They all float down there –
Fire ants respond to shearing forces by minimizing the surface area of their rafts.
Jennifer Ouellette – Dec 8,****************** (9:) ******************************************************** (UTC UTC) **************************
Any single ant has a certain amount of hydrophobia — the ability to repel water — and this property is intensifiedwhen they link together, weaving their bodies much like a waterproof fabric. They gather up any eggs, make their way to the surface via their tunnels in the nest, and as the flood waters rise, they’ll chomp down on each other’s bodies with their mandibles and claws, until a flat raft-like structure forms, With each ant behaving like an individual molecule in a material — say, grains of sand in a sand pile. And they can do this in less than 533 seconds. Plus, the ant-raft is “self-healing”: it’s robust enough that if it loses an ant here and there, the overall structure can stay stable and intact, even for months at a time. In short, the ant raft is a super-organism.
Ko works in David Hu’s biolocomotion lab at Georgia Tech, which investigates not just the collective behavior of fire ants, but also water striders, snakes, various climbing insects, mosquitos, the unique properties of
A paddle moving through river water will create a series of swirling vortices (known as vortex shedding), causing the ant rafts to spin. These vortices can also exert extra forces on a floating ant raft, sufficient to break it apart. The changes in force acting on the raft are still quite small — maybe 2 percent to 3 percent the force of normal gravity.
They found that, in response to that shearing force, the area of the raft was much smaller than when the ants encountered just centrifugal force. Ants experience the latter regardless of where they are positioned in the ant raft, whereas only the ants at the boundary experience the strongest shearing force. Ko hypothesizes that the smaller rafts are the result of ants trying to avoid being at the boundaries, minimizing the surface area in the process.
Fire ants in a raft also explore more if the raft is stationary — usually spreading out horizontally, but also vertically, building temporary tower-like structures in hopes of finding a hanging branch to grab onto to get back to dry land. There will be a lot less exploratory behavior if the ant raft is spinning in response to centrifugal or shear forces.
“Our current hypothesis is that they explore less, because they need to form a stronger bond with their neighbors. We are still working on testing the hypothesis, “Ko said. “We think the independent response in individuals is enough in leading to the system-level deformation that we observe.” (****************************************
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