Supercolonies: Can they bring peace to the world of ants?

A few weeks ago Edward O. Wilson’s début novel Anthill brought supercolonies to the attention of a wide audience, much as his 2008 book with Bert Hölldobler did for the idea of a superorgan­ism.

The terms are easy to confuse. A super­organism is a society such as a colony of ants integrated so thoroughly by the shared identity of its members that it acts as if it were an organism; a supercolony is a society that, if given the opportunity, can ex­pand in size often indefinitely by adding living space and reproductives (queens).

Supercolonies are so extraordinary that I conclude Adventures Among Ants with the Argentine ant, one of several invasive ant species forming colonies of this sort. The success of the strategy cannot be questioned: after 100 years in California, Argentine ant supercolonies still maintain extraordinarily dense populations, with about a million ants in the average suburban backyard around San Diego. Success is enhanced by dietary flexibility: after a colony exterminates local prey populations, it continues to prosper by shifting to a greater dependency on aphid honeydew.

Yet descriptions of supercolonies are confusing; in particular, there is a widespread perception that Argentine ants are bizarre for their lack of ag­gression [1]. To the contrary, I view them as the most aggressive of ants, especially toward other colonies of their kind—other supercolonies included. As AAA de­scribes, battles between Argentine ant colonies have likely raged nonstop in Califor­nia ever since more than one colony first arrived in the state a century ago.

How can there be such divergent views on aggression in the Argentine ant? From what I can tell, those who see conflict as insignificant often seem to be confusing “colonies” with “nests.”

At most sites in California, Argentine ants live in har­m­ony as far as the human eye can see. Other ant colonies occupy spaces more easily comprehensible to the human eye— whether they use one nest or many, all the ants restrict themselves to an area a few feet to a few dozen yards across [2,3]. It’s easy to question, then, how aggres­sion can be absent be­tween numerous Argentine ant nests that can be so far apart that they would be occupied by different colonies in any other ant species [4].

But whether a colony is modest in size or of supersi­zed proportions, we are unlikely to observe aggression away from the borders of its ter­ritory [5]. That is especially true for the “absolute territories” of the species that expel individuals from adjacent colon­ies as super­col­onies do [6].

A supercolony’s control of such an immense tract of land does give the average worker inside it an unusually peaceful life: Only the relatively few individuals that happen to live near a colony’s border have a chance of experiencing conflicts with another colony of their own kind. That’s because the population of a big society is likely to be huge rel­a­tive to the length of its defended borders. This also gives large societies their excess labor force—a massive pool of workers that can carry out offensive operations at little social cost, allowing for the ceaseless, no-holds-barred warfare of Argentine ants.

It is the lack of aggression among workers within a supercolony that can easily fool us about the capacity for belligerence by Argentine ant societies, which is directed largely toward outsiders, as it is for ants generally. A few years ago, before four different supercolonies were known to exist in California, no Argentine ant was thought to fight with its own kind. Now California’s supercolonies have been shown to attack each other along fronts that extend for miles. What would happen if the other Argentine ant colon­ies worldwide are eventu­ally wiped out by the Very Large Colony, which contains trillions of ants expanding their range through­out the world? With just the one supercolony remaining, and its iden­tity maintained wherever it travels, Very Large Colony would no longer face warfare with other Argentine ants. This would indeed bring peace to the world of the Argentine ant. Yet achieving this peace will have depended on the ant’s nonpareil capacity to annihilate its own kind.

Notes

1. The title for this essay is drawn from one of the papers that takes this alternative point of view: Kazuki Tsuji 2010. What brings peace to the world of ants? Myrmecological News 13:131-132.

2. Argentine ant colonies in their na­tive Argentina keep to smaller, more easily comprehensible sizes, most likely because they are penned in by numerous neighbors of the same or other hostile species, as they are as well in the American southeast.

3. Does it make a difference to the individual ant whether her society occupies the modest territory or the expansive one? Probably not. Usually a worker comes to know only a part of her colony’s foraging area, no matter how big the area is.

4. It may be more accurate and useful to think of Argentine ant colo nies as consisting not of many separate nests, but rather as an expansive anthill, in which many of the routes between nest chambers happen to be on the surface rather than hidden underground. The surface trails are in effect parts of the nest, as I described for the marauder ant with its trunk trails. (Whether one calls the frequent streams of brood-bear ing work ers, young adult ants, and queens between nest chambers along a surface trail a “migration” is pure semantics—similar pulses of traffic occur along the subter ranean routes within entirely underground nests.)

5. I am putting aside occa­sional hostilities over repro­ductive rights between nest mates in some ants, which typically occur inside the nest chambers.

6. Maintenance of truly "absolute" territories is expensive, and so foreign ants may often cross into the territories of many species. An inexpensive option is for a colony to defend resources rather than space per se, which is what honeypot ants do (AAA page 115).