Fire ants self-assemble into waterproof rafts to survive floods

Abstract

Why does a single fire ant Solenopsis invicta struggle in water, whereas a group can float effortlessly for days? We use time-lapse photography to investigate how fire ants S. invicta link their bodies together to build waterproof rafts. Although water repellency in nature has been previously viewed as a static material property of plant leaves and insect cuticles, we here demonstrate a self-assembled hydrophobic surface. We find that ants can considerably enhance their water repellency by linking their bodies together, a process analogous to the weaving of a waterproof fabric. We present a model for the rate of raft construction based on observations of ant trajectories atop the raft. Central to the construction process is the trapping of ants at the raft edge by their neighbors, suggesting that some "cooperative" behaviors may rely upon coercion.

Fig. 1.

(A) A raft of 500 fire ants, composed of a partially wetted layer of ants on the bottom and dry ants on top. (B and C) Scanning electron micrographs of links between ants in the raft, consisting of mandible–tarsus and tarsus–tarsus attachments. Note that the mandibular grip requires particular care to minimize pain to the recipient of the bite.

Fig. 2.

Water repellency of the ant rafts. (A) An individual ant’s exoskeleton is moderately hydrophobic, as shown by the contact angle of the water drop. (B) Enhanced water repellency of a raft of ants, as shown by the increased contact angle of the water drop. (C) Buoyancy and elasticity of the ant raft, as shown by attempted submersion by a twig. (D) The plastron air bubble of an ant in soap-free water. The bubble makes the ant buoyant, necessitating the use of a thread to hold it underwater. (E) An air pocket trapped in a submerged ant raft. The shimmery layer around the ants is the air–water interface.

Fig. 3.

Dynamics of ant-raft construction. (A and B) Top and side views of growth of a 3,000-ant raft. (C) Schematic of experimental setup. Ants are rolled into balls in a beaker and then placed onto a pronged stabilizer in a partially filled aquarium. (D) The trajectory of a single ant on the raft, tracked over a duration of 40 s. (E) The relation between time t and the number of ants on the bottom of the raft n(t). Data are shown for four raft sizes, characterized by the number of ants in the raft N. Solid lines are given by the predictions of our theoretical model. (Inset) The relation between the number of ants in the raft N and the proportion of ants on the bottom at equilibrium n_∞/N.