Army Ant

The name army ant (or legionary ant or marabunta[1]) is applied to over 200 ant species in different lineages. Because of their aggressive predatory foraging groups, known as "raids", a huge number of ants forage simultaneously over a limited area.[2]

army ant

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Another shared feature is that, unlike most ant species, army ants do not construct permanent nests; an army ant colony moves almost incessantly over the time it exists. All species are members of the true ant family, Formicidae, but several groups have independently evolved the same basic behavioural and ecological syndrome. This syndrome is often referred to as "legionary behaviour", and may be an example of convergent evolution.[3][n 1]

Most New World army ants belong to the genera Cheliomyrmex, Neivamyrmex, Nomamyrmex, Labidus, and Eciton.[3] The largest genus is Neivamyrmex, which contains more than 120 species; the most predominant species is Eciton burchellii; its common name "army ant" is considered to be the archetype of the species. Most Old World army ants are divided between the tribes Aenictini and Dorylini. Aenictini contains more than 50 species of army ants in the single genus, Aenictus. However, the Dorylini contain the genus Dorylus, the most aggressive group of driver ants; 70 species are known.[citation needed]

Originally, some of the Old World and New World lineages of army ants were thought to have evolved independently, in an example of convergent evolution. In 2003, though, genetic analysis of various species suggests that several of these groups evolved from a single common ancestor, which lived approximately 100 million years ago at the time of the separation of the continents of Africa and South America, while other army ant lineages (Leptanillinae, plus members of Ponerinae, Amblyoponinae, and Myrmicinae) are still considered to represent independent evolutionary events.[3] Army ant taxonomy remains in flux, and genetic analysis will likely continue to provide more information about the relatedness of the various taxa.[4]

The workers of army ants are usually blind or can have compound eyes that are reduced to a single lens. There are species of army ants where the worker caste may show polymorphism based on physical differences and job allocations; however, there are also species that show no polymorphism at all.[5] The worker caste is usually composed of sterile female worker ants.[6]

The soldiers of army ants are larger than the workers, and they have much larger mandibles than the worker class of ants, with older soldiers possessing larger heads and stronger mandibles than the younger ones. They protect the colony, and help carry the heaviest loads of prey to the colony bivouac.

The army ant syndrome refers to behavioral and reproductive traits such as obligate collective foraging, nomadism and highly specialized queens that allow these organisms to become the most ferocious social hunters.[3]

The nomadic phase begins around 10 days after the queen lays her eggs. This phase will last approximately 15 days to let the larvae develop. The ants move during the day, capturing insects, spiders, and small vertebrates to feed their brood. At dusk, they will form their nests or bivouac, which they change almost daily.[8] At the end of the nomadic phase, the larvae will spin pupal cases and no longer require food. The colony can then live in the same bivouac site for around 20 days, foraging only on approximately two-thirds of these days.[2][8] Among the army ants are some species that venture out only at night, but no adequate studies of their activities have been made.

Being the largest ants on Earth, army ants, such as African Dorylus queens have the greatest reproductive potential among insects, with an egg-laying capacity of several million per month. Army ant queens never have to leave the protection of the colony, where they mate with foreign incoming males which disperse on nuptial flights. The exact mating behaviour of the army ant queen is still unknown, but observations seem to imply that queens may be fertilized by multiple males.[13] Due to the queen's large reproductive potential, a colony of army ants can be descended from a single queen.[10]

When the queen ant dies, there is no replacement and army ants cannot rear emergency queens. Most of the time, if the queen dies, the colony will likely die too. Queen loss can occur due to accidents during emigrations, predator attack, old age or illness.[12] However, there are possibilities to avoid colony death. When a colony loses its queen, the worker ants will usually fuse with another colony that has a queen, within a few days.[6][14] Sometimes, the workers will backtrack along the paths of prior emigrations to search for a queen that has been lost or merge with a sister colony.[14][15] By merging with a related colony, the workers would increase their overall inclusive fitness.[12] The workers that merge into a new colony may cause the colony to increase in size by 50%.

