by Newell & Barber, who noted "…there is no apparent antagonism between separate colonies of its own kind". Later studies showed that these "supercolonies" extend across hundreds or thousands of kilometers in different parts of the introduced range, first reported in California in 2000, then in Europe in 2002, Japan in 2009, and Australia in 2010. Several subsequent studies used genetic, behavioral, and chemical analyses to show that introduced supercolonies on separate continents actually represent a single global supercolony. The researchers stated that the "enormous extent of this population is paralleled only by human society", and had probably been spread and maintained by human travel. Behavior 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 'super-colonies'. "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 genetically more diverse and form colonies that are much smaller than the supercolonies that dominate the introduced range.