It has long been known that when some ants are faced with a flood, they build “living rafts” that float on the water in search of land. The earliest reference I know of to this phenomenon comes from the book Insect Architecture, which was published in 1838:1
The ants consisting of the basis of this group, lay hold of some shrub for security, while their companions hold on by them; and thus the whole colony, forming an animated raft, floats on the surface of the water until the inundation (which seldom continues for longer than a day or two) subsides.
So scientists have known about this for a long time, but how such rafts manage to float has been a mystery. You see, a single ant can float on water for two reasons: First, water has surface tension, and that tension must be broken in order for something to sink. If laid carefully on the surface of water, for example, a metal needle will float. Even though the density of the needle indicates it should sink, the water’s surface tension will keep it afloat. Second, the water-repelling nature of an ant’s outer covering (its exoskeleton) causes tiny bubbles of air to cling to it. The combination of the air bubbles’ buoyancy and the water’s surface tension keeps the ant afloat, but not by much.
Now…if I start stacking ants on top of each other, only a few of them will be in contact with the water. The water’s surface tension combined with the buoyancy of the ants that are actually in contact with the water just isn’t enough to keep the whole stack of ants afloat. Thus, something else must be going on when ants get together to form rafts.
A recent report on fire ant rafts was just published online in the Proceedings of the National Academy of Sciences, USA. It tells us what is actually going on in ant rafts, which is really quite fascinating.
Nathan J. Mlot, Craig A. Tovey, and David L. Hu used time-lapsed photography to carefully watch the process by which fire ants form rafts. They found that the ants which are in contact with the water grasp each other by the mandibles or the legs, and then they pull each other together with a strength of up to 400 times their own weight! That would be like me pulling with a force of more than 82,000 pounds!2
What does this do? It allows them to trap air bubbles underneath them, and those air bubbles form a water-repellent surface that is significantly more efficient than the sum total of each individual ants’ water-repellent exoskeleton. The authors state it this way:
We find that ants can considerably enhance their water repellency by linking their bodies together, a process analogous to the weaving of a waterproof fabric.
Interestingly enough, the researchers found that when a portion of the raft sinks because of a water disturbance or an unexpected increase in weight, the ants that end up underwater start pulling even harder on each other. This forms an even more water-repellent surface, allowing that part of the raft to float again.
The authors make the point that when scientists look at nature, they generally think of water-repellency as a static thing. Either something is water-repellent or it is not. In addition, the amount of water-repellency is fixed. A leaf, for example, is water repellent because it is covered with a waxy cuticle. The water repellency is based solely on the thickness and makeup of that cuticle. However, this research shows that in some cases, the water-repellency of an organism is dynamic. It can change based on the behavior of the organism.
The authors suggest that engineers can learn from how ant rafts are formed, allowing them to build better lifejackets, boats, raincoats, etc. That’s not surprising, of course. Any engineer can learn a lot by studying the work of the Master Engineer!
1. James Rennie, Insect Architecture (London: Charles Knight and Co, 1838), p. 260
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2. Nathan J. Mlot, Craig A. Tovey, and David L. Hu, “Fire ants self-assemble into waterproof rafts to survive floods,” Proceedings of the National Academy of Sciences, USA doi: 10.1073/pnas.1016658108, 2011.
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