Worker Ants Build Rafts to Escape Floods and Protect the Vulnerable Queen

When hit by floods, ants display an extreme ability to assemble together as a group and form floating rafts using the brood's buoyancy to save themselves as well as the queen.

The study led by researchers from University of Lausanne, Switzerland, reveals the strategy used by ants to face floods and protect the queen including the larvae and pupae. The researchers observed that the ants build floating rafts and use the whole brood's buoyancy and the recovery ability to lower the rate of injury and death.  Also they place the queen in the middle of the raft and protect it from all sides.

The secret of how ants form stretchable floating rafts was revealed earlier in a study led by Zhongyang Liu, an undergraduate student in the laboratory of Georgia Tech mechanical engineer and biologist David Hu. They discovered that the fire ants re-form and realign constantly to withstand the external force that is applied on their rafts. Using their claws, jaws and adhesive pads on the legs, they emit an oily fluid that sticks to smooth surfaces. This property called viscoelasticity, helps them fight the stress and also bounce back to the previous position.

When alone in water, ants struggle to survive but when they join together they form a floating waterproof raft.  Ants forming rafts to survive is long known, but not much is known about the composition, shape and social structure of the raft. In this study the researchers discovered that the social animals get together and work in unity to boost the survival rate as well as the welfare of the group.

To gain a proper insight into the process of raft building, the researchers collected samples of ants from a flood plain in Switzerland and in a laboratory set up they induced floods for the ant population that consisted of worker ants, queen and broods (developing larvae and pupae).

The researchers noticed that during flooding, the worker ants and the broods were extremely resistant to submersion whereas the queen, the valuable nest mate, was highly vulnerable to submersion. Due to this the queen ants were placed at the center of the raft and broods were placed at the base of the raft and the worker ants used the buoyancy of these broods. The recovery ability of the workers to assemble quickly and create a raft lowers injury and death among the ants.

The highest survival rate was seen in workers and broods indicating that being placed at the base of the raft is not dangerous as believed earlier.

Dr. Purcell explained, "We expected that individuals submerged on the base of the raft would face the highest cost, so we were astonished to see the ants systematically place the youngest colony members in that positions. Further experiments revealed that the brood are the most buoyant members of the society and that rafting does not decrease their survival; thus, this configuration benefits the group at minimal cost."

The finding was published in the journal PLOS One and was funded by the Swiss National Science Foundation grant.