Army ants plugging potholes while "walking the plank"

Credit: Scott Powell, University of Bristol

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SYDNEY: While humans do their best to avoid potholes, marauding South American army ants have discovered a better solution: they plug the holes with their own bodies.

These foraging army ants form "living plugs" to fill gaps in the trail leading back to their nest, making a flatter surface so prey can be delivered at maximum speed.

"I think every road user who has ever inwardly cursed as their vehicle bounced across a pothole – jarring every bone in their body – will identify with this story," said co-author Nigel Franks of the University of Bristol in the U.K. "When it comes to rapid road repairs, the ants have their own do-it-yourself highways agency."

Predatory raids

The large colony size of the army ant means that frequent food collection is required. Certain army ants in the rainforests of Central and South America conduct spectacular predatory raids involving up to 200,000 foraging ants.

The raid always remains connected to the nest by a trail of forager traffic, along which prey-laden worker ants run back to the nest. This trail can be extremely uneven and full of "potholes" as it passes over leaves and branches on the forest floor.

Walking the plank

In the study, which is published in the British Journal Animal Behaviour, the researchers drilled holes of various sizes in wooden planks and then placed these planks in the path of the army ants. Not only did the ants form "living plugs" to fill the gaps as they "walked the plank", they also size-matched themselves to a particular hole and cooperated to fill larger holes.

Speaking to the BBC News website, co-author Scott Powell, also of the University of Bristol, explained: "The ants have a very large size range within their colony … When the ants bump into a hole they cannot cross, they edge their way around it and then spread their legs and wobble back and forth to check their fit. If they are too big, then they carry on and another ant will come along and measure itself in the same way. This carries on until an appropriately sized ant plugs the hole".

These hole-filling ants can remain in place for many hours. When the forager traffic diminishes, the well-trodden ants climb out of their holes and go back to the nest.

Collective intelligence?

The pothole-plugging behaviour of army ants has been shown to increase the average speed of prey-laden traffic and hence the daily prey intake by the colony. This more than compensates for the fact that the worker ants filling the holes don't carry any prey themselves.

"Broadly, our research demonstrates that a simple but highly specialised behaviour performed by a minority of ant workers can improve the performance of the majority, resulting in a clear benefit for the society as a whole", said Powell.