The earliest record of the Argentine ant (Linepithema humile) dates to 1847, from the Atlantic island of Madeira, then a hub for Portugal’s trade with its colonies. This is strange for a species that’s native to the Panara river drainage system in South America, and is found in parts of Argentina, Brazil and Uruguay. Perhaps the Madeira record should have been the first call for attention to be paid to an invader caught in the throes of an endless, insatiable conquest.
The Argentine ant began to hitchhike on the backs of Spanish and Portuguese explorers in the 16th and 17th centuries, eventually stowing away on ships and other transportation systems in the mid-1800s and winding up in Europe and North America.
This individually unremarkable insect has since established populations in at least 15 countries on six continents. It is resourceful, adaptable and resilient to various shocks and stresses, and frequently displaces or wipes out native ant species, precipitating ecological disruption wherever it has gone.
The most interesting thing about the Argentine ant is how it has changed in the two centuries since the first populations arrived in Europe. Today, the foreign members of the species – introduced from outside a country to inside – differ from their native cousins in the biochemical particularities that govern how ant societies are formed.
Ants are social organisms that form anonymous societies: unity doesn’t require individual recognition. Instead, ant species are characterised by a communal identity. Ants with similar genetic ancestry share chemical profiles and other cues, some of which also help identify what role each ant pays in a colony.
Most ant species are monogynous: a system of organisation in which one egg-laying queen builds a colony. A typical colony consists of the queen, her sterile female workers who forage and look after the larvae, and the drones: male ants whose sole purpose is to mate with the queen and die shortly afterwards.
During certain windows, the queen gives birth to reproductives: winged males and females that depart the nest in a swam to mate in other nests or to form entirely new colonies. This results in a feature called multi-coloniality, and each colony has a specific chemical profile shaped by the presence of cuticular hydrocarbons, usually defined by the queen.
Once an ant encounters a foreign chemical profile, it becomes aggressive. This helps maintain the colony’s integrity but results in both interspecific (between ant species) and intraspecific (within an ant species) aggression. In this organisational system, territorial boundaries are strictly demarcated but are limited in size.
However, the Argentine ant is polygynous: existing in colonies with multiple egg-laying queens. The mating season begins with a strange ritual in which worker ants kill a large fraction of existing queens.
After this, the reproductives begin mating, but instead of swarming out, female alates1 mate within the nests. As a result, nests can grow quickly in size thanks to the presence of so many queens giving birth. After a certain threshold, a queen may choose to embark on foot from a nest with a host of selected workers to establish a new colony, in a process called budding. The new nest retains the original nest’s chemical profile, resulting in the formation of a polydomous society, with multiple nests linked to one another across a geographical area. Queens and workers can move freely between them.
Unlike multi-colonial societies, a polydomous society exhibits a high degree of cooperation between different nests and a decrease in intraspecific aggression. Such behaviour is called unicolonial behaviour.
Unicoloniality lends itself to the formation of large networks of nests and colonies. However, colony size in the Argentine ant’s native range is limited by the emergence of genetically different colonies followed by intraspecific aggression. Even when unicoloniality supports the growth of super-colonies, these colonies remain much smaller than those formed by Argentine ants in non-native areas. Scientists have argued that this could be because of the presence of other, highly aggressive ant species, many of which are also successful invasive species in other parts of the world.
Argentine ants in foreign lands, on the other hand, have been known to form mind-bogglingly big colonies in the US, Europe, Australia, Northern Africa and Japan.
According to scientists, there are two reasons for this. Unlike in their native range, the Argentine ant doesn’t have to compete with other aggressive ant species for resources and can smoothly expand their colonies. Second, each super-colony across the world has its own founding population: a group of genetically similar ants that arrived from South America and founded a colony. Scientists believe they passed this genetic similarity on to their offspring, fostering the so-called founder effect.
For a long time, experts thought a super-colony in southern Europe, nearly 6,000 km long along the Mediterranean coast, was the world’s largest. In 2009, researchers from the University of Tokyo announced the discovery of a new record-holder. They had been able to link the genetic profiles of super-colonies in California and on the Japanese west coast together with the Mediterranean colony. When they brought individuals from these colonies close to each other, they cooperated instead of fighting, implying that three colonies in three continents were in fact one mega-colony spanning the globe – the largest known cooperative structure in the animal kingdom.
In effect, the Argentine ant isn’t an invasive species but an invasive community, comprising many billions of cooperative ants.
This is an evolutionary contradiction. Once a colony becomes so big that different parts of it are effectively in different environments, the ants living there typically evolve differently and show signs of aggression when brought together. How then do ants from opposite ends of the planet still work together? We don’t yet know.
New research has presented some clues. Scientists now believe unicoloniality has an inherent attribute triggered when Argentine ants are introduced to a non-native range with few threats. This attribute erases certain alleles that regulate recognition in ants, rendering the ants less discriminatory than they might be at home, and more willing to cooperate with any ant from the same species that is genetically similar.
This said, the Argentine ant has been facing stiffer competition of late. Territorial fights against the red imported fire ant (Solenopsis invicta), the tawny crazy ant (Nylanderia fulva) and the Asian needle ant (Pachycondyla chinensis) have weakened its holds in the US. The African big-headed ant (Pheidole megacephala) has prevented it from gaining ground in South Africa. In San Diego, a super-colony split in two, precipitating a battlefront many miles long in which 15 million ants died in six months of 2004.
Together with human efforts to hold such invasive species at bay and preserve endangered ecosystems, it seems the Sun may finally have started to set on the Argentine ant empire.
Kaustav Sood is a recent graduate from the history department at Ashoka University, Sonepat, and is currently interning with the National Skill Development Corporation. This article has been adapted from a research paper the author wrote while at Ashoka University.
Reproductive ants that haven’t mated↩