- Argentine ants in Southern California are engaged in ongoing battles at supercolony borders, resulting in significant fatalities.
- Research has found that these ants exhibit a form of social organization known as unicoloniality, where cooperation turns into aggression.
- Within six months, at least 15 million ants died in the fighting, indicating that aggressive behavior is related to genetic differences.
- Despite the heavy casualties, the borders of opposing supercolonies have changed little, resembling a stalemate similar to human warfare.
In Southern California, competing ants have been engaged in ongoing battles at the borders of their vast supercolonies for years, resulting in millions of casualties. The Argentine ant (Linepithema humile), native to South America, has spread globally through international trade. This species exhibits a unique social organization known as "unicoleinality," where ants from different nests cooperate rather than fight. "Certain ant species possess a unique social organization where aggressive behavior between nests is absent," researchers noted in a recent study.
Social Structure of Argentine Ants
"This form of social organization is called unicoloniality, which has only been studied in a few species, and its evolutionary origins remain unclear." However, when these ants discover new territories, they sometimes form massive supercolonies. One such colony stretches along the California coast for about 620 miles, while several smaller supercolonies occupy surrounding areas. Due to relatively low genetic diversity, they can recognize their own kind from different nests and understand that they belong to the same group. However, when ants from opposing supercolonies encounter each other, cooperation turns into aggression.
Their borders have been described by researchers as "distinct." The research team identified 16 contact zones between three supercolonies. Over six months, they monitored nine of these zones weekly and described the boundaries in a study published in Molecular Ecology as "sudden and clear." The team recorded "persistent and intense inter-nest aggression, leading to variable but generally higher mortality rates among larger workers." According to IFLScience, researchers observed fighting in 40% of field visits, with an average of 15,000 workers killed in each encounter.
Impact of the Fighting
Near Lake Hodges, researchers estimated that at least 15 million ants died during the six-month observation period. Utilizing microsatellite technology, they examined genetic differences between the same supercolony and competing groups. Within supercolonies, the differences were significantly lower than those between supercolonies. "…levels of aggression were significantly related to genetic differences, but there was no relationship between geographic distance and any internal aggression or genetic differences," the researchers wrote. Genetic differences were associated with aggressive behavior. They found high levels of gene flow within supercolonies, but almost none between supercolonies, indicating that even after prolonged contact at the borders, opposing supercolonies maintain genetic distinctiveness.
The war of territories has no clear victor. Despite casualties numbering in the millions, the borders changed little during the researchers' observation period, resembling a persistent stalemate seen in human warfare, according to IFLScience. Although the opposing armies suffered heavy losses, neither achieved any significant territorial gains. Typically, territorial animals become less aggressive towards familiar neighbors. However, according to IFL, Argentine ants may not exhibit the "familiar enemy effect," which usually mitigates such conflicts.
With little prospect for peace, encounters between opposing colonies may continue to provoke intense conflict. Researchers concluded that these findings "are inconsistent with models suggesting that relaxed ecological constraints lead to colony fusion." They also suggested that environmental cues are not the primary determinants of nest recognition in wild Argentine ant populations. The future of California ant colonies remains uncertain. IFL summarized that academic research has not extensively reviewed territorial wars in San Diego over the past two decades, leaving it unclear whether these borders have changed or if fighting continues.
Ecological Significance of the Argentine Ant Wars
The battles between supercolonies of Argentine ants reveal their unique social structure and behavioral patterns. This unicoloniality allows ants to cooperate under certain conditions but turns to aggression when facing external threats, highlighting the complexity of their social dynamics. The research indicates that despite significant mortality, the borders have changed little, which may suggest a high level of persistence in territorial defense among these ants. These findings are not only important for understanding the ecological behavior of ants but may also provide insights into the social behavior of other species.