Adventures Among Ants - Mark W. Moffett [41]
New World Eciton burchellii army ants take this trait to an extreme: their nests are called bivouacs, because the only physical structures are the bodies of the ants themselves, as a half million or more gather under a low branch and form a hanging, bushel basket–sized mass of interlinked bodies. (Other army ants form similar chains within their underground enclaves.) As Reverend Savage might have predicted from the simplicity of Eciton burchellii encampments, the colonies of this species and a few others migrate with great regularity, every day for weeks at a time. It’s thought that as their armies became more effective in the ancient past, army ants tended to exhaust the supplies of prey near their nest, forcing the evolution of such roaming behavior in response to a recurrent need for fresh hunting ground.13 No surprise, then, that one army ant species has been shown to migrate more often if the colony is underfed.14
The plainly nomadic Eciton burchellii has been a research favorite and has dominated our perceptions of army ant behavior. The evidence suggests, however, that other army ants vary in their nest movements and that the species that relocate in a regular migratory cycle represent the minority. Some early naturalists, who had the luxury (rarely afforded modern researchers) of remaining for years at a site, recorded army ant colonies staying put for many months.15
If my assessment is correct, many army ants may be no more nomadic than the many other ant species that migrate periodically, and sometimes on a regular basis.16 For species with colonies of a few individuals, relocations can commence, as they do with Bedouins, at any provocation, as appears to be true of Myrmoteras trapjaw ants whose nests in crannies in forest litter require little construction. At least some ants seem to be driven to pull up stakes by food requirements or outright hunger. A nomadic mushroom-eating ant in Malaysia, for example, changes its nest most often when its food runs low.17 With the marauder ant, the connection between migration and foraging remains uncertain, but in all likelihood this species doesn’t eat itself out of house and home as often as the more predatory army ant, permitting extended residences in one place. As with other wayfaring ants, when the need for a migration arises, the establishment of reliable thoroughfares to the new nest is the cornerstone of its success.
CREATING A NEW COLONY
Marauder ants and army ants share a common strategy for mass foraging and to some extent a proclivity for moving nests. But they have very different strategies for establishing new colonies. In most garden-variety ants, the young queen flies from her mother colony to mate. The foundress snaps off her wings upon arriving at her destination and then digs a nest chamber. In it, she rears her first crop of workers unassisted. (In some species the queen forages at this stage, but more commonly she doesn’t leave her chamber and survives off her body fat.) As soon as these few ants mature, they take over all the labor and begin the first tentative foraging expeditions, leaving the queen to lay eggs, basically the only task she will accomplish for the rest of her life.
The queen of an army ant colony, in contrast, does not grow wings or fly away. Instead, through a system known as fission, one of the queen’s daughters inherits half the colony and takes it as her own; the other half goes its own way with the original queen. Because even a start-up colony has thousands, if not hundreds of thousands, of workers, army ants never have to deal with problems of a meager labor supply. From its inception, a colony can always count on a huge contingent for its raids.18
How marauder colonies get their start, however, is a mystery. This much I know: their queens are tough, and they are excellent runners during migrations, but otherwise