Aside from humans, there are few architects more impressive than ants, says Will Newton. These diminutive designers are known for constructing some of nature’s most spectacular structures, from interconnected, subterranean cities spanning 6000km, to pizza-sized rafts capable of floating down rivers for several weeks.
As a group, ants comprise more than 14,000 species, all of which live in complex, cooperative colonies that, in some cases, are made up of hundreds of millions of individuals. Together, these colonies construct impressive structures, with ‘workers’, ‘drones’, and ‘soldiers’ working, and sometimes sacrificing themselves, at the behest of a ‘queen’ (or multiple queens in the case of ‘polygynous’ colonies).
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Many species of ants are known for their architectural talents, but none are quite as impressive as fire ants (Solenopsis invicta). These ants, which are native to South America but now invasive in the US, Australia, and China, are the Michelangelos of the ant world, known for constructing awe-inspiring structures out of the strangest of materials - themselves.
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For over a century, fire ants have been documented constructing living, waterproof rafts to survive floods. To ensure these makeshift arks don’t break up into hundreds of drifting dinghies, these ants will interlock their legs and jaws, creating strong, cohesive structures in a matter of minutes.
At the centre of these rafts lies the colony’s queen and her larvae, while at the edges other ants work tirelessly to keep the whole operation afloat. A single raft, made up of anywhere between 1,000 and 100,000 individuals, can stay afloat for several weeks, though some reports suggest they can potentially last for months. However, in order for the colony to return to restart, it must eventually reach dry land.
How do fire ants form a raft?
A 2011 study published in the Proceedings of the National Academy of Sciences (PNAS) studied this peculiar raft-building behaviour in scrupulous detail. The study’s lead author, Nathan Mlot, started by dumping balls of 1,000 to 8,000 fire ants into trays half full of water to see how they’d react.
As they hit the water, the ants spread outwards, reorganising themselves into a flattened, circular raft. When Mlot removed ants from the top layer, he observed others moving up from the bottom layer to replace them - the ants were ensuring their raft maintained a constant thickness.
A raft made up of thousands of bodies that are denser than water doesn’t seem particularly buoyant, but these ants have a trick up their sleeves; or more accurately, hairs on their sleeves (and the rest of their bodies). These hairs roughen up the surface of an ant and form a water-repellent shield, known as a ‘plastron layer’.
When fire ants scramble to form a raft, they trap tiny air bubbles between their hydrophobic bodies. Mlot worked out these air bubbles make a raft of thousands of ants 75% less dense than an individual ant and, crucially, less dense than water. These rafts are so buoyant that even when Mlot attempted to push them down with a stick, they refused to sink.
As well as repelling water, an ant’s ‘plastron layer’ also allows them to breathe underwater, effectively creating their own scuba suit. This means those unlucky enough to find themselves at the base of the raft don’t drown and can continue supporting the floating colony until it reaches dry land, or until it's ripped apart by predators.
While Mlot’s stick wasn’t able to sink a fire ant raft, bluegill, bass, and other opportunistic surface-feeding fish face no such problems. As these rafts float down rivers, fish will pick at them like they’re an all-you-can-eat buffet. The more ants that are consumed, the less buoyant the raft becomes. After a certain point, the raft will break apart and the colony will be lost.
However, fire ants may be the ones who have the last laugh. According to fish farmers in the US, large stocks of bluegill have washed up dead with bellies full of fire ants. It’s unclear exactly how these fish died, but considering the potent venom of fire ants some think they may have been ‘stung to death’ from the inside.
Like several other types of ants, such as army ants, fire ants also make living bridges. They create these structures in a similar way to the aforementioned rafts, interlocking their legs and jaws to create a bridge that others in the colony can then scurry across. These bridges can span more than 10cm and help the colony to cross difficult terrain.
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