Army ant foraging behaviours
Some army ants live a completely different existence to their non-army ant relatives. They lead a completely epigaeic existence (above ground) (Kronauer, 2009). This means that they do not make a traditional under-ground nest. Instead army ants form what is known as a bivouac. Bivouacs are literally made of the ants themselves. The colony clusters together to form a ball of ants, with the queen protected at the center (Fig 4). The formation of bivouacs was likely to have evolved when the ants first made their way above ground. It allows for them to move their nest frequently and thus exploit a larger scope of their environment for prey items (Gotwald, 2009).
Naturalist, Henry Walker Bates (1863) put it best when he said “Wherever they (army ants) move, the whole animal world is set in commotion and every creature tries to get out of their way.” The foraging behaviours of army ants usually involve mass raiding events where large numbers of worker ants scour the forest floor in search of prey. Unlike other ants, foraging is autonomous. This means that each worker ant looks for food, communicating with its fellow worker ants using a system of pheromones. This kind of foraging doesn't involve using scout ants to find prey but rather using each ant to cover more ground.(Brady, 2002) This kind of individual foraging would have evolved to allow the ants to hunt larger prey, as well as expand the amount of area that can be covered whilst foraging for food. When prey is discovered, the ants swarm and quickly immobilize it using numerous deadly stings. The prey is then either digested by the ants or, if the prey is too large, it is broken down into more manageable pieces and transported back to the bivouac.
There are two species specific patterns of raiding: column and swarm (fig 5). Swarm raids, as the name suggests are when the ants form columns that lead to a fan like shape and eventually to a large swarm group at the front of the party. Column raids are characterized by numerous columns of scouring ants. These two raiding strategies may have evolved separately to compensate for differing prey organisms of different species (Gotwald, 1995). Despite being formidable Predators, there are species of army ants that have very poor eyesight only being able to see movement and detect some light. This suggests that ancestors of these species lived a completely subterranean existence and lost the need for completely functional eyes (Gotwald, 1995).
There are two species specific patterns of raiding: column and swarm (fig 5). Swarm raids, as the name suggests are when the ants form columns that lead to a fan like shape and eventually to a large swarm group at the front of the party. Column raids are characterized by numerous columns of scouring ants. These two raiding strategies may have evolved separately to compensate for differing prey organisms of different species (Gotwald, 1995). Despite being formidable Predators, there are species of army ants that have very poor eyesight only being able to see movement and detect some light. This suggests that ancestors of these species lived a completely subterranean existence and lost the need for completely functional eyes (Gotwald, 1995).
Fig 5. Distinct raiding patterns of army ants. A being swarm raids note the structure of the raid: has several columns leading to a fan anf then the swarm front. and B being column raids note the structure is lacking a singular swarm but is composed of higher numbers of columns
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