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...

The amazing acid-proof ant

Most terrestrial insects actively avoid liquids, particularly water, because they are too small to escape from its surface if they fall in, owing to the strong hold that the liquids surface tension holds over them. Even larger insects are no different and dislike water, even though some of the bigger species may be able to paddle around to some degree and eventually escape (if they are not eaten by a bird or a fish first).

The diver ant, Camponotus schmitzi, however is a species of carpenter ant that goes against the insect social norm and actually depends on water to survive. Well, depends on acid that is; not water... The fearless ant lives in an exclusive mutualistic relationship with Nepenthes bicalcarata, a species of  carnivorous pitcher plant that lives in Borneo, where it actually dives into the plants digestive juices in order to scavenge for its prey!

A small group of diver ants waiting underneath the lip of N. bicalarata until a insect drops into the acid pool at its base. The ants will then 'liberate' the plant of its meal, eating it themselves. Although this action robs the plant of its meal initially, the ants mainly scavenge large insects and throw back what they don't eat. This also helps to remove debris from the acid pool, keeping it fresher and therefore more attractive to potential prey animals; meaning the ants also benefit the pitcher plant.

If you've never heard of pitcher plants before then you may be confused as to why they have a pool of acid in their base, but it's actually fairly simple - pitcher plants, like the more famous Venus Fly Trap, are carnivores that live by feeding off insects that they have lured into their trap. Whereas species of Venus Fly Trap clamp insects that have landed on their surface and then actively secrete acid to digest them, pitcher plants use a 'pitfall trap' mechanism. This involves a small hole in their top that their prey must climb into to reach the attractive-smelling puddle of liquid in their base. However, this liquid is actually a mixture of a powerful acid and other digestive juices and once the prey species has crawled inside, it is prevented from escaping by tough downward-pointing spines. Unfortunately, the hapless animal then faces a slow and grisly death where it is digested alive.

Nepenthes sumatrana, a species of pitcher plant that can be found throughout the Phillippines. Its bright colouration and sweet smell help to attract its prey. Some of the larger species of pitcher plant, like those found in the Amazon Rainforest, can even attract and eat mice or frogs as prey!

The observation of diver ants has found that they spend most of their time hidden under the lip of N. bicalarata and run down into the base of the plant when an insect falls into its deadly pool of acid and digestive juices. Amazingly, the ants then dive head-first into the acid and work together to pull out the now dead arthropod and begin to eat it in safety on the shore of the pool. Therefore, the ants still get a meat meal but do not have the same risk of injury that most predators face while killing their prey.

Or do they? This strange hunting method seems to have a major and very deadly risk - obviously, the pitcher plants acid should also kill the ants! Scientists are still baffled by why it doesn't and it is thought that the short exposure time of the ants to the digestive juices allows them to survive, along with some other physiological adaptation that helps provide them with some immunity to the corrosive effects of the acid.

One physiological adaptation that has been identified however, is that the ants have a fluid on their outer cuticle (layer of skin) that makes them less water-repellent and therefore, helps to negate the action of surface tension. This allows the ants to dive underwater and leave the acid pool freely and without this chemical surfactant, they would be trapped on the surface of the acid pool like other insects; eventually dying.

A unit of C. schmitzi workers dragging a dead insect (either a grasshopper or cricket) from the pitcher plant's deadly digestive juices.

What is also interesting, is that the ants appear to sprint through the acid rather than swim; using the same tripod sequence as when they move on land - they move the first and last legs on their left side along with the middle leg on their right side and then the front and last leg of their right side with the middle of their left. This simple method of swimming suggests that the ants have evolved fairly recently, since the ancient and truly aquatic insects have all developed much more complex patterns of swimming than this.

Thus, the ingenuity and unique adaptations of diver ants allow them survive in a very hostile organism that by all rights, should kill the ants; turning them into its next meal...