Adventures Among Ants - Mark W. Moffett [192]
12. GP Markin 1968, Nest relationships of the Argentine ant, Iridomyrmex humilis, J. Kans. Entomol. Soc. 41: 511–516.
13. Jump dispersal within a supercolony could increase genetic homogenization, however.
14. KK Ingram, DM Gordon 2003, Genetic analysis of dispersal dynamics in an invading population of Argentine ants, Ecology 84: 2832–2842.
15. This mental experiment is the colony equivalent of a “ring species,” in which individuals of a geographically variable species breed freely everywhere, but individuals at the extremes of the range are so different that they can’t interbreed.
16. It is still possible that ants might show more subtle animosity or favoritism within a colony, for example, by preferring to exchange food with genetically closer individuals (perhaps kin, but see n. 9).
17. Violence within otherwise affable supercolonies occurs in only one situation. Each spring, for reasons unclear, the workers mass-execute the queens, sparing only enough of them to maintain the high rate of colony growth. It’s an exception that proves the rule: social integrity is reflected in how well the ants manage conflict when it arises. There seems to be no resistance, and the colony operates smoothly as its queens are butchered—even the queens don’t protest. These mass executions were first described in GP Markin 1970, The seasonal life cycle of the Argentine ant, Iridomyrmex humilis, in southern California, Ann. Entomol. Soc. Am. 63: 1238–1242.
18. Clarence Day, This Simian World (New York: Alfred A. Knopf, 1920), p. 10.
19. Neil Tsutsui, quoted in M Shwartz 2004, Scientists challenge report of one Argentine ant supercolony flooding California, Stanford Report, 7 April.
20. ND Tsutsui, AV Suarez, DA Holway, TJ Case 2000, Reduced genetic variation and the success of an invasive species, Proc. Natl. Acad. Sci. 97: 5948–5953. A slightly different “genetic cleansing” hypothesis claims that, with the nests of colonists reaching extraordinary densities, any hostilities are inordinately costly, making it advantageous for the ants to evolve to lose the capacity to identify and discriminate against non-kin; see T Giraud, JS Pedersen, L Keller 2002, Evolution of supercolonies: The Argentine ants of southern Europe, Proc. Natl. Acad. Sci. 99: 6075–6079. While much has been made in published reports of the high genetic diversity in Argentina compared to that in invasive populations, almost certainly this is an artifact of genes being sampled largely between colonies in Argentina versus within single colonies in California. I think it’s likely that colonies of Argentine ants have the capacity to expand indefinitely, regardless of their genetic diversity.
Genetic losses could give a colony another kind of edge: workers from supercolonies with a lower genetic diversity, especially the Very Large Colony, exhibit quick attacks in combat with other supercolonies, using the “shock and awe” offensive approach we saw in the marauder ant. Perhaps their stripped-down recognition signals make the workers faster at distinguishing friend from foe; see ND Tsutsui, AV Suarez, RK Grosberg 2003, cited in n. 9.
21. The exception would be the colonies from one site in Argentina that each have a single, local nest; see NE Heller 2004, Colony structure in introduced and native populations of the invasive Argentine ant, Linepithema humile, Insectes Soc. 51: 378–386.
22. Argentine ants may be less inbred in Argentina, where the smaller territories mean the weak-flying males are more likely to reach neighboring