Ants Use Buoyant Larvae and Pupae as Base For Living Ant Rafts

Posted on February 27, 2014

Ants building a raft

Some ants species, including fire ants (Solenopsis invicta) and Formica selysi, build living rafts to escape during a flood. Researchers led by Jessica Purcell from University of Lausanne, Switzerland conducted a series of experiments where they slowly raised the water level on a group of ants in the lab. The researchers found that ants tend to place the brood at the bottom of the raft. The queen is placed in the middle and protected all around by rafting ants.

The researchers say ant larvae and pupae are more buoyant than worker ants and help support the living ant raft when placed at the base. The researchers also found that worker ants on rafts with brood used as the base are able to recover more quickly when reaching land, which is important for the overall survival of the ant colony.

Dr. Purcell said in a statement, "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."

Here are videos of 60 worker ants building a raft in different experiment. In the second experiment, the ants were also given wood cylinders to work with. The ants used all the brood in the base. They did not initially use the wood for the raft, but did add some wood pieces to the perimeter of the raft once it was afloat. The video is played at 64x speed. Take a look:

National Geographic has a longer video of a bigger fire ant raft here. This video shows the ants pulling the larvae back out of the water once they reach dry ground. There is one drawback to being on the bottom of the raft as fish feed on the floating live ant raft from underneath.

Photo: Jessica Purcell