An insect’s guide to slavery

Having studied ants for the past 4 years or so, I can say with some confidence that whatever invention of human culture you can think of, ants did it first.

Bridges engineered to connect point A to point B? Check. Agriculture? Check and double check. Suicide bombing? Check. Poetry? Nobody’s discovered it yet, but that doesn’t mean they’re not doing it.

But one of the most intriguing behaviours of a handful of ant species is the habit of turning other ants into slaves.

That is, a colony of one species steals eggs and larvae (and in a few rare cases, adults) from a colony of another species, brings them home, and then uses them to gather all the food, raise the children and keep the nest clean.

This form of parasitic behaviour carries clear advantages for the slave-making ants, namely the chance to avoid work, and fairly clear disadvantages for the enslaved ants, which lose their offspring and sometimes a significant portion of their workers if they resist a raid. Research into this area has revealed that slave ants brought up in the slave-making colony ‘imprint’ on their new dictators – much like some newly hatched birds imprint on what they assume to be their parents – helping to prevent mutiny in the slave-makers’ nest.  (Although yes, sometimes the slaves do rebel and kill off their overlords.)

But a curious matter is just how a slave-making ant from one colony can walk into another ant’s nest and walk out again carrying a bundle of eggs or larvae without Ant Wars breaking out and killing everyone. Ants are not peaceful creatures, and although some species flee before intruders, many defend their nests with a combination of acid sprays, painful stings and limb-tearing jaws. A small team of scout ants searching for new slaves therefore faces a potentially fatal, or at least horribly maiming, situation on walking into an enemy nest… So how do they pull it off?

Just last week, an answer to this question was offered in an article published in the Journal of Evolutionary Biology. According to researchers at the University of Mainz in Germany, at least one species of ant is using its own brand of mind-bending drugs to turn its victims on each other.

The researchers used two sorts of ant in their study: Temnothorax, a relatively aggressive little ant, and its common slave-making parasite, Protomognathus. To find out how the slave-makers might be overpowering their victims, the researchers first extracted chemicals from secreting glands in Protomognathus. Then, they isolated an unfortunate worker from each Temnothorax colony, doused it with the chemicals as if it had been sprayed by the slave-maker, and put it back into the colony.

The rather gruesome effect of this chemical cocktail was to make other Temnothorax ants from the same colony start treating the doused worker as an extreme threat (or more graphically, “stinging, biting, holding or dragging”).

The researchers also found that although Temnothorax workers weren’t oblivious to them, slave-making Protomognathus ants were much more likely to escape the nest unharmed in cases where Temnothorax were occupied attacking their own, weird-smelling nestmates.

In combination, then, the results indicate that the intruding slave-makers spray Temnothorax workers with a sort of aggression-inducing chemical, which causes them – by mechanisms unknown – to start attacking each other. The slave-maker can then escape during the confusion, leaving its bewildered victims to fight until the chemical wears off.

This little trick comes under a broad category of behaviours referred to as parasitic manipulation – by messing with a victim’s normal behaviour, the parasite gains an advantage. It’s thought that all sorts of insects and their (usually insect) parasites become locked in evolutionary ‘arms races’ to control behaviour. The parasite continually evolves to manipulate its victim, just as the victim struggles to evolve resistance.

But the peculiarity of ants’ highly social, complex behaviour, and the ways in which this behaviour parallels or contrasts with our own, have always made them particularly interesting to researchers and the public alike.

And thus I leave you with the words of one Dr Henry McCook who, at a friend’s garden get-together in New Jersey in 1887, became absorbed in the daily struggle between slave and slave-making ants. His account of a slave-making ant scouting for new nests is published (in the third person, as was the fashion back then) in the Proceedings of the Academy of Natural Sciences of Philadelphia under the title, “Modification of Habit in Ants through Fear of Enemies.”

… Just then a single Sanguine [slave-making] warrior approached the spot. It walked around the nest, which was indistinguishable from the surrounding surface; sounded or felt here and there with its antennae; passed over the very door into which the Schauffuss [usually enslaved] ant had disappeared, and although its suspicions were strongly awakened, it at last moved away.

The speaker felt satisfaction that the Sanguine depredator had thus been baffled and that the instinct of home protection had proved too much for the wretched kidnapping cunning. However, his pleasure was somewhat clouded by the reflection that the slave-making scout would probably be back before long, accompanied by the host of its fellows, and do its work more surely.

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