The Living Bridge

Now that the Greenfield Bridge is gone over the Parkway East, my neighbors are joking that we need a zip line to get to Schenley Park.  If we were army ants, we could build a bridge of our bodies to solve the problem.

Native to the tropical rainforest, army ants are famous for their foraging habits.  The colony has no permanent home and is always on the move like an army, killing and eating other insects and raiding their nests.  The columns of workers are so focused on their task that they overcome obstacles by building living bridges of their bodies across the gaps.

How big a gap will the ants bridge? How do they modify it for different conditions?  To learn more about their behavior, Matthew Lutz of Princeton University and Chris Reid of the University of Sydney studied Eciton hamatum in Panama.  Their study techniques are shown in the video above.

For instance, they inserted a V-shaped apparatus into army ant paths and varied the angle — 12, 20, 40, and 60 degrees — to see what the ants would do.  It turns out that ant bridges are more sophisticated than anyone knew.

Ant bridges are typically 10-20 ant-lengths but the ants don’t start building at the widest spot.  Instead, they start from the narrowest point and make the bridge longer to shorten the overall path.  They also give up on a bridge if it ties up too many workers to make one. As Iain Couzin, Lutz’s graduate adviser, explains:

“They don’t know how many other ants are in the bridge, or what the overall traffic situation is. They only know about their local connections to others, and the sense of ants moving over their bodies,” Couzin said. “Yet, they have evolved simple rules that allow them to keep reconfiguring until, collectively, they have made a structure of an appropriate size for the prevailing conditions.

Could we humans bridge the Parkway East with our bodies?  No.  We aren’t long enough and we don’t have enough legs to hold onto each other.  Besides, some of us are afraid of heights.

Army ants aren’t afraid of heights. They’re blind (!) and have no idea how far they’d fall if they failed.


(*)Read more about the study here at Princeton University news.

(New Scientists YouTube video showing the study conducted by Matthew Lutz, Princeton University, and Chris Reid, University of Sydney)

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