Masters of subjugation

Slavery is unarguably a terrible thing that has sadly ruined the lives of countless human beings throughout our violent history. Slaves were used to build the Egyptian pyramids, constructed the Mayan temples and were even used as a source of profit for many European empires during their colonial histories. Fortunately however, slavery is now illegal and has been banned for centuries in many countries. Great Britain for example, passed the Slavery Abolition Act in 1833 that banned slaves being possessed or sold anywhere within the British Empire and its dependencies. Due to its negativity and historical importance, the concept of slavery isn't new to anyone. What may surprise you however, is that humans are not the only animals to have enslaved others and certain species of ants run their entire colonies using only conquered workers!

Rather unimaginatively, such species have been termed 'slavemaker ants' and can be found across the Americas, Europe and some parts of Asia. Slavemaker ants are usually rare, but highly successful species and colonies of around 3, 000 of the ants can have as many as 60, 000 slaves working for them that cater for their every need! In fact, the genealogy of slavemaker species shows just how successful they are as most species are completely unrelated to each other. This fact suggests that their behaviour has evolved independently on several different occasions, so enslaving others must be a beneficial and rewarding way for ants to live. 

Formica sanguinea is a species of slavermaker ant that is native to the British Isles, being found most commonly in the Scottish Highlands and in the south-east of England. The species is unusually large, growing to over a centimetre long, and organises itself into multiple 'platoons' of about 100 workers before an assault. Eggs and workers from the attacked nest are carried back to their 'mother nest' to be enslaved with pheromones secreted by the slavemaker queen.

Although subjugating behaviour is clearly advantageous for slavemaker ants now, why it evolved in the first place is confusing. This is because colonies of slavermaker ants are typically very small and prefer to attack only the biggest and most strongly guarded colonies of other species. As well as carrying high risk for the attacking ants, which may be wiped out in the attack (thus ending the colony as all of its members are required for each assault), the urge to enslave has come at a price and slavemaker ants are incapable of running a colony by themselves. In fact, slavemaker workers seem to have completely forgotten the basic foraging, building and nursing behaviours seen in all other species of ant and only their queen seems to be able to function normally (who lays new eggs of slavemakers).

The loss of these normal behaviours have come at the gain of new ones however, and slavemakers have many different tactics for invading nests. Their tactics are usually used to reach one of two different goals: some species invade an existing nest and take up residence there, while others destroy a nest and carry their unborn young/survivors back to their own queen.

Slavemaker invasion of existing nests is usually carried out using 'distraction' techniques. In such assaults, the majority of slavemaker workers attack the nest to provide a diversion for the colony and draw their soldier class into focusing on them. The pre-mated slavemaker queen then uses the ensuing medley to sneak into their nest with a small raiding party and kills their actual queen, whom she then mimics. This is accomplished by rolling in her pheromones (the chemicals that ants use to identify those of their own colony from hostile invaders) and thus, when the assault is over, the workers of the nest are none the wiser to the exchange and care for the slavemaker queen as their own while she produces more slavemaker ants.

Slavemaker species that seek to completely destroy a nest often have clever biological tricks up their sleeves as their numbers are normally too few to win in an all-out assault. Saying this, it should be noted that some species of slavemaker ant like F. sanguinea have grown to be very large and thus, do rely merely on brute force. Most species don't however, and perhaps the most interesting method used is seen in some South American species, which secrete a chemical that causes ants to abandon their nest. Once empty, the slavemaker workers simply enter it and take the unhatched pupae, carrying them back to their own 'mother nest'. Once these ants hatch, they accept the slavemaker queen's pheromones as their own and spend their lives serving her and her nest.

So there you have it! Ants that conquer the nests of other species and enslave their workers for their own colony's survival! Unlike human slavery however, this is unlikely to stop and ants will likely continue to master the art of subjugation for years to come...