogs, lizards, and small birds. In the wet season, they also target broods of smaller ant species and wasp nests. Given that army ants are capable of consuming nearly all animal life in their path, their nomadic lifestyle gives them steady access to new hunting grounds. When faced with a raiding army ant horde, insects and other small animals must flee or be overwhelmed and consumed. Only a few invertebrates—such as termites in their protected mounds, certain ticks and stick insects with chemical defenses, and thick-shelled beetles or spiny caterpillars—can survive a roving swarm. Food is brought back to the bivouac to be shared with all colony members.

Ecological importance, threats, and conservation

More than 300 species of animals depend—at least in part—on the Eciton army ant for food. Although several passerine birds eat army ants directly and certain mites use the ants as their hosts, most of the associated animals rely on Eciton army ants indirectly. For example, nearly 30 bird species, particularly antbirds, prey exclusively on insect species attempting to move out of the path of an army ant swarm, a food source also used by lizards, toads, and even fish when the opportunity arises. Similarly, a number of parasitoid flies exploit the chaos of a horde on the move to lay their eggs on fleeing insects that would otherwise fend off such attacks. Some unusual ant butterflies feed only on the feces of birds that follow Eciton hordes. One species of rove beetle (Ecitophya simulans) has such advanced physical, chemical, and behavioral mimicry that it can live undetected among Eciton army ants and exploit their food and larvae. The extinction of Eciton army ants, even at a local level, would have broad and rippling effects throughout many forest animal communities.

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Although Eciton army ants have long been protected by the dense and often isolated forests in which they reside, deforestation and habitat fragmentation in the late 20th and early 21st centuries have begun to put this species at risk. Studies suggest that a single Eciton army ant colony requires 30 hectares (74 acres) of shady, densely forested land during its lifetime, and such human activity reduces the contiguous area available to foraging colonies. Suggestions to preserve and sustain Eciton army ant colonies include the conservation of wild lands, the creation of protected forest corridors to connect fragmented forests across plantations and other cleared lands, and economic incentives for shade-grown coffee and other agroforestry endeavors.