How to Build an Ant Bridge: Start Small
You know when you’re out walking with a big horde of your friends and
you come to a chasm you can’t step across, so a bunch of you clasp each other’s
limbs and make yourselves into a bridge for the rest to walk on? Eciton army ants do this. And they’re not the only ants
that build incredible structures out of their strong, near-weightless
bodies. Weaver ants make chains between leaves by holding onto each other’s
waists. Fire ants cling together to form rafts and survive flooding.
Ants build these structures democratically, without any leaders. (Or
language, or tools.) To learn more about how they do it, scientists went into
the forests of Panama. Led by Chris Reid of the New Jersey Institute of
Technology and Matthew Lutz of Princeton University, researchers designed an
apparatus to test ants’ bridge-making skills. Eciton hamatum army
ants need to build bridges because they’re constantly on the move. They swarm
an area of the forest, devour any bugs they find there, and then move
on. When they come to a gap in the leaves covering the forest floor, they
bridge it to keep the troops moving quickly. Bridges can be just a few
ants, or hundreds of them.
The researchers built a device like a miniature, raised
roadway with a sharp kink in it. Then they found an army ant trail and put
their device right in the middle. They used leaves and sticks that were already
covered with the ants’ pheromones to guide the insects onto their device. If ants walked all the way along this artificial road, they would have
to make a wide detour from their original path. But if they built a bridge
across the angle, they could continue marching in more of a straight line. A
hinge let the scientists widen or narrow the angle the ants would have to
navigate.
The army ants successfully shortened their detour by building bridges
across the gap. But they didn’t start at the widest part. Instead, they built their bridge at the skinny end of the angle,
where it took only a few ant bodies. Then they gradually scooted the bridge
toward the wider end, adding ants as they went. Although the
ants bridged the gap handily, they didn’t manage to stretch across the very
widest part. Reid and Lutz think this is because of tradeoffs that come
with building a footbridge made of soldiers. The more ants are part of
a bridge, the fewer are left to carry stuff across it, for example. And
the ants maintained a steady ratio of length to width in their bridges, so as
bridges got longer, they also needed to get wider.
Once all the troops are across, an ant bridge breaks apart and
marches away too. Being part of a living bridge is, presumably, a
thankless job for the army ants. But it won’t be long before they get to
walk on someone else’s back.
Image and videos: Courtesy of Matthew Lutz, Princeton University, and Chris Reid, University of Sydney.
Reid CR, Lutz
MJ, Powell S, Kao AB, Couzin ID, & Garnier S (2015). Army ants dynamically
adjust living bridges in response to a cost-benefit trade-off. Proceedings
of the National Academy of Sciences of the United States of America PMID: 26598673
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