Babies go on the bottom in the social structure of ant life rafts


The State Column, | February 20, 2014

When ants make rafts the babies and workers go on the bottom to make the raft more buoyant and protect the queen.


It is not unusual for social animals to work together for the good of the group when faced with catastrophic danger. Ants though seem to take this to a whole other level according to a study published in PLOS ONE by the University of Lausanne, Switzerland’s Jessica Purcell.

According to Purcell’s research, when faced with a flood, ants will not only make rafts with their bodies but the raft seems to be constructed based on a sort of social hierarchy with the queen ant on top and in the middle, protected on all sides.

The rafting behavior in ants has been observed for a long time. Researchers have long marveled at the speed and coordination with which ants can construct a living raft as flood waters appear. When constructing these rafts, the ants weave themselves together so swiftly and tightly that water cannot penetrate it. The queen, in other words, likely does not even get wet. Photographs and video of this behavior among fire ants can be found via the Georgia Institute of Technology’s Ant Lab.

While the behavior itself has been well documented though, little was known about the composition, shape and social structure of these rafts. To study the behavior, researchers collected ants from a Swiss flood plain and brought them back to replicate flood conditions in the laboratory.

“It was an interesting contribution. No one had really looked at this idea of the brood as a flotation device,” said David Hu, a mechanical engineer at the Georgia Institute of Technology who was not involved in the study told the LA Times. “It adds a level of sophistication to the rafts that was previously not understood.”

The flood conditions were simulated using ant populations containing different combinations of worker ants, broods (developing larvae and pupae) and queens. In addition to the social structure, the buoyancy and recovery ability of the ants was also observed.

The worker ants used the brood ants at the base for buoyancy to create a raft. Using the young as the base for the raft may seem cruel to humans but researchers found that both the brood ants and the worker ants had a high rate of survival. From the perspective of an ant colony the most important thing is to protect the queen, without whom there can be no new brood or worker ants.

Researchers believe that placing the brood ants at the base may aid in keeping the colony together during a flood and is not as dangerous as previously assumed by scientists.

“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,” said Dr. Purcell.

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