Supercolonies, or, What’s a society?

Mark “Dr. Bugs” Moffett has recently written a review of the literature on certain ants that live in so-called supercolonies:

Mark W. Moffett. Supercolonies of billions in an invasive ant: What is a society? Behavioral Ecology (2012). doi: 10.1093/behecoo /ars043, First published online: April 20, 2012. Full text here (PDF).

As the name suggests, supercolonies are large, with millions to perhaps a trillion members. They can also live in discontinuous sites, sometimes located on several continents, as a result of human transport.

What’s “at issue,” Moffett says, “is what we should consider a colony (or society).” However we may wish to characterize colonies from the outside, what’s functionally important is how individuals in societies distinguish between insiders and outsiders, for that’s the distinction that allows colonies to exist as functioning entities. Ants do not recognize one another as individuals.

The recognition system that ants use for identification with a colony and rejection of aliens is based on shared cues, typically a colony-specific odor blend generated by queens or workers (though environment has its influences: Crozier and Dix 1979; d’Ettorre and Lenoir 2010). As a result, ant colonies remain tightly knit without each individual necessarily having been in direct contact with every one of its nestmates.

Compare these “anonymous societies,” as I call them, with the societies of nonhuman vertebrates such as dolphins, elephants, cooperative breeding birds, and primates like the chimpanzee, where societies are defined by members recalling each other individually to know who is in their group and who is not.... I suggest calling these “individual recognition societies.” As a general rule such societies have at most 100 members.

The point of distinguishing between individual and anonymous recognition strategies, then, is colony size. Individual recognizers must recognize each an every member of the colony as individuals; anonymous recognizers do not. The colony size of individual recognizers is thus limited by the capacity of individuals to recognize one another. There is no such limitation on the colony size of anonymous recognizers.

Of course, we humans are individuals recognizers, and we’ve come to live in societies much larger than 100. And we do so by augmenting individual recognition in various ways. As an extreme example, consider passport control at border crossings. The officials who let travelers cross national borders certainly do not recognize individual travelers. Rather, they verify the validity of an official document that generally includes a photograph of the individual presenting that document. The stamps and seals on that document, in effect, play the role that odor blends play in ant colonies.

And with that, I turn things over to Moffett, first with the abstract for the article (which I’ve broken into segments) and then with a passage or two from the article itself.

Enjoy.

Abstract: All societies are characterized by the capacity of their members to distinguish one another from outsiders. Ants are among the species that form “anonymous societies”: members are not required to tell each other apart as individuals for the group to remain unified. Rather, each society depends on shared cues recognized by all its members. These cues permit societies to reach populations in the low millions in certain ant and termite species, and to grow indefinitely populous, expansive, and possibly long lasting in a few other ant species, which are described as having supercolonies.

Anonymous societies are contrasted with “individual recognition societies” such as those of most vertebrates, which are limited to a few individuals by the necessity that the members individually recognize each other. The shared recognition cues of ants provide clear criteria for defining colonies and are what enables a supercolony to remain a single society no matter how large it becomes.

I examine the often conflicting ideas about the best studied ant with supercolonies, the Argentine ant (Linepithema humile). Its invasive supercolonies, containing in some cases billions of workers and queens spread over hundreds of square kilometers, can be most parsimoniously understood as single colonies that have had an opportunity to expand across regions of suitable habitat because of a lack of well-matched competitors.

This capacity for unrestricted growth is the defining characteristic of supercolonies. There is no evidence that the local patchiness of nests and patterns of worker and food traffic within these wide-ranging populations are so invariant that supercolonies do not exist but instead are collections of numerous independent nest clusters that should be called “colonies.” Nor is there evidence for the hypothesis that invasive supercolonies have been able to grow large and successful overseas only as a result of evolving through genetic drift or selection to become fundamentally different from the smaller colonies typical of the species’ region of origin around northern Argentina.

Three other Argentine ant colonies vie with the Large Supercolony for the land near San Diego, however. They collide along centimeters-wide borders that extend for kilometers: each month, more than a million ants die in battles between 2 of the colonies alone (Thomas et al. 2007). It is a death sentence for an ant to move just beyond its colony’s territory onto ground controlled by one of these competitors (Figure 1). The same would be true if that ant came upon a fledgling Argentine ant colony offloaded from a ship Argentina. In short, at no stage in colony growth is there ambiguity as to the limits of the colony unit. The ants show a universal lack of social strain or dysfunction toward other colony members, and a clear attack response to outsiders, even after their colony range has expanded across continents.

From the article itself, concerning size and location:

Indeed, the invasive colonies of Argentine ants are the largest recorded societies of multicellular organisms. Among the supercolonies of this species spreading globally, Large Supercolony (as it is known in California, where it might contain a trillion individuals: Moffett 2010) is the champion, spanning 1000 km from San Francisco to the Mexican border in California, 6000 km in Europe, 2800 km in Australia, 900 km on the North Island of New Zealand, and ever-widening regions of Hawaii and Japan …. Carry an Argentine ant worker, queen, or male within or between any of these regions and it merges with the ants living there with at most a subtle initial pause to inspect them …. It joins the local labor force without a hitch because it is still home, in a sense—- Large Supercolony controls the entire expanse.

Three other Argentine ant colonies vie with the Large Supercolony for the land near San Diego, however. They collide along centimeters-wide borders that extend for kilometers: each month, more than a million ants die in battles between 2 of the colonies alone (Thomas et al. 2007). It is a death sentence for an ant to move just beyond its colony’s territory onto ground controlled by one of these competitors (Figure 1). The same would be true if that ant came upon a fledgling Argentine ant colony offloaded from a ship from supercolony as different colonies, typically when puzzling over how so many ants can live together harmoniously at a specific site.

Individual ants considered as cells:

Moreover, in most healthy ant societies, identification once learned is permanent and nontransferable (though at the same time remarkably adaptable, e.g., Moffett 2010, p. 215). Unlike chimpanzees or elephants, which have social mechanisms for group transfers, adult queens and workers are not able to move between colonies, except as parasites (e.g., chapter 12 of Hölldobler and Wilson 1990). Their unbreakable group identity makes ants in colonies powerful analogs of cells in bodies. This is the superorganism idea sensu Moffett (2010): Ants identify each other using chemical cues on their body surfaces, and in a healthy society, they invariably avoid or kill alien ants with different cues; cells identify each other by means of chemical cues on their surfaces, with the immune system attacking any cells with different cues (thought to originally have been hydrocarbons [Fernandez-Busquets and Burger 2003], as they are in the social insects).

One colony?

The root of much of the confusion about Argentine ants is that the “supercolonies confound our notions about societies, populations, and species like nothing else” (Moffett 2010). Consider how Argentine ants establish independent colonies. With no mating flight to allow a queen to start a nest with an identity separate from that of her natal colony, an intriguing possibility is that no truly new Argentine ant colonies ever arise, except as follows: Geographically isolated populations of the same colony might evolve to shift the genetic basis of their identity to the extent that the groups would start to kill each other if they came into contact again (Moffett 2010, p. 218; as may be occurring on the island of Corsica, which is occupied by what appears to be a long isolated part of the continental Europe portion of the Large Supercolony: Blight et al. 2010). Each Argentine ant colony, both in Argentina and abroad, potentially lasts indefinitely (by spreading locally through budding, or long distance through jump dispersal) as a “closed breeding unit” ..., rejecting both queens and males from outside colonies ... and possessing its own diagnostic genetically based characteristics .... Therefore, the colonies appear to take independent evolutionary paths, virtually as sibling species ....