Army ants are a captivating group of insects, renowned for their predatory behaviors and unique nomadic lifestyle. These highly organized social creatures move across landscapes in vast, coordinated swarms, impacting other insect populations and the broader environment. Understanding these ants reveals how cooperation and sheer numbers create a powerful force.
Unique Characteristics
Army ants encompass over 200 species, primarily classified within the subfamily Dorylinae. A distinguishing trait is their obligate nomadism; unlike most ant species, they do not construct permanent nests. Their colonies are constantly in motion, driven by their immense size and predatory habits.
Colonies can range from hundreds of thousands to tens of millions of individuals, all descending from a single queen. This large population enables their predatory nature, where they hunt cooperatively in massive foraging groups, overwhelming prey. Their workers are often blind or have reduced eyes, relying heavily on chemical signals and touch to navigate and coordinate.
Nomadic Lifestyle and Raids
The life of an army ant colony cycles through two distinct phases: nomadic and statary. During the nomadic phase, which lasts about 15 days, the colony moves almost daily, driven by the need to feed their rapidly developing larvae. Each night, they form a temporary nest called a “bivouac,” constructed entirely from the ants’ own bodies, protecting the queen and brood. These living structures can house hundreds of thousands of workers linking together to form a protective ball.
The nomadic phase is also defined by their foraging raids. Thousands, or even millions, of ants stream out of the bivouac in coordinated hunting swarms or columns. These raids can cover large areas, with ants overwhelming and immobilizing prey using their numbers and stings. They capture various arthropods, breaking down large prey into smaller pieces for transport back to the bivouac. After about 15 days, the larvae pupate, signaling the start of the statary phase.
The statary phase, lasting two to three weeks, is a period of reduced activity when the colony remains in a single bivouac location. During this time, the queen’s abdomen swells, and she lays a new batch of eggs, sometimes millions per month. Prey collected by the workers is fed to the queen, supporting her egg-laying capacity. This phase concludes when the new generation of workers emerges from their cocoons, prompting the colony to resume its nomadic foraging cycle.
Colony Organization
Army ant colonies exhibit a complex social structure with a clear division of labor among different castes. At the heart of the colony is a single, wingless queen, responsible for all reproduction. She can live for several years, constantly laying eggs to maintain the colony population. Males, known as drones, are winged and leave the colony to mate.
The majority of the colony consists of sterile female workers, which display polymorphism, meaning they come in different sizes and forms. Smaller workers tend to the brood and the queen within the bivouac. Larger workers perform tasks such as foraging, carrying prey, and defending the colony. Soldiers are recognizable by their larger bodies and mandibles, which they use for protection and dismembering prey. This caste system and the cooperative efforts of millions of individuals allow the colony to function as a cohesive “superorganism.”
Role in Ecosystems
Army ants are predators within their habitats, influencing the populations of other insects and small invertebrates. Their mass raids can reduce arthropod densities in the leaf litter by as much as 50%. This role benefits the ecosystem by maintaining balance. While carnivorous, they also scavenge on carrion and occasionally consume plant material.
Their presence creates ecological interactions, notably with “ant-following” birds. These birds follow the ant swarms, catching insects and small vertebrates that are flushed out by the ants’ advancing front. This relationship is commensal, where the birds benefit from the ants’ activity without impacting the ants. Army ants contribute to nutrient cycling and soil aeration as they move through the environment, disturbing leaf litter and soil, which promotes decomposition and nutrient availability for plants.