The whole colony of army ants can consume up to 500,000 prey animals each day, so can have a significant influence on the population, diversity, and behaviour of their prey.[17] The prey selection differs with the species. Underground species prey primarily on ground-dwelling arthropods and their larvae, earthworms, and occasionally also the young of vertebrates, turtle eggs, or oily seeds. A majority of the species, the "colony robbers", specialize in the offspring of other ants and wasps. Only a few species seem to have the very broad spectrum of prey seen in the raiding species. Even these species do not eat every kind of animal. Although small vertebrates that get caught in the raid will be killed, the jaws of the American Eciton are not suited to this type of prey, in contrast to the African Dorylus. These undesired prey are simply left behind and consumed by scavengers or by the flies that accompany the ant swarm. Only a few species hunt primarily on the surface of the earth; they seek their prey mainly in leaf litter and in low vegetation. About five species hunt in higher trees, where they can attack birds and their eggs, although they focus on hunting other social insects along with their eggs and larvae. Colonies of army ants are large compared to the colonies of other Formicidae. Colonies can have over 15 million workers and can transport 3000 prey (items) per hour during the raid period.[13][18]

When army ants forage, the trails that are formed can be over 20 m (66 ft) wide and over 100 m (330 ft) long.[18] They stay on the path through the use of a concentration gradient of pheromones. The concentration of pheromone is highest in the middle of the trail, splitting the trail into two distinct regions: an area with high concentration and two areas with low concentrations of pheromones. The outbound ants will occupy the outer two lanes and the returning ants will occupy the central lane.[18] The returning worker ants have also been found to emit more pheromones than those leaving the nest, causing the difference in concentration of pheromone in the trails.[19] The pheromones will allow foraging to be much more efficient by allowing the army ants to avoid their own former paths and those of their conspecifics.[17] Scaffolds structure has been observed when workers carried heavy prey food to inclined surface. Walking ants are prevented from falling by other ants.[20]

Many species of army ants are widely considered to be keystone species[25] due to their important ecological role as arthropod predators [26] and due to their large number of vertebrate and invertebrate associates that rely on army ant colonies for nutrition or protection.[27][28][29] During their hunt, many surface-raiding army ants are accompanied by various birds, such as antbirds, thrushes, ovenbirds and wrens, which devour the insects that are flushed out by the ants, a behavior known as kleptoparasitism.[30][31] A wide variety of arthropods including staphylinid beetles, histerid beetles, spiders, silverfish, isopods, and mites also follow colonies.[32] While some guests follow the colony emigrations on foot,[33][34][29] many others are phoretically transported, for example by attaching themseles on army ant workers such as the histerid beetle Nymphister kronaueri.[35] The Neotropical army ant Eciton burchellii has an estimated 350 to 500 animal associates, the most of any one species known to science.[29]

Historically, "army ant" in the broad sense referred to various members of five different ant subfamilies. In two of these cases, the Ponerinae and Myrmicinae, only a few species and genera exhibit legionary behavior; in the other three lineages, Ecitoninae, Dorylinae, and Leptanillinae, all of the constituent species were considered to be legionary. More recently, ant classifications now recognize an additional New World subfamily, Leptanilloidinae, which also consists of obligate legionary species, so is another group now included among the army ants.[36]

A 2003 study of thirty species (by Sean Brady of Cornell University) indicates that army ants of subfamilies Ecitoninae (South America), Dorylinae (Africa) and Aenictinae (Asia) together formed a monophyletic group, based on data from three molecular genes and one mitochondrial gene. Brady concluded that these groups are, therefore, a single lineage that evolved in the mid-Cretaceous period in Gondwana,[n 2] so these subfamilies are now generally united into a single subfamily Dorylinae, though this is still not universally recognized.[38] However, the unification of these lineages means that the only subfamily that is composed solely of legionary species is Leptanillinae, as Dorylinae contains many non-legionary genera.

A fundamental gap in our knowledge of army ant biology is the question of whether this army ant syndrome of behavioral and reproductive traits resulted from a unique set of evolutionary events, or instead, evolved convergently in multiple lineages. Army ants constitute three well defined taxonomic subfamilies (9), two restricted exclusively to the Old World (Aenictinae and Dorylinae) and the other to the New World (Ecitoninae). The prevailing view holds that the army ant syndrome originated several times in independent lineages restricted to the New World and Old World, respectively (3, 10). This polyphyly hypothesis, widely cited in the literature, is founded primarily on the assumption that army ants originated after the breakup of Gondwana, and thus must have evolved independently on separate continents. If true, this would imply multiple origins of army ant behavioral and reproductive adaptations, with their similarities due to convergence. 041b061a72