page_title: Argentine ant
section_title: Behavior
hierarchical_section_title: Argentine ant / Behavior
caption_attribution_description: Español: La hormiga argentina (Linepithema humile) es una especie invasora proveniente del norte de Argentina. Se introdujo en Europa y Norteamérica por vía humana y es un problema medio ambiental enorme debido a su fuerte competencia. (gracias Roberto).English: Argentine ant (Linepithema humile) is an invasive species from northern Argentina. It was introduced into Europe and North America by humans and is a huge environmental problem due to strong competition. (Thanks, Roberto.)
context_page_description: The Argentine ant, formerly Iridomyrmex humilis, is an ant native to northern Argentina, Uruguay, Paraguay, Bolivia and southern Brazil. It is an invasive species that has been established in many Mediterranean climate areas, inadvertently introduced by humans to many places, including South Africa, New Zealand, Japan, Easter Island, Australia, Europe, Hawaii, and the continental United States.
context_section_description: They have been extraordinarily successful, in part, because different nests of the introduced Argentine ants seldom attack or compete with each other, unlike most other species of ant. In their introduced range, their genetic makeup is so uniform that individuals from one nest can mingle in a neighboring nest without being attacked. Thus, in most of their introduced range, they form supercolonies. "Some ants have an extraordinary social organization, called unicoloniality, whereby individuals mix freely among physically separated nests. This type of social organization is not only a key attribute responsible for the ecological domination of these ants, but also an evolutionary paradox and a potential problem for kin selection theory because relatedness between nest mates is effectively zero." The Very Large Colony, which covers territory from San Diego to beyond San Francisco, may have a population of nearly one trillion individuals.
  Conflict does occur between members of different supercolonies. In 1997, UC San Diego researchers observed fighting between different Argentine ants kept in lab, and in 2004 scientists began to map out the boundaries of the different supercolonies that clashed in San Diego. On the border of the Very Large Colony and the Lake Hodges Colony thirty million ants die each year on a battlefront that covers many miles. While the battles of other ant species generally constitute colony raids lasting a few hours or skirmishes that occur periodically for a few weeks, Argentine ants clash ceaselessly; the borders of their territory are a site of constant violence and battles can be fought on top of hundreds of dead ants. Fights may be halted by adverse weather such as rain.
  In contrast, native populations are more genetically diverse, genetically differentiated (among colonies and across space), and form colonies that are much smaller than the supercolonies that dominate the introduced range. Argentine ants in their native South America also co-exist with many other species of ants, and do not attain the high population densities that characterize introduced populations.
  In a series of experiments, ants of the same colony were isolated and fed different diets. The hydrocarbons from the diet were eventually incorporated into the cuticle of the subjects. Those that had the same diet appeared to recognize one another as kin. Those who had at least some overlap in dietary composition also appeared to react non-aggressively to one another. These interactions contrasts drastically with the groups that fed on completely different sources, such as those who lived off flies and those that fed on grasshoppers. The groups appeared to have incorporated hydrocarbons that were not similar to the others and created an unfamiliar identity cue. These groups reacted violently towards each other. This suggests that dietary factors affect the recognition cues for colony members.
caption: Argentine ants on a dead horse-fly
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