Adventures Among Ants - Mark W. Moffett [78]
A weaver ant grasping a larva that is dispensing a silk thread to bind leaves for a nest in Queensland, Australia.
MAPPING AND DEFENDING AN EMPIRE
Weaver ants are excellent nest builders, but they excel equally in transportation and communication. Their colonies employ a flexible network of routes between population centers and valued resources, much as human civilizations have done since ancient times.11 The ants travel via trunk trails, marked by pheromones produced by the rectal gland, an organ unique to these ants. Trails are made more durable by droplets of worker excrement, which they deposit in a wide swath along the thoroughfares. The droplets harden like shellac when dry, creating a sort of “blacktop” that renders the path extremely persistent.12 The workers can rediscover lost routes even after months of torrential rain or drought.
The workers use different forms of communication to convey different messages.13 Before a trail is reinforced through time and usage to become a highway, it is likely to be ignored unless the ants producing it provide additional signals. As a worker deposits trail markings from her rectal gland, she employs a combination of signals—jerky moves, gentle touches, regurgitation—to identify whether her chemical path goes to some distant food, a leaf construction site, a newly finished nest, fresh territorial space, or an enemy confrontation. An ant returning from an unexplored part of the hinterlands, for example, lays a trail while shaking her body at passersby. Recruited ants, following the trail, enter the unoccupied area, then do what any self-respecting dog would do: they mark the space by relieving themselves. In this situation the fecal droplets are the ant version of urine and serve to identify the society, rather than the individual (as is the case with a dog), and claim the area in the colony’s name. Given that the marks persist for months, their use along trails and at borders most likely serves a long-term strategic function: the colony that first marks a site has the edge in later conflicts.
When an ant contacts an enemy worker, she rushes back to familiar territory, encouraging others to follow her by conducting mock fights in which she stands tall and jabs at her fellow workers as if to bite them. Like a dog’s bared teeth, this serves a warning function, in this case communicating with nestmates about the battle to come or already afoot. Her movements are one of several symbolic ant behaviors, actions normally associated with a practical activity, such as fighting, that have been ritualized into communication signals about that activity.14
Among weaver ants in Australia, combatants are usually recruited from “barrack” nests at the territory perimeter near the outer crowns of trees or at the base of their trunks. These guard posts look like normal nests but rarely hold brood, being occupied instead by expendable elderly ants. “It can be said,” write Edward O. Wilson and Bert Hölldobler, “that while human societies send their young men to war, weaver-ant societies send their old ladies.”15
Once the battle is joined, the ensuing conflict involves thousands of workers circling one another, high on their legs, with raised abdomen and open jaws. When a worker seizes one of the foe, she releases a substance from a second gland unique to weaver ants. This “short-range” pheromone secreted from the sternal gland diffuses and dissipates quickly, like an emergency flare, inciting nearby ants to rush to her aid. (She uses the gland to similar effect when seizing prey.) Mortal conflicts do not continue indefinitely; eventually, a “no-ant’s land” emerges between colonies.16 Defined by the fecal droplets, these perimeters can remain in place for a long time. Similar buffer zones have been common among both humans and aggressive species such as chimpanzees in hotly contested areas with large populations. Other social species don’t defend a specific property. The honeypot ants of the American Southwest fight not so much over land as over shifting patches of food.17 This