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By Root 523 0

task is normally made up of the ant colony’s own workers. But why work? If we consider the slavemaker colony as a superorganism, it’s as if a creature obtained its arms not by growing them but by grafting on fully formed limbs that it pulled off someone else and can replace whenever it needs to, something like the horticulturalist who grafts a branch of one apple tree onto the trunk of another. Apart from the effort required to make this graft—or steal the slaves—in the first place, the outcome is cost free and pays off handsomely as long as each slave brings in more food than the meat she would have provided had the slavemakers eaten her instead of letting her live. By this measure, a slave probably nets the colony a profit in a few days, with an hour-long raid yielding many thousands of hours of services by the ants enslaved.

The graft has the most value when the initial cost to the colony of procuring it is least. Capturing pupae is the key. Although nursing the larvae is labor intensive, pupae—ants at the nonfeeding stage between larva and adult—yield a new slave crop quickly and with almost no effort. In fact, the slaves raise only pupae to become new slaves; the few eggs and larvae sacked by their masters become their food.13 These snacks reduce the number of mouths the colony has to feed. There’s no explanation for why the slaves make this decision about what to eat. But the slaves even consume the pupae if they are hungry enough, as always regurgitating a portion of each meal to the Amazon workers.

When honeypot ants engage in ritualized combat over food, the colony that draws fewer big workers retreats, often without injury. By standing on a pebble, the worker at right is “cheating” to appear larger than she is, driving off a larger opponent near Portal, Arizona.

These observations are a clue that sustenance and slave raiding are linked: after all, raids bring in the slaves that will in turn bring in the food (or, at the whim of other slaves, end up as food themselves). Amazons raid less often when there is a food glut, which makes sense, given that a well-fed colony probably already has an adequate supply of enslaved foragers.14 Future research should resolve whether raids are prompted by hungry slavemakers and slaves. If they are, eating more of the booty may be a means of sustaining a colony during the time between procuring brood and turning it into foraging slaves.

SEASONAL STRIKES

Slavemakers are commonplace in the temperate zones, even in suburban backyards. But their absence in tropical climes is an enigma. There are theories, of course. One has to do with numbers of ants. Temperate-zone ants frequently subject to enslavement, such as Formica and Temnothorax, tend to be superabundant and yet easy to attack. Despite the large number of ants in the tropics, few tropical species are as plentiful as Formica and Temnothorax, and the ones that are, like the weaver and marauder ants, are extraordinarily well defended.

I once traveled a few weeks on the Paria Peninsula of Venezuela and in the Arima Valley of Trinidad with Robin Stuart, an expert on the North American slavemaker ants that reside in acorns. We spent our time watching little brown ants called Nesomyrmex come and go from nests in the hollow twigs of roadside shrubs—while trying to ignore the roaring logging trucks at our backs. These weren’t attractive locations, but Robin and I had decided Nesomyrmex were promising candidates for enslavement, and it would be a coup for us to find ant slavery in the tropics.

Nesomyrmex interested us not only because they are common and innocuous but also because they are cousins to the temperate Temnothorax, or acorn ant. By occupying a number of acorns, a single acorn ant colony is fragmented into isolated housing units (polydomy), each of which may have one or more egg-laying queens (polygyny). Both these attributes may increase the openness of the colony to invasion by outsiders, including slave raiders. Robin and I found that Nesomyrmex colonies are likewise characterized by polydomy and polygyny, and