Ants, like humans, are social organisms that work collectively in the face of adversity to protect themselves and their brethren. Floodplain-dwelling ants adeptly link themselves together and pile up forming structures resembling rafts to avoid drowning during floods. But researchers previously observed that they don’t do so randomly. Each member of the ant colony has a specific place in the raft according to their function and social position. The queen, not surprisingly, goes in middle of the raft, surrounded by the workers at the sides and the top, and the brood (larvae and pupae, the youngest members) strangely occupy what appears to be the most precarious—and potentially deadly—position in the raft: the base.
Jessica Purcell and her team from the University of Lausanne, Switzerland decided to investigate this unusual behavior by collecting Formica selysi ants from a Swiss floodplain and performing experiments in the lab. In particular, they wanted to find if the broods were sacrificed to protect the other, more valuable members, of the group. Ant colonies were divided into different groups, each containing the same number of workers but different numbers of pupae (brood) and queens. Then the team induced the ant groups to form rafts by simulating flood conditions in the lab after which they measured the buoyancy of the workers and brood as well as the survival of brood after rafting.
When brood ants were present, worker ants chose them over wood pieces to form the base of the raft and piled up in three to four layers on top of them. Unexpectedly, the researchers found that both brood and workers were highly resistant to submersion as most of them survived, despite being submerged for hours at the base. So the position may not be that dangerous after all. And it turns out that larvae and pupae are more buoyant than worker ants, partly explaining why they are chosen for the base, and therefore may serve as a more durable floatation platform. There is another advantage of placing brood at the bottom: workers from rafts with brood had fewer unresponsive workers during raft recovery compared with those without brood.
This study demonstrates that raft formation is highly coordinated among ant colonies to ensure maximal survival of all members of the colony.
Purcell J, Avril A, Jaffuel G, Bates S, Chapuisat M (2014) Ant Brood Function as Life Preservers during Floods. PLoS ONE 9(2): e89211. doi:10.1371/journal.pone.0089211