In Southern California, rival ants warred at the borders of their massive supercolonies for years, with casualties soaring into the millions.
Native to South America, Argentine ants (Linepithema humile) have spread worldwide through international trade.
This species has an unusual form of social organization known as unicoloniality, in which ants from different nests cooperate rather than fight.
“Some species of ants possess an unusual form of social organization in which aggression among nests is absent,” researchers stated in a recent study.
“This type of social organization, called unicoloniality, has been studied in only a handful of species and its evolutionary origins remain unclear.”
However, when these ants found new territory, they sometimes formed enormous supercolonies. One in coastal California extends approximately 620 miles long, while several smaller supercolonies occupied surrounding territories.
With relatively low genetic diversity, they recognized one another from different nests and knew they belonged to the same group. However, when ants from rival supercolonies met, cooperation turned to aggression.
Their borders are “well-defined”
Researchers identified 16 contact zones between three supercolonies. Over six months, they monitored nine of them weekly. They described these boundaries as “abrupt and well-defined” in a new study published in Molecular Ecology.
The team documented “consistent and intense intercolony aggression that resulted in variable, but often large, levels of worker mortality.”
According to IFLScience, researchers observed fighting at 40 percent of site visits, with an average of 15,000 worker ants killed per battle. Near Lake Hodges, researchers estimated that at least 15 million ants died during the six-month observation period.
Using microsatellites, they examined genetic differences between ants within the same supercolony and those in competing groups. Differentiation was substantially lower within supercolonies than between them.
“…Levels of aggression and genetic differentiation were significantly correlated, but no relationship existed between geographic distance and either intraspecific aggression or genetic differentiation,” the researchers wrote.
Genetic differences were associated with aggression.
They found high levels of gene flow within supercolonies, but little to no gene flow between them, suggesting that rival supercolonies remained genetically distinct even after prolonged contact along their borders.
A territorial war with no clear winner
Despite the millions of casualties, the boundaries showed little movement during the researchers’ observation period, akin to a prolonged stalemate in human wars, according to IFLScience. Though opposing armies suffered heavy losses, neither made any significant territorial gains.
Typically, territorial animals become less aggressive towards their familiar neighbors. However, Argentine ants may not exhibit the “dear enemy effect,” which tends to quell these confrontations, according to IFL.
Without the possibility of peace, encounters between rival colonies may continue to trigger violent confrontations. The researchers concluded that their findings were “inconsistent with a model of relaxed ecological constraints leading to colony fusion.” They also suggested that environmental cues were not the primary determinants of nestmate recognition in wild Argentine ant populations.
The future of California ant colonies remains uncertain.
IFL concludes that academic research has not extensively revisited the San Diego territorial wars in about two decades, leaving it unclear whether the boundaries have shifted or the fighting continues.