Humans aren't the only species that have had to deal with the issue of slavery. Some species of ants also abduct the young of others, forcing them into labouring for their new masters. These slave-making ants, like Protomagnathus americanus conduct violent raids on the nests of other species, killing all the adults and larva-napping the brood.
When these youngsters mature, they take on the odour of their abductors and become the servants of the enslaving queen. They take over the jobs of maintaining the colony and caring for its larvae even though they are from another species; they even take part in raids themselves. But like all slave-traders, P.americanus faces rebellions.
Some of its victims (ants from the genus Temnothorax) strike back with murderous larvae. Alexandra Achenbach and Susanne Foitzik from Ludwig Maximillians Universty in Munich found that some of the kidnapped workers don't bow to the whims of their new queen. Once they have matured, they start killing the pupae of their captors, destroying as many as two-thirds of the colony's brood.
Ants that are targeted by slave-makers take massive hits to their colonies and they are under intense pressure to resist these marauders. But all the defences discovered so far happen before the raids have been successfully completed. They involve better fighting skills, quicker reaction times when enemies are spotted, hastier escapes and so on.
Some scientists have suggested that strategies like this would be impossible to develop because the enslaved workers are caught in an evolutionary trap. Far away from their own colony, and sterile themselves, there is no way for them to increase their reproductive success. But Achenbach and Foitzik have rejected this idea - their conclusion is that by conducting assassinations within their new home, they severely reduce the slave-makers' numbers and their ability to conduct raids. That safeguards the future of their relatives.
Achenbach and Foitzik collected 88 colonies of the slave-making P.americanus ant that had abducted workers from three species of Temnothorax. They found that the workers clearly care for the larvae, and nearly all of them were raised until their pupated. But at that point, the slaves' behaviour changed dramatically, taking on a more homicidal bent.
Two-thirds of pupae died before they hatched. The mortality rate was even higher (83%) for pupae containing queens, but very low (3%) for those containing males. The duo saw that the captives were deliberately killing the healthy pupae. In about 30% of cases, as in the photo, the workers would gang up to literally pull the developing ants apart. Another 53% of the pupae were killed by neglect, by workers who moved them out of the nest chamber.
These murders were solely the acts of the slaves. No P.americanus worker ever lifted a mandible against its own pupae. Nor are the deaths a reflection of a generally poor standard of care on the part of Temnothorax. In their own colonies, the majority of pupae hatched, with just 3-10% dying before that happened.
This rebellion takes its toll on the slave-makers and may explain why the nests of P.americanus tend to be very small. The captives may never reproduce themselves, but they do their part for their relatives back home by crippling the workforce of the slave-makers. These indirect benefits are particularly pertinent to Temnothorax ants because a single colony can occupy many different nests - a family of sisters spread out over a large area. If one nest is raided, it pays to ensure that none of the related nests are targeted later.
Other species use similar strategies to reject attempts by parasites to usurp their parental efforts. For example, cuckoos all over the world lay eggs with varyingly strong resemblances to the eggs of other birds, in an attempt to turn them into unwitting foster parents. Hosts develop sharper recognitions skills and cuckoos develop more strikingly matched eggs. In Australia, this arms race has escalated to the point where the superb fairy wren has adopted a different strategy - it ignores the egg and recognises the young bird, killing it if it turns out to be a bronze cuckoo.
At the moment, P.americanus is on the losing end of a similar evolutionary arms race with its hosts. But it's an old parasite with a long history with Temnothorax. Achenbach and Foitzik believe that the rebellious slaves are a recent countermeasure against the problem of slavery and the impetus to evolve resistance is now on the slavers. They need to develop counter-adaptations to prevent most of their brood from dying at the jaws of their captives. The war is not yet over.
Reference: Achenbach, A., & Foitzik, S. (2009). FIRST EVIDENCE FOR SLAVE REBELLION: ENSLAVED ANT WORKERS SYSTEMATICALLY KILL THE BROOD OF THEIR SOCIAL PARASITE Evolution, 63 (4), 1068-1075 DOI: 10.1111/j.1558-5646.2009.00591.x
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