All ants on deck! Ants have shown something akin to veteran boatmen qualities when surviving flooding on their raft of interlinked bodies. In subsequent floods, they take up the same specialist positions as on their last rafting excursion – the first time such memory has been demonstrated in ants.

When individuals come together and work as a team they can overcome obstacles that seem insurmountable when considered from a single individual’s point of view. This is as true for social insects as it is for humans. Ant colonies are so well coordinated that worker ants can link their bodies together to form physical structures such as bridges, rafts and bivouacs. These allow ants to overcome environmental obstacles which might otherwise overwhelm their colony.

Alpine silver ants (Formica selysi) are commonly found on the floodplains of central and southern Europe, and are well known for their ability to form living rafts to carry their colonies to safety during floods. It may be a very simple rule (‘hang on to the ant nearest to you!’) that galvanises the colony to form a raft once rising water is detected. But within an ant colony, workers specialise in different tasks; such as foraging, caring for the brood or defence. And it now appears that similar patterns of specialisation occur when worker ants have to assemble a raft, suggests a new study from the University of Lausanne.

By simulating floods in the laboratory, researchers found that silver ant workers occupy specialised positions when building a raft; the same individual would consistently choose a place on the top, middle, base or side of the raft. The position which an ant chooses in the raft might be decided by factors such as age, body size or task specialisation. Larger workers responsible for foraging may ride on the top of the raft, while smaller nurse ants stay at the base of the raft with the eggs, larvae and pupae; for example.

The study also found that the structure of the rafts and behaviour of the workers was markedly different when ant pupae were included in the flood simulations. The pupae were placed on the base of the rafts, which were broader and contained fewer layers of workers than rafts without pupae. Pupae and larvae are large, and more workers have to be placed at the base of the raft to hold the wriggling juveniles together. So the structure of the raft and arrangement of workers has to change.

Remarkably, in subsequent trials without the presence of pupae, the workers’ behaviour and the structure of the raft reflected their previous experience of rafting with their young. This may be the first time that individual memory has been shown to play a role in the structure of such assemblages.

“These elaborate rafts are some of the most visually stunning examples of cooperation in ants,” said author Jessica Purcell. “They are just plain cool. Although people have observed self-assemblages in the past, it’s exciting to make new strides in understanding how individuals coordinate to build these structures.”

In a previous study in 2014, the researchers demonstrated that worker ants would protect their queen during flooding by placing her at the centre of the raft, and the pupae at the base of the raft. The pupae are the most buoyant members of the colony, and the arrangement ensures that the raft remains afloat. Workers and pupae at the bottom of the raft were found to have high survival rates, so arranging themselves in this manner is both sensible and effective in ensuring the survival of most of the colony.

references

Avril, A., Purcell, J., & Chapuisat, M. (2016). Ant workers exhibit specialization and memory during raft formation. The Science of Nature, 103(5-6), 1-6.

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