Fire ants can form impressive rafts made up of hundreds of ants. However, rather than this being the result of collective social behaviour as was previously thought, experiments suggest the insects are forced together by a fluid phenomenon called the Cheerios effect. If there are 10 or more nearby ants on water, the effect becomes too strong to overcome.
When an ant nest gets flooded, ant colonies can survive by banding together to form rafts. These are buoyant enough to float for weeks on end and other than the ants at the very bottom, most ants in the raft stay dry.
at Princeton University in New Jersey and his colleagues looked at how fire ants (Solenopsis invicta) behave when placed on water.
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Over a series of 72 experiments, they placed groups of between two and 158 ants into a small aquarium. When there were only a few ants, they approached each other without coordinating their motion. Typically, they pushed each other away by flailing their legs, rarely forming rafts. If they did, the rafts lasted less than 2 minutes.
However, when the researchers added 10 or more ants, the insects intertwined their bodies into more stable rafts. These rafts lasted until the team stopped the experiment after 5 minutes.
To try to understand what was going on, Ko and his colleagues developed a computer simulation where they approximated the ants as small spheres. They gave the spheres similar properties to the ants 鈥� such as drag 鈥� by measuring comparable values in the ants first.
The simulation confirmed that 10 ants is a threshold number for forming stable rafts. Ko says that this is mostly due to the so-called Cheerios effect. Named after the cereal, the Cheerios effect describes how small objects on the surface of a fluid are pushed together by forces that minimise how much the surface gets distorted. When there are 10 or more ants, the Cheerios effect is more powerful that the ants pushing each other away and so forces them to bunch up into a raft, says Ko.
at the University of Louisville in Kentucky says that though we think of ants as social animals, they rarely opt to be very close to each other in nature. Even when species like army ants coordinate to form bridges, they touch each other minimally and deliberately instead of 鈥渉ugging鈥�, he says.
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