Ants make their rafts float by putting the buoyant babies at the bottom

If you decided to build a raft, where would you put your young? Would you stick them in the middle of the raft to protect them, or place them around the base of the raft to help it float? That depends on whether you're an ant.

Many species of floodplain-dwelling ants form rafts to surf the flood waters, and a new paper published in PLOS ONE looks specifically at the position of ants of different castes in those rafts. The researchers collected members of Formica selysi and examined how they form rafts when ants of different castes and different buoyant materials are available. They expected that the ants would protect the most vulnerable or valuable members of the group by placing them in the center of the raft, and they did find that the queen was always placed at the center of the raft out of the water. However, they were surprised to find that members of the brood were invariably placed at the base raft, even if there were enough adult workers to keep the brood items out of the water.

So why put baby on the bottom? For one thing, the researchers observed very little mortality in these rafts, suggesting that, even after being submerged for long periods of time, the brood items are not at risk for death. The brood items also happen to be more buoyant than the adult ants, suggesting they may serve as better flotation devices. Another possibility is that it's important to keep a certain proportion of adult worker ants above the water, so that, once the need for the raft has passed, they can recover more quickly and get to back to work while the brood rests from its rafting adventure.

Photo from PLOS ONE.

Ant Brood Function as Life Preservers during Floods [PLOS ONE via PhysOrg]