Admire ants

Enthusiasts have long admired the shipbuilding skills of fire ants (Solenopsis invicta). Especially in South America, the animals navigate rivers with raft-like structures that consist solely of members of their own species.

Sometimes only 500, sometimes millions, of the social insects come together to survive floods by swimming or to explore the world on week-long journeys and to open up new settlement grounds. So far, however, a technical question has not been clarified: Why don't they go under?

Some of the ants, which are up to 4.5 millimeters long, can walk over water - thanks to its surface tension and individual air bubbles adhering to the body. But if thousands of ants stand on top of each other, their collective weight should, according to the laws of physics, actually lead to destruction.

Only now have engineers led by Nathan Mlot from the Georgia Institute of Technology clarified the secret of the unsinkable ant rafts (PNAS, online) by first shock-freezing the animals with liquid nitrogen and then examining them in detail with a scanning electron microscope. In doing so, they found that the insects cling to each other with their mouthparts and hind legs so tightly that an extremely dense structure is created that no water can penetrate.

The air between the ants then provides the necessary buoyancy. The entirety of the animals forms a cooperative air mattress, so to speak, which even has a decisive advantage compared to man-made watercraft: it can repair itself.

When the researchers removed individual ants from the swimming pool, ants immediately crawled up from below to close the gap and thus preserve the thickness of the raft and its load-bearing capacity. The speed with which the animals joined together to form a raft was also impressive: in the laboratory tests with 1,000 to 7,000 ants, these only took around 100 seconds.

According to the authors of the study, these properties of the ant raft once again support the thesis that ant colonies cooperate so well that they can also be understood as individual superorganisms: They can assemble themselves and also heal themselves, as is otherwise only possible in the animal world from individual, knows closed organisms.