Adventures Among Ants - Mark W. Moffett [141]
In Queensland, Australia, the jumping spider Cosmophasis bitaeniata lives in weaver ant nests by taking on the same scent the ants use to identify their colony.
The ability to distinguish self from other (and friend from foe) has been a theme of evolution since the inception of life, beginning in earnest with the aggregation of cells into organisms and continuing with the grouping of organisms into societies (or “superorganisms,” in societies with a sharply defined sense of self). Evolutionary turning points often require the components at one level of complexity (the bodies in a society, the cells in a body, even the parts of a cell) to come together to establish a group that takes on an identity of its own—a process of social bonding universal in nature from microbes on up.6 When its identity is signaled clearly by all its constituents, an organism such as an ant or a society such as an ant colony is easily recognized, giving it a clear individuality.7
THE SCENT OF KINSHIP
Regardless of their usual hostility toward outsiders, colonies can be widely inclusive—sometimes without being tricked. Consider the carpenter ants that share space with acrobat ants in South American ant gardens, or the leafcutter ant and its fungus, which require each other absolutely; in these cases both species benefit from the association, and they (especially the latter pair) evolve together as integrated parts of one society.8 Parasites, however, must trick the other species to join its society, to the detriment of the unwitting partner. Sometimes a society will trick an individual: in slavemaker colonies, a kidnapped worker accepts her captors as nestmates despite differences that seem obvious when we watch the ants through our microscopes. Actually, this acceptance is the least of it: to avoid fights within the nest, every slave and slavemaker must accept all the other slaves as well, no matter when or where they were captured. To further confuse matters, a slavemaker occasionally captures more than one kind of slave, yet there are no fights even with three or more species living in the same nest. None of them have a problem identifying the others as nestmates.
The slave example suggests ants can learn to recognize nestmates even when they come from a different environment or are genetically distinct. This must also be true for Argentine ants, whose colonies range over diverse environments and contain the progeny of multiple queens. Although all those queens arise within a single colony, their offspring exhibit some genetic differences.9 Either the ants come to accept the varied odors present, or the odor cocktail of each colony member is diluted by food exchange or grooming between ants. That could be enough for the nestmates to achieve the same average scent—the only scent they need to function in their society.10
The changing assortment of offspring of the colony’s many queens must create a changing scent profile, so Argentine workers must be able to refine their ability to distinguish friend and foe as they age. This is also seen in their adaptable response to enemies outside the colony. Familiarity with outsiders breeds contempt in Argentine ants, which do not exhibit the dear enemy phenomenon. In one experiment, workers were at first able to touch members of a foreign colony by sticking their antennae through a mesh barrier; afterward they attacked more ferociously than if they had met the enemy on the battlefield for the first time.11
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