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Slavery in Ant Hives

20 Nov

It’s a story about raiders, slavers, and war. There are chemical weapons, propaganda, and kidnapped children. An imposter kills the queen and takes her place. A group of slaves revolts and begins to kill the slaveowner’s children. Most of it takes place in the underground. No, literally. Under ground. It’s a story about ants.

Ants, you see, are social creatures and several different taxa, totaling over 200 species, have evolved complex symbiotic relationships, including parasitism. But unlike the physiological parasites I discussed before, these parasites don’t steal nutrients from their host.

They steal offspring – they’re kidnappers.

Not pictured: The tiny ransom note

This kind of parasitism, known as dulosis, starts with the parasites start by finding a large, healthy host hive. Like thieves targeting millionaires, healthy hives have the most to lose and so make the most valuable targets. Then, once the parasites have amassed together, they attack.

The attacking force is usually smaller than the host hive, but they have a secret weapon. Ants have terrible eyesight and can’t hear very much, so they communicate through smell. There are signals for ‘family’, for ‘enemy’, for ‘food’, and so forth. The parasites exploit this by bombing the host hives with propaganda chemicals that say ‘PANIC’. Immune to their own weapons, the parasites march through the scattered, confused defenses until they find the host’s pupae (an immature form, similar to a caterpillar’s cocoon). The parasites grab as many as they can and escape back to their own nest.

Normally you might expect that the pupae would be eaten, but they’re not. Instead the parasites induct the pupae into the hive as slaves. Raised by the parasites, the slaves don’t know the typical ‘family’ or ‘enemy’ smells and so accept the parasites as kin. They feed, clean, and work for them as if they were sisters. Some parasite species have gotten so good at capturing slaves that they’ve become totally dependent on their hosts. They can’t even feed themselves, instead devoting themselves wholeheartedly to more attacks and more raids.

Even their queens have become dependent on the host species. In at least one species of parasite, Polyergus breviceps, the newborn queens will seek out healthy host nests, infiltrate them, kill the host queen, and take their place, coating themselves in the scents of the deceased queen. The workers don’t know any better, eventually raising her children as their sisters.

However, this entire relationship is tenuous. The propaganda signals, tricking the pupae, and infiltrating healthy hives all depends on the parasite being able to perfectly mimic the host species’ language of smells. If the panic signal doesn’t work, the host hive will destroy the attacking force. The two species have thus become locked in an evolutionary game of catch up. The hosts are evolving new, different signals and the parasites evolve to mimic them. Even their control over the slaves may depend on it. Already-captured slaves will sometimes begin to sabotage the parasites hive by killing the parasite’s own pupae – it’s thought that this behavior stems from an imbalance in the chemical race.

This evolutionary relationship seems to be unique to the complicated social systems found in ant hives. Any sufficiently complex system will have emergent patterns of behavior and exploitation, but this level of sophistication and dependence is remarkable. Many different scientists have examined this in the past and it continues to fascinate others.

***

Sources:

Carroll, C. R. and D. H. Janzen. 1973. Ecology of Foraging by Ants. Annual Review of Ecology and Systematics 4; 231-57.

Pohl, S. and S. Foitzik. 2010. Slave-making ants prefer larger, better defended host colonies. Animal Behaviour 81; 61-68.

Wilson, E. O. 1974. Leptothorax duloticus and the beginnings of slavery in ants. Evolution 29; 108-1

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Posted by on November 20, 2012 in Uncategorized

 

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